Alternaria

Current temperatures and high relative humidity create conditions more conducive to disease development. That has been very true for cases of Alternaria. We have seen reports of Alternaria increase dramatically in recent days, with at least one report of an incidence inside a high tunnel with drip irrigation, where Alternaria is not very common. While the germination of Alternaria spores occurs across a relatively broad temperature range of approximately 46–90°F, disease development is most favorable under warm conditions (68 to 86 °F). The presence of free moisture on leaf surfaces, or >90% relative humidity, is essential for spore germination and successful infection.

Alternaria impacts a wide range of crops including solanaceous and cruciferous plants. The most common symptom is yellow, dark brown to black circular leaf spots with target-like concentric rings. Lesion centers may fall out. Individual spots coalesce into large necrotic areas and leaf drop can occur. Lesions can occur on petioles, stems, flowers, flower pedicels, and seed pods.

Alternaria (aka Early Blight) in Tomatoes

Commonly found early in the season, but can occur at any point during the growing season, early blight will start on the lower leaves as dark brown spots that start small and grow larger. The stems and fruit may also develop symptoms with dark sunken lesions that will also exhibit the concentric rings. On the stem, the infected area may be more elongated than would be seen on the leaves or fruit. 

Alternaria in Crucifers

Infections of broccoli and cauliflower heads can lead to complete deterioration of the heads and impact marketability.

Management Best Practices

• Avoid overhead irrigation if possible. When irrigation is needed, do it in the morning
• Control weeds, especially those in the family Brassicaceae
• Avoid working in fields when foliage is wet
• Promptly incorporate plant debris after harvest, or mow if tillage is not possible in late fall crops
• Start with certified disease-free seed or plan to use a hot water treatment for future seasons
• Practice a 3-year crop rotation with all brassica crops. Use proper plant spacing and row spacing for good air circulation.

Use Cornell’s Disease Resistant Vegetable Crop Varieties as a resource when selecting seeds. The New England Vegetable Management Guide also has related information in its Field Tomato and Brassica crop section. Double Nickel and Oso are examples of fungicides that can help control the spread of Alternaria.

Botrytis Blight

Another damp, high temperature, and humidity-loving disease commonly found in tomato crops - particularly those grown in high tunnels - is Botrytis Blight. Symptoms include tan to brown lesions on leaves and stems that can expand and cause wilting. It also causes brown, dying flowers, spreading the infection to fruit. Soft, watery rot on fruit, or whitish rings called "ghost spots," reduce market quality. Botrytis Blight is often referred to as gray mold due to the grayish-white, fuzzy mold that appears on infected plant parts. The fungus Botrytis cinerea spreads through spores carried by wind, rain, or air currents.

• Take measures to improve air circulation. This can be accomplished by increasing plant to plant/row to row spacing and continual pruning of plants
• Currently there are no known B. cinerea resistant tomato cultivars
• Apply fungicides preventatively prior to dense canopy growth, rotate fungicides to manage resistance
• Avoid overhead irrigation, minimize plant wetness
• Do not work on wet plants, prune plants in the early afternoon allowing wounds to dry quickly
• Ventilate high tunnels or greenhouses to maintain relative humidity at less than 90%
• Practice good greenhouse sanitation, scout, bag, and remove infected plants

Cross-Striped Cabbageworm

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