Preparing for Powdery Mildew in Summer Squash and Fungal Leaf Spots in Tomatoes

The forecast for Connecticut calls for hot, humid weather with scattered thunderstorms and possible heavy downpours in the coming days. These conditions are important for vegetable disease management. High humidity and dense crop canopies favor powdery mildew development in cucurbits, while thunderstorms and leaf wetness favor splash-dispersed tomato leaf spots such as early blight/Alternaria and Septoria. 

Summer squash: powdery mildew

Fungicides should be applied at the first sign of disease (or earlier with some products). Begin scouting for powdery mildew at fruit initiation. On cucurbits, the disease has a white powdery appearance and infects both the top and bottom of the leaf, making it more difficult to control with non-systemic fungicides. However, powdery mildew fungi tend to become resistant to systemic fungicides such as Topsin-M (Group 1) and Cabrio, Flint Extra, Quadris, and Sovran (Group 11). Resistances to Group 1 and Group 11 fungicides have resulted in the removal of these classes of fungicides from recommendations, with the exception of Pristine which is a combination product. Resistance to the DMI fungicides (Rally, Procure - Group 3) is also widespread; use Rally or Procure at the high labeled rate only. 

The most effective contact fungicides are sulfur, mineral oil, and chlorothalonil. For organic production, options include sulfur, mineral oil/JMS Stylet-Oil, potassium bicarbonate, neem oil, Regalia, Double Nickel, and similar labeled products, but many organic materials work best preventively and require excellent coverage.

With the forecasted heat, be careful with sulfur and oils. Sulfur can injure plants when temperatures approach 90°F, and oils can also cause crop injury under hot or drought-stressed conditions. If using sulfur, oil, or other organic contact materials, apply during cooler parts of the day and follow label restrictions on temperature, crop safety, and mixing intervals.

Tomatoes: Alternaria/early blight and Septoria leaf spot

Tomato growers should also prepare for early blight and Septoria leaf spot, especially in fields with a history of these diseases. Both diseases often start on lower leaves and move upward. Rain splash, wet foliage, dense canopy, and old crop debris all increase risk. Stake and prune where appropriate, mulch to reduce soil splash, avoid overhead irrigation if possible, maintain good airflow, and remove lower diseased leaves when practical.

In fields with a history of early blight or Septoria, begin fungicide applications before disease is evident, usually when first fruit are about half grown or around the first week of July. However, if disease is already present or if repeated storms and long leaf wetness occur earlier, begin earlier. Where possible, use TOMCAST or NEWA disease-forecasting tools to time sprays based on local weather.

Preventive fungicide options for conventional tomatoes include protectant materials such as chlorothalonil and mancozeb, copper products, and premixes or rotational partners. Chlorothalonil and mancozeb are important protectants because they are multi-site fungicides and useful for resistance management. For organic tomatoes, copper products are the main preventive option, with some biological products labeled for suppression; however, organic materials are protectants and will not “clean up” established disease.

Under wet, humid, stormy conditions, maintain a regular protective schedule, commonly 7–10 days depending on disease pressure, product label, rainfall, and forecast. Shorten intervals when disease pressure is high or after heavy rain if the label allows. Extend intervals only when weather is dry and disease risk is low. Focus on protecting new growth and maintaining lower-canopy coverage, where these diseases usually begin.

Continue to be on the lookout

The following pests that were covered in the previous pest alerts and are still relevant in current conditions:

• Allium Leafminer
• Colorado Potato Beetles
• Cucumber Beetles
• European Corn Borer
• Potato Leafhoppers
• Cabbage Root Maggot
• Onion Thrips
• Mites