Growing tomatoes in Pennsylvania means battling a formidable lineup of pests and diseases — and two stand out as crop-killers. Late blight (Phytophthora infestans), the same pathogen that devastated Irish potato crops in the 1840s, can reduce a healthy Pennsylvania tomato patch to rotting plants in 5–7 days during cool, humid July and August weather. Equally destructive is the Brown Marmorated Stink Bug (Halyomorpha halys), which first established its U.S. population right here in Allentown, Pennsylvania around 1998 and has become a major pest in the mid-Atlantic ever since. The good news: with early identification and a preventive spray schedule timed to Pennsylvania’s pest pressure calendar, you can protect your crop and harvest a full season of ripe tomatoes.
Beyond these two major threats, Pennsylvania’s warm summers and humid conditions create ideal breeding grounds for early blight, Septoria leaf spot, and a parade of insects — tomato hornworms that can defoliate a plant overnight, aphids that transmit destructive viruses, flea beetles that riddle young transplants with holes, and whiteflies that thrive in wet years. This guide walks you through identification, treatment options (both organic and conventional), resistant varieties suited to Pennsylvania zones 5a–7a, and a month-by-month spray calendar so you know exactly what to monitor and when to act.
Pennsylvania Tomato Pest & Disease Pressure Calendar
Month-by-month pest and disease risk in Pennsylvania tomato gardens. Pressure varies by zone; this reflects statewide averages for mid-PA (zones 5b–6a).
Quick Reference: Pennsylvania Tomato Pests & Diseases
| Threat | When / Peak Month | First Signs | Best Treatment | Prevention |
|---|---|---|---|---|
| Late Blight | Jul–Sep / Peak Aug | Water-soaked gray-green lesions on leaves with white fuzzy growth on undersides; black firm rot on fruit | Copper fungicide or mancozeb every 7–10 days; remove infected plants immediately | Resistant varieties (Mountain Merit, Defiant PhR); drip irrigation; avoid overhead watering |
| Early Blight | May–Sep / Jul–Sep worst | Brown target-ring spots on lower leaves, progressing upward | Remove lower leaves; copper or chlorothalonil spray | Space plants well; consistent watering; resistant varieties (Celebrity, Mountain Fresh Plus) |
| Septoria Leaf Spot | Jun–Sep / Jul–Aug peak | Small circular spots with dark borders, tan/white centers, tiny black specks in center | Remove lower leaves; copper fungicide | Improve air circulation; avoid overhead watering; resistant varieties |
| Fusarium / Verticillium Wilt | Mid-season onward | Yellowing on one side of plant; brown discoloration in cut stems | No cure; remove infected plant and dispose | Crop rotation 3+ years; use resistant varieties (marked F and V); raised beds with new soil |
| Blossom End Rot (BER) | Jun–Aug / Jul peak | Black or brown leathery patch on bottom (blossom end) of fruit | Not a disease; improve watering consistency; add calcium amendment | Water deeply 1–2 inches per week; mulch; lime or gypsum at planting |
| Tomato Hornworm | Jun–Sep / Jul–Aug peak | Large sections of leaf eaten; dark droppings on leaves; huge green caterpillars (up to 4″) | Hand-pick; Bacillus thuringiensis (Bt) spray; spinosad | Look for white cocoons (beneficial parasitic wasps); remove debris in fall to disrupt pupae |
| Brown Marmorated Stink Bug | Jul–Oct / Aug–Sep peak | Corky white discoloration under skin and in fruit; small holes with brown rings | Row cover early season; kaolin clay; hand-pick at dusk; trap crops (sunflowers) | Remove host plants; monitor for aggregation in late summer; reflective mulch |
| Aphids | May–Sep / Jun–Jul peak | Small green insects on leaf undersides; sticky honeydew; curled leaves | Strong water spray; insecticidal soap; neem oil; beneficial insects | Plant trap crops; encourage ladybugs and lacewings; reflective mulch |
| Flea Beetles | May–Jun / at transplant | Tiny shothole damage on leaves of young plants; small black jumping insects | Row cover first 3 weeks; spinosad; diatomaceous earth | Row cover at planting; avoid direct seeding; reflective mulch |
| Whiteflies | Jun–Sep / wet years | Tiny white triangular insects on leaf undersides; sticky honeydew; yellowing leaves | Yellow sticky traps; insecticidal soap; reflective mulch | Adequate spacing; avoid overhead watering; encourage beneficial wasps |
Pennsylvania Tomato Disease Quick-ID
| Disease | First Symptom | Where It Starts | Peak Period (PA) | Key Distinguisher |
|---|---|---|---|---|
| Late Blight | Water-soaked gray-green spots on leaves | Any leaf, often upper canopy | July–Aug (cool/wet nights) | White fuzzy sporulation on leaf underside; spreads fast |
| Early Blight | Brown target-ring spots with yellow halo | Lower/oldest leaves first | Jun–Sep | Concentric rings inside spots; progresses upward slowly |
| Septoria Leaf Spot | Small circular spots, tan center, dark border | Lower leaves first | Jul–Aug (humid) | Tiny black specks (pycnidia) visible in spot centers |
| Fusarium Wilt | Yellowing on one side of plant | Lower leaves, one-sided | Jul–Aug (warm soil) | Brown discoloration inside stem when cut |
| Blossom End Rot | Brown/black leathery patch on fruit bottom | First developing fruits | Jun–Jul (first fruits) | Not a pathogen — no leaf symptoms; caused by inconsistent watering |
| Bacterial Speck | Tiny dark spots with yellow halo on leaves | Leaves and fruit surface | May–Jun (cool/wet) | Spots much smaller than early blight; rough dark spots on green fruit |
Late Blight: Pennsylvania’s #1 Tomato Killer
Late blight (Phytophthora infestans) is the disease Pennsylvania tomato growers fear most — and for good reason. It’s the same oomycete pathogen that caused the Great Irish Famine of the 1840s, and it remains a devastating threat in mid-Atlantic gardens, especially during Pennsylvania’s cool, humid July and August months. Unlike early blight, which develops slowly and can be managed with pruning, late blight can destroy the entire planting in 5–7 days once conditions turn wet and temperatures drop into the 60–70°F range at night — conditions that are common in Pennsylvania from late June through September.
