Vegetable Diseases to Scout for: Cucurbit Downy Mildew

About this time last year, our first Kentucky cucurbit downy mildew identification occurred. As the most economically important disease of cucurbits in Kentucky, it is not a matter of if commercial growers and homeowners will experience downy mildew, it is a matter of when. All cucurbits, such as cucumber, cantaloupe, summer and winter squashes, pumpkin, and watermelon are susceptible to downy mildew. The disease occurs once plants are established in the field. Since cucurbit downy mildew does not overwinter in Kentucky, each summer the pathogen must be blown north from overwintering locations in the southern U.S.  Prevention, early identification, and management will help reduce plant and yield losses.

Here are the symptoms to look for, preventative tactics, and brief suggestions on how to treat crops once disease is confirmed.

Cause, Disease Development, and Forecasting

Cucurbit downy mildew is caused by the water mold (fungus-like organism/Oomycete) pathogen, Pseudoperonospora cubensis. This pathogen requires living plant tissue to complete its life cycle and is not present during Kentucky winters. Thus, the pathogen survives winter in the southern U.S., and pathogen propagules are blown north to Kentucky on wind currents each year. The time of arrival varies from season to season.

The northward movement of the pathogen is monitored on the Cucurbit Downy Mildew Forecasting page of the IPM pipe website. This tool provides valuable information to growers regarding the spread and risk of cucurbit downy mildew. Cucurbit growers can sign up for free email or text alerts to keep them updated on the proximity of downy mildew to their individual farm.

Downy mildew is favored by rainy, humid conditions coming from a southerly direction. Though the pathogen does not infect cucurbit fruit, disease can spread extremely rapidly, resulting in defoliation and complete plant death in a matter of days during rainy conditions.

Symptoms

Cucurbit downy mildew is a foliar disease. The first symptoms are pale or bright yellow spots on the top-sides of leaves (Figure 1). These symptoms may first be observed near the crown of the plant. Over time, spots become irregular or “blocky” in appearance and develop into necrotic lesions (Figure 2).  On the undersides of leaves, lesions may appear sunken or water-soaked. When humidity is high, light to dark gray or purple sporulation (reproduction by the pathogen) may be observed (Figure 3).

Figure 1: Symptoms of cucurbit downy mildew begin as pale to bright colored yellow spots on the upper leaf surface. (Photo: Kenny Seebold, UK)

Figure 1: Symptoms of cucurbit downy mildew begin as pale to bright colored yellow spots on the upper leaf surface. (Photo: Kenny Seebold, UK)

Figure 2: Spots develop an irregular or “blocky” appearance and expand to become necrotic lesions. (Photo: Kenny Seebold, UK)

Figure 2: Spots develop an irregular or “blocky” appearance and expand to become necrotic lesions. (Photo: Kenny Seebold, UK)

Figure 3: In humid conditions, light to dark gray or purple sporulation may be observed. (Photo: Kenny Seebold, UK)

Figure 3: In humid conditions, light to dark gray or purple sporulation may be observed. (Photo: Kenny Seebold, UK)

Management Options

Cultural practices

  • Utilize disease resistant cultivars (primarily cucumber).
  • Monitor disease spread and risk through the Cucurbit Downy Mildew Forecasting page of the IPM pipe website.
  • Avoid overhead watering in order to reduce leaf wetness.
  • Plant in sunny areas with good airflow.
  • Use recommended plant spacing to facilitate air movement and leaf drying.
  • Remove and destroy heavily infected plants.

Chemical approaches

Protectant fungicides, such as chlorothalonil, mancozeb, and copper, should be applied when crops are at low risk. At higher risk levels, systemic fungicides are recommended. A number of systemic fungicide options are listed in ID-36, among them: Gavel, Orondis Opti, Previcur Flex, Ranman, Tanos, and Zampro. For management of resistance to fungicides, it is recommended to always tank-mix a systemic downy mildew fungicide with a compatible protectant fungicide. Growers can identify their individual risk levels by using the Cucurbit Downy Mildew Forecasting page and/or contacting their local Extension agent for further assistance.

As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals.

Resources

  • Cucurbit Downy Mildew Forecasting (IPM pipe website)
  • Home Vegetable Gardening (ID-128)
  • IPM Scouting Guide for Common Problems of Cucurbit Crops in Kentucky (ID-91)
  • Sustainable Disease Management of Cucurbit Crops in the Home Garden (PPFS-VG-19)
  • Vegetable Production Guide for Commercial Growers (ID-36)

 

By: Kim Leonberger, Extension Associate and Emily Pfeufer, Extension Plant Pathologist

 

Posted in Vegetables