Diseases & Pests —
Disease of the Month: Downy Mildew on Basil Pest of the Month: Fall Webworm by Heather Prince
Disease of the Month: Downy Mildew on Basil
Basil downy mildew is caused by Peronospora belbahri and is host specific. It flourishes in cooler, wet weather. Symptoms first appear as diffuse yellow areas on the top of leaves, then produces spores on the underside, giving them a dirty appearance. Under magnification, undersides of leaves appear to be covered in grey fuzz. The disease progresses quickly, with affected leaves turning brown and falling; defoliating a plant within a few days. According to Dr. Babadoost, a professor in the Department of Crop Sciences at the University of Illinois who specializes in diseases of vegetable crops, this disease is serious for Illinois growers.
Pest of the Month: Fall Webworm
Fall webworms (Hyphantria cunea) are communal leaf-eating caterpillars that spin silk tents at the tips of branches. They prefer crabapple, redbud, sweetgum, maple, and oak. As the caterpillars finish the leaves within the Fall Webworm web, they expand it every week or so, building a 2 to 3 foot haven. There are two races of fall webworm found in Illinois. The northern race caterpillars Treatment: have yellow-gray bodies with red Downy mildew is thought to overwinter in greenhouses or heads and all white moths. The travel in on basil cuttings. Spores can travel large distances southern race caterpillars have by wind and a small number will spread exceptionally fast. yellow-green bodies with black This pathogen develops resistance to fungicides quickly, so heads and adults are white with chemicals with different modes of action are required. For small black spots on the wings. commercial producers, there are a few chemical fungicides Adults emerge late spring to that are very effective against this pathogen. They require a Downy mildew on basil midsummer and begin to deposit pesticide applicator’s license and up to 17 applications per season. hairy egg masses on the underside of leaves in their preDr. Babadoost recommends alternating the following fungicide combiferred species. A few days later, the larvae hatch and begin nations at weekly intervals: to build silken webs over the ends of branches. You may • Azoxystrobin (Quadris 2.08SC (15.5 fl oz/A)) + potassium begin to see their tent-like webs when Hydrangea panicuphosphite (ProPhyt SC (4 pt/A)) • Cyazofamid (Ranman 400SC (3 fl oz/A)) + potassium lata is blooming. As larvae feed, they leave the large veins phosphite (ProPhyt SC (4 pt/A)) or midribs behind. While fall webworms may cause client • Revus 209SC (8 fl oz/A) plus potassium phosphite alarm, their damage is more aesthetic than life threatening ProPhyt SC (4 pt/A)) as they have many natural enemies. We do not have effective compounds for managing downy mildew in organic basil production. Treatment: For homeowners, chlorothalonil and copper fungicides demonstrate If control is desired, prune branches to remove the web minimal effectiveness. Removing and destroying infected plants is and dispose. Bt (Bacillus thuringiensis kurstaki) treatments best. Inspect plants before purchase. Full sun, good air circulation, may be sprayed when webs appear on trees. The webs are and watering the base of plants are good preventative measures. water resistant, so tear open the web before application. Resistant basils may be infected in optimal conditions. Dr. Babadoost Treatments effectively target caterpillars, but not adults, so reports that basil downy mildew is less virulent in red or purple basils. be sure to spray when caterpillars are active.
University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519
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Additional resources:
The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 The Landscape Contractor September 2020
Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972