Identifying Late Blight on Tomato Leaves and Fruit
Late blight has a distinctive appearance that sets it apart from other tomato diseases. On leaves, look for water-soaked gray-green lesions that often start on the leaf margins or tips and expand rapidly. The key identifying feature is the white fuzzy growth (sporulation) on the undersides of affected leaves — this is where the fungus releases millions of spores into the air. The lesions have an oily or water-soaked appearance, not the dry brown rings of early blight. On stems, you’ll see black or brown streaks and cankers. On fruit, the disease causes firm, dark brown or black rot that often starts at the top (blossom end) and spreads inward; the flesh beneath the skin becomes mushy and may smell sour.
The white mold on the leaf undersides is the dead giveaway. If you see it, assume late blight and act immediately. The spores that produce this white fuzz are what spread the disease via water — rain splash, overhead watering, even fog and dew keep the fungus happy. In Pennsylvania’s humid mid-summer climate, conditions for late blight spread are near-perfect, which is why many PA gardeners have experienced the heartbreak of a thriving tomato patch turning to mush in less than a week.
Why Late Blight Explodes in Pennsylvania
Three factors make Pennsylvania ideal for late blight epidemics. First, Pennsylvania’s summer nights often dip into the 50–70°F range in July and August, particularly in zones 5a–6a, which is the optimal temperature range for Phytophthora infestans reproduction. Second, the humid conditions, frequent rain, and dew cycles of a Pennsylvania summer keep leaf surfaces wet — the fungus needs wet conditions to form spores and spread. Third, many PA gardeners plant susceptible tomato varieties because they taste great (like Brandywine or Cherokee Purple), but offer no resistance to late blight. Together, these three factors create what plant pathologists call “blight weather” — and it strikes Pennsylvania nearly every summer.
The Pennsylvania State University Integrated Pest Management program monitors late blight conditions statewide. When temperatures fall below 72°F and humidity rises above 90% for consecutive days in summer, conditions are right for late blight reproduction. This happens predictably in late July and August in most of Pennsylvania.
How Fast Late Blight Spreads
Late blight is an oomycete (not a true fungus), and it spreads with alarming speed. The white spores on the leaf undersides release zoospores that travel in water — either rain or overhead irrigation — and infect new plants within hours of landing on wet foliage. One infected plant can infect the entire row in 3–4 days if conditions remain humid. The disease can also overwinter in plant debris, soil, and potato crops, so if late blight hit your garden last year, it’s likely to return this year unless you’ve removed all infected plant material and rotated your planting location.
Treatment: Fungicides and Plant Removal
Once you spot late blight, move fast. There is no cure for an infected plant — you cannot “save” a tomato plant with late blight by pruning off the infected leaves. Your only option is to remove the entire plant and dispose of it in the trash (not your compost pile). Leaving even a small piece of infected stem or fruit will continue to produce spores and spread the disease.
If you catch it on just one or two leaves in the early stages, you might try an aggressive spray schedule with copper fungicide or mancozeb, applied every 7–10 days as a preventive. These fungicides work best as a preventive barrier; they’re far less effective once the disease is established. Apply copper fungicides at the first sign of blight conditions (cool nights, high humidity, rain forecast), not after you see lesions. Once lesions are visible, the fungus is already established inside the leaf tissue, and fungicides can’t reach it.
For conventional growers, chlorothalonil is highly effective for late blight prevention, with a 7–10 day spray interval. For organic gardeners, copper sulfate or copper hydroxide fungicides are the gold standard. Bacillus subtilis (like Serenade) offers some protection but is less reliable than copper. Mancozeb (for conventional growers) is excellent for late blight prevention and can be rotated with copper to prevent fungicide resistance.
Apply any fungicide thoroughly, making sure to coat the undersides of leaves where spores form and where the disease spreads. Early morning or late afternoon application is best; avoid spraying during hot afternoon sun, which reduces fungicide effectiveness and can burn leaves.
