Vegetable Crop Diseases and Interventions

Most of the diseases described in this chapter are common throughout North America. Many, such as late blight, are found worldwide. As with insect pests, I categorize interventions for plant disease into minimal impact, moderate impact, and heaviest impact, based on their potential to disrupt ecological systems. I include information and research about suppressive soil microorganisms because these microorganisms are a natural part of a functioning and healthy soil organic matter system. These beneficial microorganisms suppress disease in several ways: They improve plant health so plants can better resist disease, and they directly compete with and attack disease microorganisms. I also focus on suppressive soil microorganisms because many researchers and farmers are experimenting with commercial formulations of these microorganisms, which are now or will become available for farmers and gardeners.

Note that for some diseases, I list only minimaland moderate-impact interventions. This is because the heaviest-impact interventions available to organic growers, such as copper fungicides, are not really worth the ecological impact risk. For other diseases, there are no moderate-impact options available. The “Physiological Disorders and Nutrient Deficiencies” section beginning on page 260 covers deficiencies and other growth problems that may sometimes be mistaken for disease.

Alternaria Diseases: Early Blight, Leaf Blight, Purple Blotch

Alternaria is a common fungal genus with many species; individual species attack different plant families. For example, Alternaria cucumerina attacks melon and cucumber, A. solani attacks tomatoes and potatoes, and A. porri prefers onion and garlic. Alternaria causes leaf spotting and wilting. Leaf spots progress from older leaves to newer ones. Circular spots start as small, sunken, water-soaked areas and progress to purple to brown spots surrounded by yellow rings that create a diagnostic bull’s-eye pattern seen in figure 11.1. Entire leaves may die and drop from the plant. Spots are also found around stem ends on fruits and on the plant stem. This fungus is spread by splashing water and by walking through plants when wet. Alternaria survives in plant debris and may be spread by insects, or wind, and by rain/irrigation. Spores germinate in several hours during wet, highhumidity conditions. Infection is encouraged if water remains on plant tissue for longer than a couple of hours. Optimal temperatures for infection are 60 to 80°F (16–27°C) (depending on the

Angular Leaf Spot

Alternaria Diseases: Early Blight, Leaf Blight, Purple Blotch

reportedly improve tomato growth and suppress early blight disease.1

• Fruit rot on field-grown chili peppers: Application of Trichoderma harzianum IMI 392432 to pepper seeds before planting significantly suppressed disease caused by A. tenuis 2

• Tests at Cornell University report suppression of Alternaria on basil using an organic product containing the beneficial bacterium Streptomyces lydicus.

• Tests at Cornell University report suppression of Alternaria on brassica crops using an organic product containing the beneficial bacteria Bacillus amyloliquefaciens strain D747.

HEAVIEST-IMPACT INTERVENTION

Alternaria species) plus high humidity. The disease is encouraged by low soil fertility.

HOSTS

Depending on the fungus species, cucurbits (cucumber, squash, melon, and pumpkin), solanaceous crops (eggplant, pepper, potato, and tomato), onion, and other vegetables such as peas and cabbage.

MINIMAL-IMPACT INTERVENTION

Keep water off foliage; irrigate with soaker hoses or drip irrigation. Follow 3- to 4-year rotations between susceptible crops in the same plant family. Remove infected leaves. Maintain balanced nutrient levels and high soil organic matter levels to enhance native soil biological control microorganisms.

Suppressive Soil Microorganisms

Some specific soil microorganisms have shown suppression of Alternaria species.

• Early blight on tomato: Bacillus amyloliquefaciens strain F727 has shown some suppression of A. solani. Plant-growth-promoting rhizobacteria

Sulfur (or as a very last resort, copper) can be sprayed when temperatures are between 55 and 85°F (13–29°C) and weather is wet, to protect leaves from infection.

Angular Leaf Spot

Angular leaf spot (Pseudomonas syringae) is a bacterial disease causing dark, angular spots between leaf veins. One good diagnostic is that tear-shaped droplets ooze from infected tissue. As the leaf dries, tissue tears and shrinks. Fruit may have circular spots or rotted areas. The bacteria overwinters in plant debris and can persist for more than 2 years on dry leaves.

Hosts

Cucurbits (cucumber, squash); a similar strain infects beans.

MINIMAL-IMPACT INTERVENTION

Follow 2- to 3-year rotations between cucurbit crops. Avoid wetting foliage with irrigation water. Use resistant cultivars when possible. Plant on raised beds. Excessive nitrogen fertilizer increases disease severity. Angular leaf spot is also seed-borne; treat seed for 20 minutes with 120°F (49°C) water.

Angular Leaf Spot

Maintain balanced nutrient levels and high soil organic matter levels to enhance native soil biological control microorganisms.

SUPPRESSIVE

Soil

MICROORGANISMS

Beneficial soil bacteria have been reported to induce systemic resistance against angular leaf spot disease of cucumber.3 (See the section “Epiphytic Bacteria and Fungi” on page 141 for a discussion of induced systemic resistance.)

HEAVIEST-IMPACT INTERVENTION

As a very last resort, if a crop is already showing symptoms and wet weather is expected, copper can be sprayed to protect leaves.

Anthracnose

Small, depressed, and circular fruit spots are the most common symptom of anthracnose, which is caused by Colletotrichum species fungi. Spots become much larger, with centers ranging from tan or orange to brown or black. Anthracnose prefers temperatures of around 80°F (27°C) and wet conditions. The disease is seed-borne and can also overwinter in the soil. Rain splash spreads anthracnose spores from infected soil, plant debris, and/or fruits onto susceptible plants.

Hosts

Vegetables, members of the cucurbitaceae family, and peppers.

MINIMAL-IMPACT INTERVENTION

Rotate crops, allowing a 2- to 3-year gap period between susceptible crops (including strawberry). Avoid wetting foliage with splashing irrigation water. Use resistant cultivars when possible. Plant on raised beds. Use straw or living mulch or plastic mulch to avoid splashing of soil onto plants. Avoid excessive nitrogen fertilization.

HEAVIEST-IMPACT INTERVENTION

During wet, warm weather, copper can be sprayed to protect leaves as a very last resort if anthracnose is a common problem for your crops.

Aster Yellows

Aster yellows is a disease caused by an organism called a phytoplasma. It is spread only by leafhopper feeding. This disease causes twisted distorted new growth (including leaflike petals) and a yellowing/reddening of leaf tissue; it also causes hairy roots in carrots. Leafhoppers that spread the disease overwinter in warmer regions. After feeding on infected plants, they migrate north and can then transmit the disease through continued feeding on uninfected plants. Peak infection periods are in late summer and early fall.

Hosts

Carrot, celery, and various flowers, including asters and zinnia.

MINIMAL-IMPACT INTERVENTION

Plant resistant cultivars when possible. At the Frontier Herb Company Experimental Farm in Iowa, planting several crop species together in growing beds greatly reduced the incidence of aster yellows.

HEAVIEST-IMPACT INTERVENTION

Control leafhoppers that spread this disease by applying insecticidal soap plus neem.

Bacterial Spot or Bacterial Blight

Leaves, stems, and fruit infected with bacterial spot (Xanthomonas campestris) develop brown, circular spots or specks, but without concentric zones (as for fungal leaf spot diseases). On beans there is a diagnostic yellow ring around the brown spots. The