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Dealing With Vegetable Diseases: Fungi

By Karen Newcomb

Tags: Mildews, Rust, Rot, Canker, Scab, Fungal Leaf Spot, Wilt, Karen Newcomb,

Karen NewcombFungi are minute non-green plants that exist everywhere in the garden soil. One pound of soil contains up to 225 million of them.  Like bacteria, some fungi break down organic matter into nutrients that can be used by vegetables. In contrast, other fungi attack live plants. With the exception of rots, fungal diseases tend to start with a sunken dark area that is later bordered by yellow, tan or light green.

You will find eight general types of fungal infections in your garden: mildews, rusts, rots, cankers, scab, spots, wilts and smuts.

Mildews fall into two groups – powdery and down mildew. Powdery mildew shows up as superficial white to light grayish patches on the upper surfaces of leaves and on buds. Plants infected with downy mildews have pale green or yellow areas on the upper leaf surfaces, with light gray or purplish patches below. The leaves wilt, wither and die. Seedlings may wilt and collapse. Both mildews affect a wide number of vegetables. Mildew attacks are most severe in cool, humid or wet weather and are common in areas with cool nights and warm days.


Mildews. Courtesy David B. Langston, University of Georgia

Prevention or natural controls: Rotate crops; avoid overhead sprinkling; plant resistant varieties.

Chemical control: Downy mildew needs a copper base product to get rid of it. Use Actinovate, a high concentration of a patented beneficial bacterium on a 100% water soluble powder. Effectively suppresses/controls a wide range of soil borne diseases. Found in Bountiful Gardens seed catalog. Also found in Totally Tomatoes catalog. Bonide Copper Fungicide QT is a copper fungicide to control early and late blight, leaf spot, downy mildew, anthracnose, and other fungal diseases. Found in Park Seed catalog. Also found in Totally Tomatoes catalog. Powdery mildew needs to have a sulfur spray applied when mildew first appears. Safer Brand 3-in-1 Ready to use Garden Spray works as a sulfur-based fungicide, insecticide and miticide. Can be found in Harris Seeds catalog.

Rust sometimes appears as bright yellow, orange, red, reddish-brown, or black powder pustules (blisters) on the underside of the leaves. This is a complicated disease, since the forms that attack vegetables require two different plants (called alternate hosts) to complete the life cycle.  The rust that attacks corn completes its alternate life cycle on oxalis. In extreme cases, plants attacked by rust wither and die.

Corn Rust

Corn Rust. Courtesy Meg McGrath, Cornell University, Ithaca, NY

Prevention and natural controls: Destroy nearby weeds that show rust; collect and destroy infected plants when first seen.

Chemical control: Safer Brand 3-in-1 Ready to use Garden Spray works as a fungicide, an insecticide and a miticide. Harris Seeds catalog.

Rot is not one general disease but several. Plants with root rot may gradually or suddenly lose vigor, and their leaves will become pale or yellow. The actual root decay may be mushy and spongy, caused by both fungi and bacteria. In many cases, nematodes (microscopic worms) provide the wound by which root rotting fungi and bacteria enter. When seedlings rot, wilt, collapse, and die before or after emergence, it is called damping off. Fruit rot often starts as one or more spots that enlarge to include a portion or all of the fruit. Rots can attack practically all the vegetables in your garden.

Tomato Rot

Tomato Bottom-End Rot. Courtesy Missouri Botanical Garden plant information

Prevention, natural controls: Plant in well-drained soil; collect and destroy infected material and plant debris; keep fruit off soil.

Chemical controls: You can treat seed with captan or use in seedbed before planting.

Cankers are dead areas on the stem. They are oval or irregular in shape, often sunken or swollen, and typically discolored. Some completely girdle the stem. The plants are often stunted.  Blackleg of cabbage falls within this category and is probably one of the most serious vegetable diseases.

Bacterial Spots

Bacterial Canker Spots. Courtesy Missouri Botanical Garden plant information

Prevention, natural controls:  Destroy infected plants; use four-year rotation; purchase healthy-looking plants.

Scab, as the name suggests, usually appears as roughened, crust-like raised or sunken areas on the surface of leaves, stems, fruit, roots or tubers. The leaves may wither and drop early. Scab is caused by a few bacteria and a wide range of fungi.


Scab. Courtesy T.A. Zitter, Cornell University, Ithaca, NY

Prevention, natural controls: Practice long crop rotation; plant resistant varieties; remove weeds.

Chemical control: Use Liquid Copper Fungicide as a preventive measure preceding a rain to prevent fungal spores from landing on leaves and establishing themselves. Can be found in Johnny’s Seeds catalog.

Fungal leaf spots vary in size, shape and color. The centers of the spots may fall out, and they may also enlarge to form big blotches. Wet seasons, high humidity, and water splashed on foliage increase the incidence of leaf spots. Certain leaf spots have special names such as black spot, tar spot, and anthracnose.

Fungal Leaf Spots

Fungal Leaf Spot. Courtesy Iowa State University Plant Disease Clinic

Prevention, natural controls:  When severe, collect and burn infected material.

Chemical control:  Safer 3-in-1 Ready to use Garden Spray is an insecticidal soap with sulfur-based fungicide. Approved for organic gardening. Harris Seeds.

Wilt may seem like one disease, but it is basically three. Each type invades and plugs up the water and food conducting vessels inside the plants. Bacterial wilt will be discussed in my bacteria blog. These wilts are sometimes confused with root rot. Fusarium and verticillium wilt are fungi. Don’t let these names bother you; just wilt will do. Plants attacked by both verticillium and fusarium wilt are usually stunted and yellow. The wilting starts at the base of the stem and proceeds upward. You can buy fusarium and verticillium wilt-resistant seeds in most seed catalogs.

Bacterial Wilt

Bacterial Wilt. Courtesy Missouri Botanical Garden plant information

Prevention, natural controls: Use resistant varieties; practice long rotation; collect and destroy infected plants.

Smuts and sooty molds produce massive amounts of black sooty spores. Smut is a fungus disease that produces dark brown to black sooty-looking spore masses inside swollen whitish blisters. Sooty mold shows up as unsightly superficial dark brown or black blotches on leaves, fruit and stems. It can be removed easily by rubbing and causes little damage to most plants.

Corn Smut

Corn Smut. Courtesy T.A. Zitter, Cornell University, Ithaca, NY

Prevention, natural controls:  Pick off and burn infected parts before blisters open; grow resistant varieties. 

My next blog will talk about bacteria.


Cornell University scientist Meg McGrath is studying basil downy mildew and is asking any gardener who has had this problem to be a “citizen scientist” and help in her research. Meg is trying to find out how wide spread this disease has become. If you have experienced basil mildew in your garden, and you want to help in Meg’s research, she can be reached at

Basil Downy Mildew

Basil Downy Mildew. Courtesy Meg McGrath, Cornell University, Ithaca, NY

Basil Downy Mildew

Basil Downy Mildew. Courtesy Meg McGrath, Cornell University, Ithaca, NY

Basil Downy Mildew

Basil Downy Mildew. Courtesy Meg McGrath, Cornell University, Ithaca, NY

Basil Downy Mildew Sporation

Basil Downy Mildew With Sporulation. Courtesy Meg McGrath, Cornell University, Ithaca, NY

© Copyright by Karen Newcomb