Resistant Varieties for Pennsylvania
The best defense against late blight is to grow resistant varieties. If you’ve had late blight before, plant only varieties with the “Late Blight Resistance” rating on the seed packet or marked with “LB” or “PhR” (Phytophthora infestans resistance). Pennsylvania-tested resistant varieties include:
- Mountain Merit — a vigorous determinate with good taste and strong late blight resistance; excellent for PA zones 5b–6b
- Defiant PhR — mid-sized slicing tomato with excellent late blight resistance and good flavor
- Iron Lady — early maturing determinate with late blight resistance, suited to zone 5a
- Legend — heirloom-style tomato (not quite as fancy as Brandywine, but nearly) with excellent late blight resistance
- Plum Regal — a paste tomato with strong late blight and early blight resistance
Late Blight Prevention Strategy
Rather than wait for late blight to show up and then panic-spray, implement a preventive program from late June onward in Pennsylvania:
- Use drip irrigation or soaker hoses — keep the foliage completely dry. Overhead watering creates the wet conditions that late blight thrives in. Water only at the soil level, early in the morning.
- Space plants for air circulation — crowded plants stay wet longer and create a microclimate that favors late blight. Space determinate varieties 2 feet apart, indeterminate varieties 3 feet apart.
- Begin fungicide sprays in late June — don’t wait for symptoms. If late blight is in your area, start preventive sprays on a 7–10 day schedule in late June through August.
- Scout regularly — check the undersides of leaves at least twice a week during humid periods. Catch the first lesions before they spread.
- Prune lower leaves — remove leaves touching the soil or within 12 inches of the ground. These leaves are most likely to encounter soil-borne spores.
- Remove all plant debris in fall — Phytophthora infestans can overwinter in infected tomato and potato debris. Till soil in late fall to bury any remaining plant pieces.
- Rotate crops — plant tomatoes in a different location each year, at least 3 years away from where blight occurred.
If late blight is a recurring problem in your Pennsylvania garden, the most reliable solution is to grow resistant varieties, use drip irrigation, and begin preventive fungicide sprays in late June — don’t wait for symptoms.
Early Blight: A Manageable But Persistent Threat
Early blight (Alternaria solani) is less devastating than late blight, but far more common in Pennsylvania tomato gardens. While late blight strikes in cool, humid summer weather and can destroy a plant in days, early blight develops slowly over weeks, allowing you time to respond with pruning and fungicide sprays. It’s also more forgiving — a plant infected with early blight can often be saved by removing affected leaves and staying on top of sprays, something you cannot do with late blight.
Identifying Early Blight
Early blight starts on the lowest leaves and works its way up the plant, which is how it got its name — it’s “early” in the season relative to late blight, not because it shows up early on the plant (though it often does). Look for brown circular or irregular spots with concentric “target ring” patterns — the lesions look like a bullseye or tree rings. The rings are alternating bands of tan and dark brown. The spots start small (1/4 inch) and grow to 1/2 inch or larger. As the disease progresses, leaves yellow around the lesions and eventually drop, leaving bare stems.
Early blight favors warm, humid conditions (70–85°F) and spreads via water splash from rain or overhead watering, just like late blight. However, it develops more slowly and usually doesn’t kill the plant — it just defoliates it progressively, reducing yield. In Pennsylvania, early blight typically appears in late May or June and worsens through August and September.
Early Blight Treatment
Remove lower leaves aggressively. Once early blight appears, prune off all infected leaves and any leaves touching the soil or lower branches within 12 inches of the ground. Do this immediately — don’t wait. The spores of Alternaria solani live in plant debris and soil, so removing infected foliage is your first line of defense. Prune in the morning when the plant is dry, and sanitize your pruners between cuts (dip in a 10% bleach solution or rubbing alcohol) to avoid spreading the fungus from plant to plant.
After pruning, begin fungicide sprays on a 7–10 day schedule with copper or chlorothalonil (for conventional growers). Mancozeb is also very effective for early blight. For organic growers, copper sulfate or copper hydroxide fungicides work well. Sulfur (for fungal control) can help, though sulfur and copper should not be mixed.
Space plants well — at least 2 feet apart for determinate varieties — to ensure good air circulation. Use drip irrigation or soaker hoses to keep foliage dry. Improve air circulation by pruning lower leaves and branches ruthlessly. The goal is to create a microclimate around the plant where leaves dry quickly after rain or watering.
Resistant Varieties to Early Blight
Celebrity and Mountain Fresh Plus are widely available Pennsylvania-garden varieties with solid early blight resistance. If early blight is recurring in your garden, prioritize planting these varieties alongside your favorites.
Septoria Leaf Spot: A Persistent Summer Disease
Septoria leaf spot (caused by Septoria lycopersici) is extremely common in Pennsylvania’s humid summers and is often confused with early blight because both diseases affect tomato leaves. The key difference: Septoria creates tiny circular spots (often just 1/8 inch across) with very fine dark borders and pale tan or white centers, and the centers contain tiny black specks (pycnidia) that look like pepper grains. Early blight, by contrast, creates larger spots (1/4 to 1/2 inch or more) with prominent concentric rings.
