May showers bring June crop diseases

Author: 
Neith Little, Andrew Kness, Dr. Kathryn Everts

Published 6/6/17

Neith Little, Urban Agriculture Extension Educator
Andrew Kness, Harford County Agriculture Extension Educator
Dr. Kathryn Everts, Professor and Extension Specialist, Plant Pathology

The past month has been full of calls about seedlings and transplants growing poorly and showing disease symptoms.

That may be related to the weather. Preliminary data from Baltimore weather station reports 5.64 inches of rain, 1.65 inches more than the monthly normal. More importantly, we’ve had a lot of drizzly days, with 16 days that saw at least 0.01 inches of rain (NOAA). This has kept things consistently damp, and the light intensity low, which is the perfect environment for many vegetable diseases.

When seedlings have been dying, or just failing to thrive, damping off is the most likely suspect (Fig 1). There are several types of fungi and fungal-like pathogens that can cause damping off, and it can affect any type of seedling. Transplant seedlings can be protected by avoiding the temptation to over-water and using clean potting media, flats, and tools (Penn State Extension).

Cucumber seedlings affected by damping off

Figure 1: Cucumber seedlings affected by damping off. Note the overly moist potting media.
(Image credit: Michelle Grabowski, University of Minnesota Extension--Horticulture, Bugwood.org)

A second problem has also occurred this past month. Neith has seen some tomato plants with visual symptoms indicative of early blight (Fig. 2). As usual prevention is key:

  • Choose resistant varieties. There are no cultivars with complete resistance to early blight, but some have greater tolerance than others, so select cultivars accordingly.
  • Use pathogen free seed. If you save seed, consider trying the hot water seed treatment (Morton et al. 2016).
  • Remove and properly dispose of old crop residue that could serve to overwinter the pathogen that causes early blight.
  • Remove any weeds and/or volunteer plants that can harbor the pathogen. (Note that late blight, another very devastating tomato disease, can overwinter on tomatoes and potatoes, and also weeds in the nightshade family.)
  • Increase ventilation and airflow to facilitate the drying of leaves. Leaf wetness is your enemy when trying to fight many fungal (and bacterial) diseases.
  • Ensure plants have the proper fertility. Nutrient-stressed plants are weaker and more prone to infection. Nitrogen-deficient plants are particularly susceptible to early blight.
  • Sanitize pruning equipment often.
  • Whatever you do, do NOT over-head water your tomatoes, even when they’re small transplants. Also, do not irrigate at night or during days with prolonged periods of cloud cover.

Early blight leaf symptoms

Figure 2: Early blight leaf symptoms. Note the concentric rings surrounded by yellowing foliage.
(Image credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org)

If prevention fails and you think your tomatoes might be infected with early blight, there are a few treatment options:

  • Consider sending a sample to the UMD Plant Diagnostic Laboratory for diagnosis. Being certain what you’re dealing with will help you choose the right treatment.
  • If tomato transplants are infected and another source of transplants is available, the best course of action may be to destroy the infected plants and replace them.
  • Fungicides available for commercial vegetable production, including greenhouse production, can be found at the University of Maryland Extension Vegetable Production website in the Mid-Atlantic Commercial Vegetable Production Recommendations: tomato section pp F206-F224 , early blight fungicides p. F223. For organic growers, certain copper fungicide formulations and a Bacillus subtilis spray product are available and OMRI listed (University of Maine Extension 2010, OMRI).

Unfortunately, fungicides can only prevent the spread of infection, so they are most useful before the plant is infected, or when the infection is still small and localized.

Caution should be taken in organic production when using repeated applications of copper products, as over-application can lead to copper toxicity and contamination of soil, water, and off-target toxicity to other organisms. That does not mean do not use copper products. It means take the time to calculate the proper application rate and follow recommended rotations of at least three years between tomato crops in the same field or bed.

Relatively cheap fungicides that have some protectant and curative properties against early blight are products containing mancozeb and chlorothalonil; however, they require repeated application every 7-10 days for as long as environmental conditions remain conducive for disease. The Qoi fungicides, or strobilurins (FRAC group 11), have good activity against early blight but should be used sparingly in a spray program to prevent resistance because these products have a single mode of action. The SDHI fungicides (FRAC group 7) also have good activity against early blight.

When using a pesticide, always read and follow the label instructions.

References:

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