Vegetable Disease Update: July 26, 2018

The current relentless rain is expected to intensify foliar bacterial diseases on many crops. Scouting for diseases and protection of crops will be critical in the upcoming days.
Vegetable Disease Update: July 26, 2018 - News

Updated:

Angular leaf spot on the upper surface of a cucumber leaf. Photo: Beth Gugino, Penn State

General Updates

  • Still no new reports of late blight since the confirmations on tomato in York County and on potato and tomato in southern Lancaster County a couple of weeks ago. If you suspect late blight, please contact your local Penn State Extension office or let Beth Gugino know via email at or by phone at 814-865-7328. For the latest reports check USBlight.org.
  • The recent unsettled weather has been highly favorable for the spread of cucurbit downy mildew on cucumber and also cantaloupe, and it is likely that many growers are struggling to keep their crops protected due to waterlogged soils. Scouting over the next few days as the weather breaks will be critical. If you suspect cucurbit downy mildew on your farm, please contact your local Penn State Extension Office or let Beth Gugino know via email at or by phone at 814-865-7328. Every confirmed report of downy mildew enables us to improve disease forecasting accuracy for the benefit of cucurbit growers not only in Pennsylvania but all along the east coast. Even reports that are made from previously reported counties. The latest information on reports of cucurbit downy mildew can be found at the CDM ipmPIPE website.
  • There have been numerous reports of Septoria leaf spot on tomato both in commercial field and home gardens. The disease will likely increase both within and between plants as the spores are dispersed in the rain splash and by wind-driven rain.
  • Be on the watch for Phytophthora blight in cucurbits, peppers, and tomatoes!

Vegetable Bacterial Diseases

The continued unsettled weather pattern which has resulted in the recent frequent and unrelenting rains will likely exacerbate foliar bacterial diseases on many crops ranging from angular leaf spot on cucumber to bacterial spot and speck on tomato. Bacterial pathogens that are associated with the leaf surface are easily splash dispersed during rain events, and the long durations of leaf wetness allow ample time for the bacteria to enter into the leaves through stomata (open pores that enable the leaf to “breathe”) and small microscope wounds.

Once the bacteria are inside the plant tissue, the application of copper-based or other microbial-based products are ineffective. These products are active on the surface of the leaves and on internal plant tissues and thus need to be used preventatively and as a protectant. The latter is further hindered by the excessively wet field conditions making it difficult to get equipment into the field to spray. Products such as Actigard, Regalia, and LifeGard that function to boost the plant’s defense response are most effective when applied earlier in the season. Each may take several days to upregulate defense responses in the plant so waiting until symptoms are visible will limit their efficacy. These products are also used most effectively when coupled with copper-based products regularly during the growing season. Keep in mind that some crops like cantaloupe or muskmelon are sensitive to copper and can result in marginal yellowing of the leaves.


Copper phytotoxicity on cantaloupe. Photo: Beth Gugino, Penn State

As much as possible, avoid working the plants when they are wet. This can be challenging when harvesting in the morning for afternoon markets. Since diseases are more likely to spread from older to newer plantings avoid working in both on the same day or divide up your labor, so those harvesting for the market are not working tying younger plantings in the same day.

Having an accurate diagnosis is important. Pennsylvania residents can submit samples to the Penn State Plant Disease Clinic. However, the diagnosis can only be as good as the sample. Be sure to read the plant sample submission instructions before sending your sample.

Diseases such as angular leaf spot and downy mildew can look very similar on cucurbit crops especially cucumber. Keep records of the specific crops, cultivars and bacterial diseases that are affecting your crops. In some cases, host resistance is available (e.g., bacterial spot of pepper) or you may want to consider selecting a different cultivar or seed source if you are experiencing problems over multiple seasons. Most bacterial disease management strategies are preventative and focus on sanitation. When outbreaks occur, it is important to review your notes from the season and reflect where you could make improvements in sanitation for next season.


Bacterial leaf spot of pepper. Photo: Beth Gugino, Penn State

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