Vegetable Diseases to Scout for: Anthracnose of Cucurbits

Cucumber, cantaloupe, and watermelon growers in Kentucky should have their eye out for anthracnose this week. Initial anthracnose infections occur during this time in the growing season in Kentucky, and preventative practices are essential to reducing disease incidence. Prevention, early identification, and management will help reduce plant losses.

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

Cause and Disease Development

Anthracnose on cucurbits is caused by the fungal pathogen Colletotrichum orbiculare.  This pathogen can infect cucumber, gourds, squash, and pumpkin, but is most common on cantaloupe and watermelon. This pathogen may originate from infested seed, transplants, weeds, or persist in association with crop debris from a previous year. Fungal spores are spread via water splashing, such as rain or irrigation. Anthracnose is favored by rain, warm temperatures, and high humidity. Once established, the disease can spread rapidly in dense plantings, and in severe cases, plants collapse.

Symptoms

Anthracnose begins as a foliar disease; however, all aboveground plant parts may become infected. The first symptoms appear as small, circular lesions on leaves, particularly in the inner parts of the foliar canopy. Over time, lesions enlarge to become large tan to brown spots (Figure 1). These spots may coalesce, resulting in extensive blight. Stem lesions are tan-brown, sunken, and may be elongated. Lesions may also develop on maturing fruit and first appear as small, circular, sunken areas. Throughout fruit development, lesions expand to become the size of a quarter or larger (Figure 2). Under humid conditions, fruit lesions may darken and salmon-pink spores may be observed. Symptoms can differ slightly on watermelon: leaf lesions tend to be smaller, somewhat angular (particularly at leaf veins), and darker in color. On fruit, lesions may be cracked and irregularly shaped.

Figure 1: Leaf lesions first appear as small, circular lesions on leaves, but overtime, lesions enlarge to become large tan-to-brown spots. (Photo: Kenny Seebold, UK)

Figure 2: Lesions on fruit begin as small, circular, sunken areas and expand to become the size of a quarter or larger. (Photo: Julie Beale, UK)

Management

Cultural practices

  • Select pathogen-free seed or varieties with resistance to anthracnose. Multiple pathogen races exist, so varieties rated for resistance to multiple races will be more robust against the disease.
  • Rotate fields out of cucurbits for at least two growing seasons.
  • Remove plant debris and weeds from the growing area.
  • Avoid overhead watering to reduce leaf wetness.
  • Use recommended plant spacing to facilitate air movement and leaf drying.

Chemical approaches

Begin a protectant spray program utilizing chlorothalonil or mancozeb no later than vine touch. Sprays may be applied every 7 to 10 days, or ideally with the help of MELCAST, a melon disease forecasting system. Include copper if bacterial disease becomes an issue. Within 4 weeks of transplant (earlier if symptoms develop), introduce a systemic fungicide into the program. Systemic fungicides with good activity for anthracnose on cucurbits include those in FRAC groups 1, 3, 7, and 11, as well as pre-mixes including these groups.

As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals. Also, some chemicals may be phytotoxic to melons under certain conditions; carefully read product labels prior to application.

Resources

  • Anthracnose of Cucumber, Muskmelon, and Watermelon (Purdue Extension publication, BP-180-W)
  • Foliar Disease Control Using MELCAST (Purdue Extension, PB-67-W)
  • IPM Scouting Guide for Common Pests of Cucurbit Crops in Kentucky (ID-91)
  • Sustainable Disease Management of Cucurbit Crops in the Home Garden (PPFS-VG-19)
  • Home Vegetable Gardening (ID-128)
  • Vegetable Production Guide for Commercial Growers (ID-36)

 

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

 

Posted in Vegetables