Identifying Septoria Leaf Spot
Septoria starts on lower leaves and progresses upward, just like early blight. The spots are more uniform and smaller than early blight lesions, with less prominent rings. The tan or whitish center of each lesion contains the tiny black fruiting bodies (pycnidia) of the fungus — this is the diagnostic feature. Septoria is favored by warm, wet conditions (68–77°F) and spreads via water splash, so it thrives during Pennsylvania’s humid June through September growing season.
Septoria Treatment and Prevention
Management is essentially identical to early blight: remove lower leaves, improve air circulation, use drip irrigation, and spray with copper or mancozeb on a 7–10 day schedule. Copper fungicides are particularly effective against Septoria. Chlorothalonil (for conventional growers) is also highly effective. Focus on keeping the foliage dry — this disease absolutely requires wet conditions to spread, so good cultural practices (drip irrigation, pruning lower leaves) are your first defense.
Clean up all infected plant debris in fall. Septoria lycopersici overwinters in plant material, so removing all tomato residue from your garden in late fall reduces the spore load for next season.
Free PA Planting Calendar
Zone-specific · 4 pages · Instant download
Get the exact dates for your Pennsylvania zone — when to start seeds indoors, direct sow, transplant, and harvest. Built around your local frost window, not a generic national average.
- Wall chart with all key dates
- Seed-start schedule (50+ crops)
- First & last frost reference
- Soil temp cheat sheet
Fusarium and Verticillium Wilt: Soil-Borne Killers
Fusarium wilt and Verticillium wilt are soil-borne fungal diseases that affect tomato plants mid-season and onward. Unlike foliar diseases like late blight or early blight that spread through air and water, Fusarium and Verticillium live in the soil and infect the plant through its roots. Once established in the vascular system (the plant’s internal “water pipes”), there is no cure — the plant will decline and die.
Identifying Wilt Diseases
The symptoms of Fusarium and Verticillium wilt are similar: a healthy-looking plant suddenly begins to yellow and wilt, often on one side of the plant first. The wilting happens despite adequate soil moisture — a key sign that the problem is vascular, not just dry soil. If you cut the stem and look at the inside, you’ll see brown discoloration in the vascular tissue (the woody part) — this brown ring is the fungus clogging the plant’s water-conducting tissue.
Fusarium causes permanent wilting that doesn’t recover, even if you water heavily. Verticillium wilt may come and go with the time of day (the plant wilts during the heat of the day and partially recovers at night), but over time the plant succumbs. Both diseases are more common in warmer soils (above 70°F), which is why they typically appear mid-to-late season in Pennsylvania.
No Cure: Remove and Rotate
Once you confirm wilt disease, there is no fungicide that will cure an infected plant. Your only option is to remove the entire plant, including as much of the root ball as possible, and dispose of it in the trash (not compost). Do not pull it out and leave the soil undisturbed — the fungus will remain in the soil for years.
The prevention strategy is crop rotation and resistant varieties. Do not plant tomatoes in the same location for at least 3 years if Fusarium or Verticillium has been confirmed. The fungi can survive in soil for many years, but rotating crops for 3+ years depletes the spore load.
When selecting tomato varieties, look for varieties marked with “F” or “V” (or “FF” or “VV”) on the seed packet — these indicate resistance to Fusarium wilt and/or Verticillium wilt. Mountain Merit, Defiant PhR, and Legend all carry wilt resistance.
Consider using raised beds or containers with fresh, pasteurized potting soil if wilt has been a problem. This is one of the few situations where growing tomatoes in containers or raised beds with new soil gives you a real advantage — you’re avoiding the contaminated garden soil entirely.
Blossom End Rot: Not a Disease, But a Cultural Problem
Blossom end rot (BER) is one of the most common tomato fruit problems in Pennsylvania, and it’s frequently mistaken for a disease. It is not a pathogen — it’s a physiological disorder caused by inconsistent watering disrupting calcium uptake. Understanding this distinction is key, because the cure is not fungicide but better water management.
What Blossom End Rot Looks Like
BER appears as a dark brown or black leathery patch on the blossom end (bottom) of the tomato fruit — the end opposite the stem. The patch starts small and can grow to cover 1–2 inches of the fruit. The tissue beneath the skin becomes sunken and feels soft. Internally, the fruit may be partly rotted. Affected tomatoes are often smaller than normal and may have a bitter taste. BER typically appears on the first fruits to set in early summer (June–July in Pennsylvania) and becomes less common as the season progresses and plants mature.
Why Blossom End Rot Happens
Tomato plants need consistent calcium availability to develop properly. Calcium is not very mobile in plant tissues — once a fruit is set and begins to develop, the plant directs most water and nutrients to the actively growing stems and leaves, not the fruit. If soil moisture fluctuates dramatically — say, the plant is drought-stressed for several days and then flooded with heavy rain — the disrupted water flow also disrupts calcium transport to the developing fruit. The growing fruit doesn’t get the calcium it needs, and the blossom end (which is the last part of the fruit to reach full size) is particularly vulnerable. The result is a sunken, leathery, brown lesion.
