Downy mildew on Plants

Symptoms

Downy mildew shows up as fluffy mats of spore growth on the underside of leaves. The fluffy mats are a colony of sporangia. They usually have a bluish tint, but their color can range from white to gray to purple.

Small yellow dots appear on the upper leaf surface, directly opposite the spore growth underneath. Yellow dots on top and fluffy growth below mark the same spot on a leaf. In plants such as roses and grapes, an oily-looking spot on the leaf can be an early sign.

On roses, the upper-leaf lesions start pale green to yellow. They then turn purplish red to dark brown and necrotic, and they often follow the leaf veins. Roses with these lesions can also drop their leaves early.

Leaf, stem, flower and fruit tissue can all become infected. The damage can be severe and rapid, and plants left untreated can die within a week.

Causes

Downy mildew is caused by oomycete pathogens in the family Peronosporaceae. These are water molds, not true fungi.

Cool temperatures between 58 and 78°F (14-26 °C) combined with relative humidity above 85% near the plant or leaf surface favor infection. Leaves that stay wet give spores the chance to infect within 8 to 12 hours. Spore production needs relative humidity of 90% or higher.

Different pathogens target different plants. Hyaloperonospora parasitica infects Brassicaceae such as broccoli, Brussels sprouts, cabbage, cauliflower, kale, radish and turnips. Pseudoperonospora cubensis infects cucurbits such as squash, pumpkin, melons and cucumber. Peronospora sparsa infects roses, blackberries, raspberries and other Rubus species.

Peronospora belbahrii infects basil. Plasmopara obducens infects impatiens. The hosts in this list range from vegetables such as cabbage and cucumber to ornamentals such as roses and impatiens, and to berries and basil.

Diagnosis

The clearest sign is on the underside of the leaf, where downy mildew forms fluffy mats of spore growth. The mats are usually bluish, but they can look white, gray or purple. A yellow dot on the upper surface fits this problem when spore growth sits directly opposite it underneath. The dots and the underside growth appear as a pair, so look for both.

On roses, lesions that follow the leaf veins and turn purplish red to dark brown also fit. Infection of stems, flowers and fruit also fits, as does rapid damage that can kill a plant within a week.

Weather also fits when temperatures sit between 58 and 78°F (14-26 °C) and humidity near the leaf is above 85%. Wet foliage matters too, since spores infect within 8 to 12 hours. Basil, impatiens, roses, cucumbers and cabbage each have a named downy mildew pathogen. No look-alike problems are named in the facts behind this page.

Treatments

Cultural steps come first. Choose disease-resistant cultivars when you select plants to grow. Avoid overcrowding, overwatering and poorly drained soil. Use drip or soaker irrigation instead of overhead watering, and water early in the morning so foliage can dry during the day.

Remove and dispose of infected plant material by burning it or sealing it in a closed container. Do not compost it, since home composting rarely reaches temperatures high enough to kill the spores. Sterilize garden tools between use on infected and uninfected plants. Remove plant debris, especially in fall, to keep the pathogen from overwintering.

Space plants adequately and prune the host canopy and nearby or overhead vegetation to improve air circulation and keep humidity down. Avoid wetting foliage where feasible, using drip or low-volume microsprinkler irrigation instead of overhead sprinklers. Promptly remove and dispose of infected foliage to reduce inoculum.

Neem oil can be applied as a preventive treatment in the evening. It is not guaranteed to eliminate the disease, and it kills bees and other insects that contact it. A copper-based fungicide can also be applied preventively and repeatedly, though it is not guaranteed to eliminate the disease. Copper treatments can have a negative effect on some plants.

Apply fungicides preventively, and rotate products periodically because some downy mildew populations have developed fungicide resistance. The product label is the legal instruction, so follow it for the crop and site being treated. This page gives no dose, rate or spray interval.

Prevention

Cool temperatures between 58 and 78°F (14-26 °C) with relative humidity above 85% near the leaf surface favor infection. Keep humidity down by spacing plants adequately and pruning the host canopy and nearby or overhead vegetation.

Wet foliage lets spores infect within 8 to 12 hours, so keep leaves dry where you can. Drip or soaker irrigation and early morning watering let foliage dry during the day. Avoid overcrowding, overwatering and poorly drained soil.

Choose disease-resistant cultivars when selecting plants. Sterilize tools between infected and healthy plants. Clear plant debris, especially in fall, so the pathogen cannot overwinter. Infected plant material should be burned or kept in a closed container, not composted.

Plants most often affected

Downy mildew is recorded on rose, blackberry, boysenberry, cucumber, cantaloupe, basil, impatiens, broccoli and cauliflower. Hosts include caneberries, rose, various herbaceous ornamentals, and many fruit, grain and vegetable crops. Peronospora sparsa also infects and damages pineapple guava and rose. These plants range from ornamentals and berries to herbs and vegetables.

• Balsampear
• Blackeyed Susan
• Cantaloupe
• Common Hop
• Field Pumpkin
• Garden Cornflower
• Garden Cucumber
• Garden Lettuce
• Garden Onion
• Spinach
• Sweet Basil
• Virginia Creeper