Agricultural Reference
Crop Disease Guide
Identify diseases affecting New Jersey crops with our visual reference. 18 common diseases across blueberries, peaches, tomatoes, grapes, corn, soybeans, and more — with treatment timing so you know exactly when to call for drone spraying.
Early Blight
Caused by Alternaria solani, early blight produces dark concentric rings on lower leaves first. Left unchecked it defoliates plants and reduces yields by up to 50%.
Signs to Watch
- Dark brown spots with concentric rings (target pattern)
- Yellowing leaves starting from the bottom
- Stem lesions near the soil line
When to Spray
Apply fungicide at first sign of spots, typically mid-June through August.
Late Blight
The same pathogen (Phytophthora infestans) that caused the Irish Potato Famine. Spreads explosively in cool, wet weather and can destroy a field in days.
Signs to Watch
- Water-soaked gray-green lesions on leaves
- White fuzzy mold on leaf undersides in humid conditions
- Dark, firm rot on fruit
When to Spray
Preventive sprays before rainy periods; immediate treatment at first detection.
Powdery Mildew
A white, powdery fungal coating that thrives in warm, dry days with cool nights. Reduces photosynthesis and weakens vines and trees over the season.
Signs to Watch
- White powdery patches on leaf surfaces
- Curling or distorted new growth
- Premature leaf drop
When to Spray
Begin applications at bud break; repeat every 10-14 days through fruit set.
Downy Mildew
Plasmopara viticola attacks leaves, shoots, and fruit clusters. It favors wet springs and can devastate untreated vineyards in a single season.
Signs to Watch
- Oily yellow-green spots on upper leaf surface
- White-gray downy growth on leaf undersides
- Brown, shriveled fruit clusters
When to Spray
Apply before bloom when shoots are 6-10 inches; repeat after rain events.
Anthracnose
Colletotrichum species cause sunken, dark lesions on ripe fruit. Warm, humid conditions accelerate spore production and fruit rot during harvest.
Signs to Watch
- Sunken, circular lesions on ripe fruit
- Salmon-pink spore masses in wet weather
- Twig dieback on woody plants
When to Spray
Spray at bloom and pre-harvest; critical during warm, rainy stretches.
Bacterial Spot
Xanthomonas bacteria cause water-soaked spots that turn brown and papery. Splashing rain spreads the pathogen rapidly through fields.
Signs to Watch
- Small, dark, water-soaked spots on leaves
- Raised, scab-like lesions on fruit
- Shot-hole appearance as leaf tissue falls out
When to Spray
Copper sprays before symptoms appear; treat after storms in warm weather.
Cedar Apple Rust
Gymnosporangium juniperi-virginianae alternates between cedar and apple hosts. Bright orange spots on apple leaves reduce tree vigor and fruit quality.
Signs to Watch
- Bright yellow-orange spots on upper leaf surfaces
- Tube-like projections on leaf undersides
- Gelatinous orange galls on nearby cedar trees in spring
When to Spray
Fungicide at pink bud through petal fall; 3-4 applications total.
Gray Mold (Botrytis)
Botrytis cinerea is the most common post-harvest rot in NJ. Cool, damp conditions during bloom cause blossom blight and later fruit rot.
Signs to Watch
- Gray, fuzzy mold on flowers or ripening fruit
- Brown, water-soaked tissue beneath mold
- Wilting and collapse of flower clusters
When to Spray
Apply at early bloom and again at full bloom; critical in wet springs.
Mummy Berry
Monilinia vaccinii-corymbosi is the most economically important blueberry disease in NJ. Infected berries shrivel into hard, gray 'mummies' that overwinter on the ground.
Signs to Watch
- Wilted, brown shoot tips in early spring
- Berries that turn salmon-pink, then gray and hard
- Shriveled mummified berries on the ground or bush
When to Spray
First spray at green tip; repeat at 10% bloom and full bloom.