Inconsistent watering is the #1 cause of BER. Other factors that increase BER risk include:
- Excessive nitrogen fertilizer (which promotes lush foliage at the expense of fruit calcium uptake)
- Low soil calcium (in acidic, sandy soils)
- Soil salt buildup (which interferes with calcium uptake)
Preventing Blossom End Rot
The cure is consistent watering. Water deeply — 1–2 inches per week — at the soil level (using drip irrigation or soaker hoses), not overhead. Consistent watering maintains steady soil moisture and steady calcium transport. Do not allow the soil to dry out between waterings. Apply a 3–4 inch mulch layer (straw or shredded leaves) to buffer soil moisture and keep roots cooler and moister during hot spells.
At planting time, add calcium amendment to the soil — this is especially important in acidic Pennsylvania soils. You can use ground limestone (lime), gypsum, or crushed eggshells. A light dusting of agricultural lime mixed into the soil around the planting hole helps prevent BER. If your soil test shows calcium is adequate, you still benefit from the consistent watering — calcium uptake improves dramatically with steady moisture.
Once BER appears on fruit, there’s no cure for that fruit — remove and discard it. But by implementing consistent watering and mulching, new fruits developing later in the season will be protected. Don’t apply excess nitrogen, which makes the plant grow vigorously and can worsen BER by shifting the plant’s priorities away from fruit.
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Pennsylvania Tomato Pest Quick-ID
| Pest | What You See | When (PA) | Damage Type | First Response |
|---|---|---|---|---|
| Tomato Hornworm | Large green caterpillar (up to 4″); black & white markings; black horn | Jun–Aug | Rapid defoliation; chewed fruit | Hand-pick; look for white cocoons (leave if present) |
| Brown Marmorated Stink Bug | Brown shield-shaped bug, ½″; marbled pattern | Aug–Sep (peak) | Corky white areas inside fruit; cat-facing | Kaolin clay; row cover; hand-pick at dusk |
| Aphids | Clusters of tiny soft insects (green/yellow/black) on leaf undersides; sticky honeydew | May–Jun; Sep | Curled leaves; virus transmission | Strong water spray; insecticidal soap |
| Flea Beetles | Tiny black jumping beetles; shothole damage on young leaves | May–Jun (transplants) | Shothole defoliation of seedlings | Row cover for first 3 weeks; spinosad |
| Whiteflies | Triangular white insects that fly up when disturbed; dusty white leaf undersides | Jul–Aug (wet years) | Leaf yellowing; honeydew; sooty mold | Yellow sticky traps; insecticidal soap |
| Cutworms | Transplants cut off at soil level overnight | May–Jun | Entire transplant severed | Cardboard collars around stems at planting |
Tomato Hornworm: Pennsylvania’s Iconic Caterpillar
The tomato hornworm is arguably Pennsylvania’s most infamous tomato pest — a huge green caterpillar that appears seemingly overnight and can defoliate an entire plant in a matter of days. Two species are common in Pennsylvania: Manduca quinquemaculata (tobacco hornworm) and Manduca sexta (tomato hornworm). Both are devastating, reaching up to 4 inches long at maturity, and both can destroy the foliage of a mature plant in 48 hours. The good news is that hornworms are easier to manage than diseases — they’re large, visible, and responsive to organic controls.
Identifying Hornworm Damage
Hornworm damage is unmistakable: large, ragged sections of leaves are missing, often leaving only the stem and the thickest veins. Look for dark pellet-like droppings (frass) on leaves below the damage — this is the hornworm’s feces and is a reliable clue to the pest’s location. The caterpillar itself is large (up to 4 inches), bright green, and often has white or light-colored stripes running along its body. At the rear end, there’s a dark horn-like projection (the “hornworm” part). Despite their size and visibility, hornworms can be surprisingly hard to spot because they’re perfectly camouflaged against the green foliage.
Hornworm damage typically starts in June in Pennsylvania as eggs hatch, peaks in July and August, and may occur again in a second generation in late August and September. The caterpillars feed for 3–4 weeks before pupating.
Hand-Picking: The Simplest Control
Hand-picking is the most effective control for hornworms. Scout your plants regularly (especially in early morning or at dusk, when hornworms feed actively) and pick off any caterpillars you find. Drop them into a bucket of soapy water. This sounds tedious, but most home gardeners only encounter a few hornworms per season, so it’s very doable. Wear gloves if the thought of handling a 4-inch caterpillar bothers you — they’re harmless to humans, though they may regurgitate a smelly green liquid when stressed (a defense mechanism).
Check under and inside the canopy, not just the outer leaves. Hornworms like to hide in the interior of the plant where it’s darker.
Beneficial Wasp Cocoons: A Sign of Natural Control
If you’ve been spotting hornworms but notice that some of them have tiny white cocoons attached to their backs, that’s actually great news. These white rice-grain cocoons are left behind by parasitoid wasps (Ichneumon and Braconid wasps) that lay their eggs inside the hornworm larva. As the wasp larvae develop, they consume the hornworm from the inside, eventually emerging and spinning their cocoons on the caterpillar’s back. A hornworm covered in wasp cocoons is already dead or dying — it can no longer feed or move effectively.
