Symptoms Of Tomato Blight, Powdery Mildew and Black Spot

The most common garden fungal infections, blight, powdery mildew, and black spot, can be devastating to your fruits and vegetables. Here’s a primer on symptoms and prevention.

Consistently wet, humid, or damp weather creates a perfect environment for fungal infections of plants. Fungi spores grow rapidly in persistent moisture and at the very least create stress on the plant, leaving them vulnerable to secondary infections. Early prevention is the best defense because once you actually spot symptoms, it’s usually a losing battle. Here’s what you should look for:

Symptoms of Powdery Mildew

Powdery Mildew appears as white or gray powdery spots on the stems and leaves of your plants, which grow larger every day. The fungus covers the entire plant quickly if untreated.

Symptoms of Black Spot

Black Spot affects roses and fruit trees and is very common in apples and pears. As its name implies, the disease starts as small black spots on the tree’s leaves which grow larger as the season progresses. Black Spot frequently spreads to the fruit bud as well and will show on the fruit as it matures, resulting in a fruit neither pleasing to the eye nor the taste buds.

Symptoms of Tomato Blight

Tomato Blight takes two forms: Early Blight and Late Blight.

• Early Blight: 1/4″ – 1/2″ diameter spots form on older mature leaves at the base of the plant. Spots have tan centers with concentric rings and yellow halos around the edges. The spots enlarge and coalesce, and leaves become yellow eventually drop from the plant. Fruit becomes discolored. No mold appears.
• Late Blight: Lower leaves are the first to show spots that look water-soaked. Spots begin as pale green on the upper side of the leaf, usually near the edges and tip. Eventually spots appear on the underside of the newer leaves that turn brown to black with a green border and a velvety and water-soaked appearance. A fuzzy ring of mold may appear on the underside of the leaves. Leaves shrivel and die and the disease spreads rapidly.

Tips to avoid Tomato Blight

• Plant tomato varieties resistant to blight. These would be marked as LB (late blight resistance) and AB (Alternaria/Early Blight resistance) on plant tags or seed packets.
• Use mulch under your seedlings. Mulch suppresses fungal spores that may have overwintered in your soil.
• Plant varieties well adapted to your area (heirlooms).
• Leave more space than usual between your seedlings. Airflow is essential for helping plants to dry quickly after a rainstorm. If the plants spend less time wet, they’ll be less susceptible to blight and other fungal infections.
• Plant your tomato seedlings on the opposite side of the garden from where blight occurred previously.
• Practice crop rotation and never plant tomatoes in the same bed 2 years in a row.

Treatment for Tomato Blight, Powdery Mildew and Black Spot

If you discover symptoms of tomato blight, powdery mildew, or black spot, act immediately:

• Prune out any affected foliage, stems or fruit and trash it (do not compost).
• Treat your plants with a fungicide that is OMRI listed (approved for organic gardening) and with the frequency recommended on the fungicide’s label. Do not apply more frequently or in greater quantity than recommended as you may damage the plant or soil. Any fungicide which contains copper sulfate or copper hydroxide is usually effective. There are also new biological fungal controls which are pretty effective in my experience. As a preventative for Powdery Mildew, Baking Soda Spray or milk spray have shown some effectiveness in studies (but before symptoms appear).
• The earlier you treat the plant, the more likely you’ll get the fungus under control. It’s always a good idea to treat with an organic fungicide at the first report of symptoms in your area, even if you haven’t yet seen any symptoms yet in your own garden.