Phomopsis
Phomopsis vaccinii causes twig blight and fruit rot in blueberries. Wet spring weather drives infection through wounds and natural openings.
Signs to Watch
- Brown, wilted shoot tips with visible cankers
- Small dark spots on green fruit
- Soft rot and shriveling at harvest
When to Spray
Spray at bud swell and green tip; additional applications if rain persists.
Brown Rot
Monilinia fructicola is the primary stone fruit disease in NJ. It can destroy an entire peach crop in warm, humid summers within days of harvest.
Signs to Watch
- Soft, brown expanding lesions on ripe fruit
- Tan powdery spore tufts on fruit surface
- Mummified fruit hanging on tree branches
When to Spray
Pre-bloom through petal fall, then again 2-3 weeks before harvest.
Leaf Scorch
Caused by Xylella fastidiosa bacteria spread by sharpshooter leafhoppers. Infected plants show marginal leaf burn and gradual decline over years.
Signs to Watch
- Browning and drying of leaf margins
- Reddish discoloration in blueberry leaves
- Reduced vigor and smaller fruit over multiple seasons
When to Spray
No direct spray cure; control leafhoppers early in the season to prevent spread.
Cercospora Leaf Spot
Cercospora species cause small purple-bordered spots that expand and coalesce. Heavy infections cause premature defoliation and weaken plants for winter.
Signs to Watch
- Small spots with purple-red borders and tan centers
- Coalescing spots leading to large dead areas
- Premature leaf yellowing and drop
When to Spray
Apply fungicide when spots first appear, typically mid-summer; repeat in 14 days.
Septoria Leaf Spot
Septoria lycopersici starts on lower leaves and works upward. One of the most common tomato diseases in NJ, it thrives in warm, wet weather.
Signs to Watch
- Many small circular spots with dark borders and gray centers
- Tiny black dots (pycnidia) visible in spot centers
- Progressive defoliation from bottom up
When to Spray
Begin sprays when first spots appear on lower leaves; repeat every 7-10 days.
Northern Corn Leaf Blight
Exserohilum turcicum produces large cigar-shaped lesions on corn leaves. Severe infections during tasseling can reduce yields by 30% or more.
Signs to Watch
- Long, elliptical gray-green lesions (1-6 inches)
- Lesions running parallel to leaf veins
- Lower leaves affected first, moving upward
When to Spray
Apply fungicide at VT (tasseling) if lesions reach the ear leaf or above.
Sudden Death Syndrome
Fusarium virguliforme attacks soybean roots in cool, wet soils. Above-ground symptoms appear suddenly during pod fill, causing dramatic yield loss.
Signs to Watch
- Interveinal yellowing and browning of upper leaves
- Leaves drop but petioles remain attached to stem
- Blue-gray fungal growth on root surface
When to Spray
Seed treatments at planting; foliar fungicides are ineffective for this disease.
Frogeye Leaf Spot
Cercospora sojina creates distinctive eye-shaped lesions on soybean foliage. Warm, humid conditions in July and August favor rapid disease development.
Signs to Watch
- Circular spots with gray centers and dark reddish-brown borders
- Spots that resemble a frog's eye
- Lesions on leaves, stems, and pods
When to Spray
Apply fungicide at R3 (beginning pod) if threshold is exceeded; scout weekly.
Black Rot
Guignardia bidwellii (grape) and Botryosphaeria obtusa (apple) cause fruit mummification and leaf spots. Infected fruit becomes unmarketable.
Signs to Watch
- Reddish-brown circular leaf spots (apple)
- Light brown spots that darken and shrivel grapes
- Hard, black mummified fruit
When to Spray
Spray from bud break through 4-5 weeks after bloom; sanitation is critical.
Showing 18 of 18 diseases
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Data Sources
Disease information compiled from the 20k+ Multi-Class Crop Disease Images dataset, Crop Pest and Disease Detection research, Rutgers Cooperative Extension plant pathology guides, and USDA agricultural research publications.