If you see a hornworm with wasp cocoons, do NOT kill it. Leave it in the garden so the wasps can emerge and fly off to parasitize other hornworms. This is one of nature’s best pest controls.
Organic Spray Control: Bacillus thuringiensis (Bt)
If hand-picking isn’t feasible or if hornworm populations are high, Bacillus thuringiensis (Bt), sold as Monterey Garden Insect Spray or similar products, is the gold-standard organic control. Bt is a naturally occurring soil bacterium that produces a toxin lethal to caterpillars but completely safe for humans, pets, and beneficial insects. When a caterpillar ingests Bt, the toxin damages its gut, and it stops feeding within hours and dies within a few days.
For maximum effectiveness, apply Bt in the early morning or late evening when hornworms are feeding actively. Spray the foliage thoroughly, coating both the tops and undersides of leaves. Repeat every 7–10 days if hornworm activity continues. Bt breaks down quickly in sunlight, so repeated applications are often necessary.
Spinosad is another organic option — a fermented-product insecticide that kills caterpillars and is OMRI-certified organic. It’s a bit harsher on beneficial insects than Bt, so use Bt first if possible.
Cultural Control: Disrupt Pupation
Hornworms overwinter as pupae buried 2–4 inches in the soil. In fall, after you’ve harvested your tomatoes and removed the plants, till the soil in late October or November to a depth of 6–8 inches. This brings pupae to the surface where they’re exposed to predators, weather, and desiccation. Fall tilling is particularly effective for managing hornworms — it’s one of the main benefits of tilling, and why many PA gardeners till their vegetable beds in late fall.
Brown Marmorated Stink Bug: A Modern Pennsylvania Pest
The Brown Marmorated Stink Bug (Halyomorpha halys) is a relatively new invasive pest in Pennsylvania, but it has quickly become a major problem. The species first established its U.S. population in Allentown, Pennsylvania around 1998, arriving accidentally in shipping containers from Asia. In the 20+ years since, it has spread throughout the U.S. and has become one of the most damaging agricultural pests in North America. For tomato growers in Pennsylvania, BMSB is a serious and growing threat, particularly in zones 6a and warmer.
Identifying Brown Marmorated Stink Bug Damage
Both adult and nymph stink bugs use piercing-sucking mouthparts to feed on developing tomato fruit, creating corky white or tan discoloration under the skin of the fruit where they’ve extracted fluid and introduced saliva. The punctures themselves are often surrounded by a brown or tan halo. Multiple punctures can make the entire fruit unmarketable — the damage is purely cosmetic (the interior of the fruit is not affected), but an affected tomato looks awful and will rot faster than undamaged fruit.
Stink bug damage is distinct from other pests: it’s not the chewed-out gaps you’d see from a caterpillar, and it’s not the stippled pattern of spider mite damage. It’s localized discoloration or small punctures with a sunken or corky appearance immediately under the skin.
In Pennsylvania, brown marmorated stink bugs are most damaging in late July through September, when adults aggregate in preparation for overwintering. Early to mid-summer pressure is usually light, but as August progresses and adults complete their second generation, populations explode.
Why Brown Marmorated Stink Bug Is So Hard to Control
BMSB is a difficult pest to manage for several reasons. First, there is no truly effective organic insecticide — neem oil and other organic sprays have minimal impact on adults. Second, the stink bugs aggregate in massive numbers in late summer and are hard to kill with individual spray applications. Third, they have few natural enemies in North America (parasitoid wasps from Asia have been introduced, but aren’t yet common enough to provide meaningful control). Fourth, the adults have a very hard exoskeleton that resists many insecticides.
This is why BMSB management relies heavily on prevention and cultural practices rather than sprays.
Brown Marmorated Stink Bug Prevention and Management
Row cover at planting time is your best early-season defense. From the time you transplant tomatoes in mid-May through late June, cover your tomato bed with lightweight row cover (like Agribon). This keeps BMSB nymphs (which begin reproducing in June) off your plants. Once plants are flowering heavily and need pollinator access, remove the row cover.
Kaolin clay (Surround) can help reduce BMSB feeding damage by creating a physical barrier on the fruit. Apply it starting in early July, before adult populations peak. Reapply after rain. It’s not 100% effective, but it reduces damage.
Trap crops are an interesting option if you have space. BMSB strongly prefers certain plants (like sunflowers, amaranth, and winter squash) over tomatoes. Planting a border of sunflowers or amaranth around your tomato bed can pull stink bugs away from your tomatoes and concentrate them on the trap crop. Once the trap crop is infested, you can simply pull and dispose of the entire trap crop plant.
Reflective mulch (aluminum foil or specially designed reflective plastic) confuses BMSB and can reduce feeding on fruit. It’s not a complete solution, but combined with other methods, it helps.
Hand-picking at dusk is tedious but possible in small gardens. Stink bugs are most active and visible at dusk. You can knock aggregating stink bugs into a bucket of soapy water. This is laborious for large populations, but on smaller plantings can be effective.
