Diseases & Pests Webworms and Downy Mildew on Basil

by Heather Prince

Disease of the Month: Downy Mildew on Basil Basil downy mildew is caused by Peronospora belbahri Pest of the Month: Fall Webworm and is host specific. It flourishes in cooler, wet weather. Fall webworms (Hyphantria Symptoms first appear as diffuse yellow areas on the top of cunea) are communal leaf-eating leaves, then produces spores on the underside, giving them caterpillars that spin silk tents at a dirty appearance. Under magnification, undersides of the tips of branches. They prefer leaves appear to be covered in grey fuzz. The disease procrabapple, redbud, sweetgum, gresses quickly, with affected leaves turning brown and fallmaple, and oak. As the caterpiling; defoliating a plant within a few days. According to Dr. Fall Webworm lars finish the leaves within the Babadoost, a professor in the Department of Crop Sciences at web, they expand it every week the University of Illinois who specializes in diseases of vegor so, building a 2 to 3 foot etable crops, this disease is serious for Illinois growers. haven. There are two races of fall webworm found in Illinois. Treatment: The northern race caterpillars Downy mildew is thought to overwinter in greenhouses or have yellow-gray bodies with red travel in on basil cuttings. Spores can travel large distances heads and all white moths. The by wind and a small number will spread exceptionally fast. southern race caterpillars have This pathogen develops resistance to fungicides quickly, so yellow-green bodies with black chemicals with different modes of action are required. For heads and adults are white with commercial producers, there are a few chemical fungicides small black spots on the wings. that are very effective against this pathogen. They require a Downy mildew on basil Adults emerge late spring to pesticide applicator’s license and up to 17 applications per season. midsummer and begin to deposit Dr. Babadoost recommends alternating the following fungicide combihairy egg masses on the underside of leaves in their prenations at weekly intervals: ferred species. A few days later, the larvae hatch and begin • Azoxystrobin (Quadris 2.08SC (15.5 fl oz/A)) + potassium to build silken webs over the ends of branches. You may phosphite (ProPhyt SC (4 pt/A)) begin to see their tent-like webs when Hydrangea panicu• 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

The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972