Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
I recently wrote an update article about fungicide programs for pumpkin http://extension.udel.edu/weeklycropupdate/?p=4429. However, because many fungicides are effective on some diseases, but not others, it is important to be able to identify the diseases present in a field as you design your spray program.
Foliar Diseases
The most common foliar diseases of pumpkin are powdery mildew, downy mildew, white speck (Plectosporium), gummy stem blight and anthracnose.
Powdery Mildew
Figure 1a. Powdery mildew sporulation covering leaves and defoliating pumpkin plants.
Figure 1b. Close up image of a leaf showing the “powdery” white sporulation on the upper surface of the leaf. Note that sporulation is usually seen first on the lower leaf surface. Scout a field by looking at the under surface of 45 old leaves in a field each week. Begin targeted sprays for powdery mildew when it is first observed.
White Speck (Plectosporium)
Figure 2. White speck or Plectosporium on the leaf causes tan spindle shaped lesions which form on the veins and result in distorted leaves. Plectosporium also causes lesions on the fruit (Figure 5).
Downy Mildew
Figure 3. Downy mildew lesions are initially seen on the upper surface as angular water soaked or yellow spots (3a) that are limited by the leaf veins. The angular nature of the lesions is especially evident on the lower leaf surface where sporulation occurs (3b). Look for grey angular lesions on the under surface of leaves after dewy nights. Lesions become necrotic over time.
Anthracnose
Figure 4. Anthracnose will initially be small tan lesions with darker margins (image courtesy of B. Precheur, Ohio State Univ.). They will expand as they age and damage large portions of the leaf. They may develop small holes in the leaf. Anthracnose also causes lesions on the fruit (Figure 7).
Fruit Diseases
There are several pathogens that cause fruit rot on pumpkin. To manage fruit rot the single most important practice is to follow a good fungicide management program in the field. The same fungi that cause white speck, black rot and anthracnose also cause lesions on the leaves. If the leaves are protected from disease, the fruit will be less likely to become diseased. In addition to protecting fruit from rot, a good spray program will protect “handles” from damage and will maintain foliage health and keep sunscald at a minimum.
White Speck (Plectosporium)
Figure 5. White speck (caused by Plectosporium, formerly Microdochium) causes white or tan “pimples” on the fruit.
Black Rot
Figure 6. Black rot (caused by Didymella bryoniae the same fungus that causes gummy stem blight on the foliage) results in large grey lesions on fruit.
Anthracnose Fruit Rot
Figure 7. Anthracnose fruit rot (caused by Colletotrichum spp.) appears as smaller grey lesions on fruit.
Fusarium Fruit Rot
Figure 8. Fusarium fruit rot (Fusarium solani) causes a relatively dry fruit rot that initially appears as small white or pink spots as in this photo. Eventually however, the lesions may become black or tan because of saprophytic growth.
Southern Blight
Figure 9. Southern blight on pumpkin fruit (Sclerotinia rolfsii) appears as a fan shaped white growth embedded with small round brown “seeds”.
Phytophthora Blight
Figure 10. A young target shaped lesion (10a) of Phytophthora blight (caused by Phytophthora capsici). Large lesion where fruit was in contact with soil (10b). Close up image of P. capsisi fruit lesion with felt-like sporulation (10c).