For conventional gardeners, pyrethroid insecticides (like permethrin) are effective against BMSB, but repeated applications are necessary and can harm beneficial insects. Spray in the early morning or late evening for best results, and avoid blooms to protect pollinators.
Seasonal Management for BMSB in Pennsylvania
Early in the season (May–June), BMSB pressure is light, and row cover or kaolin clay is usually sufficient. By July, monitor fruit weekly and begin kaolin applications. August and September are when BMSB pressure peaks, and this is when you need to be most vigilant. Late in the season (October), as adults seek overwintering sites, they may migrate away from your garden entirely, relieving pressure.
Remove all host plants (weeds, garden debris) from around your tomato patch in late fall, as overwintering BMSB adults hide in plant residue.
Aphids: Vectors of Viral Disease
Green peach aphids and other aphid species feed on tomato plants throughout the Pennsylvania growing season, but their damage is as much about what they transmit as what they directly cause. Aphids transmit viruses more than they cause direct damage — particularly Tomato Mosaic Virus (ToMV) and Cucumber Mosaic Virus (CMV), which cause severe leaf distortion, mottling, and yield reduction.
Identifying Aphid Infestation
Look for small green, yellow, or black soft-bodied insects clustered on the undersides of leaves, especially on new growth. Infested plants often show sticky honeydew residue on leaves (which is aphid excrement) and may have leaves that curl, yellow, or become distorted. Ants are often attracted to the honeydew and may be visible on affected plants — where there are many ants on tomatoes, aphids are usually present.
Aphids are most common in early summer (June–July in Pennsylvania) and again in late summer (August–September). Cool, wet springs favor aphid reproduction.
Aphid Management
A strong spray of water from the hose can dislodge aphids and significantly reduce populations. Do this weekly if aphids are present. The aphids fall to the soil and can’t easily climb back up the plant.
Insecticidal soap and neem oil are effective organic controls. Spray the undersides of leaves thoroughly, where aphids congregate. Repeat every 7–10 days. Both products work best on young aphids (nymphs); mature adults are harder to kill. Early detection and intervention are key.
Encourage beneficial insects — ladybugs and lacewing larvae are voracious aphid predators. Plant flowering herbs like cilantro, fennel, and dill nearby to attract beneficial insects. Avoid broad-spectrum insecticides that kill beneficial insects; stick with targeted organic options like insecticidal soap or neem.
Reflective mulch (aluminum foil or reflective plastic) confuses aphids and can reduce their ability to locate tomato plants, thus reducing infestation rates.
Plant trap crops — nasturtiums attract aphids strongly and can serve as a sacrificial trap crop planted around the tomato bed.
For conventional gardeners, pyrethroid or neonicotinoid insecticides are effective, but neonicotinoids are harmful to bees and should be avoided or used only after blooming has finished.
Flea Beetles: Young Plant Destroyers
Flea beetles are tiny black jumping beetles that create the most damage on young transplants in May and June. They punch tiny holes (shothole damage) in leaves, giving the foliage a lacy, pockmarked appearance. On mature plants, flea beetle damage is usually cosmetic and not a serious concern. But on newly transplanted seedlings, heavy flea beetle feeding can stunt growth, delay flowering, and reduce yields significantly.
Identifying Flea Beetle Damage
Look for numerous tiny (1–2 mm) round holes scattered across the leaves of young plants, creating a shot-through appearance. If you look closely, you may see tiny black jumping beetles. Flea beetles are most active on warm, sunny days and jump away quickly if disturbed.
Flea Beetle Control
Row cover is the simplest and most effective control. Cover transplants with lightweight row cover at planting time (mid-May in Pennsylvania) and keep the cover in place for 3 weeks (through early June) or until plants are flowering. Row cover blocks flea beetles while allowing light and water through. Once plants are in heavy bloom, remove the cover to allow pollinator access.
Spinosad spray is effective against flea beetles and is OMRI-certified organic. Spray every 7–10 days starting at transplant time if row cover isn’t used.
Diatomaceous earth (DE) can help — dust food-grade DE on young plants every 5–7 days or after rain. DE is a physical desiccant that injures the waxy cuticle of flea beetles.
Reflective mulch around young plants can reduce flea beetle pressure by confusing the insects.
For conventional gardeners, pyrethroid insecticides are effective against flea beetles, particularly if applied early in the season.
Whiteflies: A Wet-Weather Problem
Whiteflies are small triangular white insects that cluster on the undersides of tomato leaves and can become problematic in wet years in Pennsylvania. They suck plant sap, which weakens the plant and can transmit viruses, and they produce copious sticky honeydew that attracts sooty mold.
Identifying Whitefly Infestation
Look for clouds of tiny white insects that flutter up when you brush the leaves. Check the undersides of leaves for clusters of small white eggs and nymphs (which look like tiny translucent scales). Affected plants may show yellowing leaves and wilting despite adequate water due to heavy feeding.
Whitefly Management
Yellow sticky traps are highly effective for monitoring and reducing whitefly populations. Place bright yellow sticky traps throughout your tomato bed at plant level. Whiteflies are attracted to yellow, and the sticky surface traps them. Replace traps when they become covered with insects.
