We’re seeing persistent fungal problems in our gardens this May. Late-season showers brought cooler, more humid weather than normal which has led to continuing problems with downy mildew, powdery mildew, leaf spot and black spot. Plants that have been affected with fungal disease for these last several weeks will be in bad shape with nutrients sapped, leaves dying and reduced fruit and flower production.
Our treatment options at this time of year are limited. It’s too late in the growing season to replant diseased cucumbers, squashes and melons or to replace deformed stunted roses; high summer temperatures will kill young transplants and seedlings (as well as the fungal spores). We can either pull out infected plants and wait to replant in fall or we can apply fungicides; home remedies are unlikely to control this season’s serious fungal infections.
Here’s what to look for and treatment options in each case:
Downy mildew – It shows as yellow or brownish spots on leaf surfaces with a grayish white fluffy growth on the undersides. It causes leaf defoliation and poor ripening primarily on grapes and members of the cucurbit family (cucumbers, melons, squashes). Remove severely infected leaves and fruit and dispose of them in the green waste bin. Apply a fungicide formulated to treat for downy mildew on the specific plant according to label directions.
Powdery mildew – It’s a problem in both humid and dry weather. Leaves and stems on infected plants are covered with a white powder. Powdery mildew affects a wide variety of plants in the garden and is spread by wind from diseased to healthy plants. Remove infected plants if possible as soon as the disease is identified and spray remaining plants with a fungicide containing chlorothalonil every seven days until the disease is controlled or hot, dry weather arrives. Apply a fungicide that contains triforine on infected roses every seven to 10 days. Moderate cases of powdery mildew can be reduced by washing the white spores off leaves or by applying neem oil or jojoba oil.
Leaf spot – It appears as small brown, yellow, or gray spots that eventually grow together into blotches on leaves; leaves may yellow and die and fruit can be discolored. The spores are spread by splashing water and wind. Strawberries are especially susceptible to leaf spot.
Moderate leaf spot damage can be tolerated and fungicides are not recommended in these cases. Remove seriously infected plants.
Black spot – This fungus causes circular black spots on leaf upper surfaces during periods of high humidity. Poor air circulation is another common contributing factor for black spot problems. Leaves turn yellow and fall, twigs die, and repeated infections can weaken or kill plants. Black spot spores are spread by rain or by overhead irrigation. Correct cultural conditions if possible by replacing overhead irrigation with ground level bubblers, drip emitters or soaker hoses and by transplanting crowded plants so that they have good air circulation. Fungicides that contain chlorothalonil or triforine can be sprayed on infected roses or apply neem oil, sulfur or bicorabonate of soda when damage is minor.
More to consider – To help control for fungal problems in your garden clean up plant debris regularly and, because fungal spores overwinter, make a thorough winter cleanup that includes mulch replacement. When replanting diseased plants, look for those that are resistant to whatever fungus killed the first plants.
Fungicide labels stress consistent, regular application and they mean it. The fungicides will not be as effective unless they’re sprayed at the proper intervals. Some fungicides must be applied before the disease is noticed. Read labels very carefully.
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Harvest vegetables and deadhead flowers regularly to encourage continued production.
- Avoid cutting lawns too severely because the resulting stress causes yellowing.
- When using herbicides for weed control, spray on a calm day and protect plants by shielding with a large piece of cardboard.
- Remove spent blooms. Cut back annuals that have stopped blooming to encourage rebloom.
- Avoid fertilizing herbs as too much fertilizer reduces flavor and fragrance.
- Spring planting season is over. Anything planted this month will need careful watering.
- Annuals and perennials: fibrous begonia (Begonia semperflorens), clustered bellflower, cockscomb (Celosia), morning glory (Convolvulus tricolor).
- Bulbs, corms, tubers: gladiolus.
- Trees, shrubs, vines: bird of paradise (Caesalpinia), bottlebrush (Callistemon), fringe tree (Chionanthus).
- Fruits and vegetables: cherry, cucumber, eggplant.
Things to ponder
- Consider mulching around trees and shrubs with 3 to 4 inches of mulch. Avoid stacking mulch against the trunk.
Source: Adapted from “A Gardener’s Companion for the Central San Joaquin Valley,” 3rd edition, currently available from the Master Gardeners for $30.
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