Insecticidal soap and neem oil work well on whitefly nymphs, though mature adults are harder to kill. Spray the undersides of leaves thoroughly and repeat every 7–10 days.
Reflective mulch reduces whitefly pressure by confusing the insects.
Good air circulation through proper spacing and pruning helps prevent whitefly outbreaks — this pest favors warm, humid, still-air conditions.
Avoid overhead watering, which creates the humid conditions whiteflies love.
Integrated Spray Calendar for Pennsylvania Tomatoes
This month-by-month spray schedule is designed for Pennsylvania gardeners in zones 5b–6b (typical mid-PA conditions). Adjust timing if you’re in zone 5a (start 1 week later) or zone 7a (start 1 week earlier). This schedule assumes you’re growing at least some susceptible varieties; if you’re growing all resistant varieties, you can reduce spray frequency.
| Month | Main Threats | Spray Schedule | Product (Organic) | Product (Conventional) | Monitoring Notes |
|---|---|---|---|---|---|
| May | Flea beetles, Aphids (light) | Row cover or weekly spinosad if beetles visible | Spinosad, Diatomaceous Earth | Pyrethroid if needed | Scout for flea beetle damage on young leaves. Remove row cover if flowers appear. |
| June | Aphids, Hornworm eggs, Early blight risk | Weekly if aphids present; begin copper if rain is frequent | Insecticidal soap, Neem oil, Copper sulfate | Pyrethroid (aphids), Chlorothalonil (early blight) | Look for first hornworm damage (missing leaves, dark droppings). Scout leaf undersides for aphids. |
| July | Late blight window, Early blight, Hornworms, BMSB, Aphids | Copper every 7–10 days if temps drop below 72°F at night; hand-pick hornworms; begin kaolin for BMSB | Copper sulfate (7–10 days), Kaolin clay, Bt (hornworms), Insecticidal soap (aphids) | Mancozeb or chlorothalonil (7–10 days), Permethrin (BMSB, hornworms) | Monitor nightly lows — if 60–70°F predicted, assume late blight conditions and spray preventively. Look for white spore patches on leaf undersides (late blight) or target-ring spots (early blight). |
| August | Late blight (PEAK), Early blight, Hornworms, BMSB (heavy), Septoria | Copper every 7–10 days as preventive; continue hornworm monitoring; kaolin for BMSB | Copper sulfate (7–10 days), Kaolin clay, Bt (hornworms) | Mancozeb or chlorothalonil (7–10 days), Permethrin (BMSB) | Peak month for late blight risk in PA. Check leaf undersides for white mold (late blight). Remove any infected plants immediately. Watch for second-generation hornworms. BMSB populations peak — monitor fruit for damage. |
| September | Early blight, Septoria, BMSB (aggregating), Hornworms (late generation) | Continue copper spray; hand-pick hornworms; monitor BMSB | Copper sulfate (7–10 days), Bt (hornworms) | Chlorothalonil, Permethrin (BMSB) | Focus on lower leaves — remove any with spots. BMSB may be aggregating heavily; consider reflective mulch or hand-picking if populations are severe. Harvest mature green tomatoes if late blight appears; ripen them indoors. |
| October | None (harvest ends) | None; focus on cleanup | N/A | N/A | Remove all plants and debris. Do NOT compost diseased plants (late blight, early blight); bag or burn them. Clean up weeds and host plants where BMSB overwinters. |
Notes: This schedule is preventive, meaning sprays are applied before disease is visible to prevent it from becoming established. Once disease is visible, it’s much harder to control. Resistance to fungicides can develop, so rotate between copper, mancozeb, and chlorothalonil (if conventional) or between copper and sulfur (if organic). Always follow label directions and respect harvest intervals (the number of days that must pass between the last spray and harvest). Early blight and Septoria thresholds: spray if you see the first lesions on lower leaves. Late blight: spray at the first sign of “blight weather” (60–70°F nights + high humidity + rain forecast), not just when you see lesions.
Zone-by-Zone Pest Pressure in Pennsylvania
Pennsylvania spans USDA hardiness zones 5a (coldest, in northeast) to 7a (warmest, in southeast). Pest and disease pressure varies significantly by zone due to differences in temperature, humidity, and season length. Use the zone selector below to see how pest pressure shifts in your area.
Frequently Asked Questions
More Pennsylvania Tomato Growing Resources
Protecting your tomato crop from pests and diseases is only half the battle. Check out these related guides to get the most from your Pennsylvania garden:
- Pennsylvania Tomato Growing Guide — Complete planting, staking, pruning, and harvesting information for every zone from 5a to 7a.
- Growing Tomatoes in Containers in Pennsylvania — Container varieties, soil mixes, watering schedules, and how to extend your harvest season.
- Tomatoes in Raised Beds: A Pennsylvania Guide — Raised bed dimensions, soil preparation, drainage, and varieties that thrive in restricted root systems.
- Pennsylvania Vegetable Gardening Hub — Guides for peppers, cucumbers, beans, squash, and other vegetable crops in your zone.
