So many things can go wrong in the garden that it's amazing gardeners keep at it year after year. Gray spots on butternut squash leaves are not uncommon; what's more, they can appear suddenly and the underlying pathogens sometimes take whole crops down quickly if left untreated. Most gray spots on the leaves of cucurbits (a family of plants that includes cucumbers, squash and melons) are caused by fungi that are easy to treat, with the right knowledge.
Angular Leaf Spot
Angular leaf spot (Pseudomonas lachrymans) appears on many cucurbits during warm, wet weather. When plant tissues swell due to excessive moisture, the warm weather encourages the bacterial populations to explode. Lesions appear irregularly throughout the plant, primarily on leaves, but can extend to stems and fruits. Lesions often begin as water-soaked or brown spots, but quickly turn gray, then white when the tissue is completely dead. P. lachrymans overwinters in plant debris and is sometimes spread by infected seeds, if they survive germination. Homeowners can successfully battle angular leaf spot by applying Bordeaux Mixture (a combination of copper sulfate, lime and water) throughout the year, destroying plant debris as soon as possible and practicing crop rotation.
Gummy Stem Blight
Gummy stem blight (Didymella bryoniae), is caused by a fungus that overwinters in plant debris and seeds. When temperatures warm and if leaves are chronically wet, the fungus reproduces quickly inside plant tissues. Aphids and striped cucumber beetles provide tissue entry points for this aggressive fungus. Young plants may show yellowing of the leaves before developing brown spots and eventually becoming necrotic with black or grey patches. Cankers form on stems of affected plants, oozing a gooey substance and girdling the stem completely. As gummy stem blight becomes more severe, black rot occurs on the fruit of many cucurbits. Where fruits are in contact with the ground, dark yellow to brown lesions form on butternut squash -- these eventually turn black during storage. Bacillus subtilis, a biological fungicide, is effective against black rot, as are a number of broad-spectrum fungicides such as the contact fungicide mancozeb and the systemic tebuconazole.
Downy mildew (Pseudoperonospora cubensis) thrives in areas where hard freezes are rare or nonexistent. This fungus, which causes angular spots that follow the contour of leaf veins, prefers cool weather and will appear when temperatures are around 59 degrees Fahrenheit and dew is frequent. The undersides of leaves are often covered in greyish down -- new spores preparing to move to other plants. While downy mildew is extremely aggressive, it only attacks leaves, making it easier to distinguish from other fungal pathogens. In areas where downy mildew is known to be present, a combination of preventive and systemic fungicides should be applied. Chlorothalonil or mancozeb are good preventives, while propamocarb, cyazofamid and several combination fungicides are effective once the fungal bodies are present.
Cucurbit scab (Cladosponum cucumerinum) first appears on leaves as water-soaked spots that gradually turn gray or white. Unlike with other fungal pathogens, these lesions are often haloed with yellow. Symptoms are similar to angular leaf spot, but cucurbit scab is more of an equal-opportunity pathogen, infecting the whole plant instead of limiting itself primarily to leaves. Butternut squash is moderately resistant to C. cucumerinum, but can still suffer with heavy fungal loads. Apply fungicides when the plants just begin to bloom unless scab is known to be widespread in the area -- in that case, apply fungicide once the first true leaves appear for optimum protection. Maneb, mancozeb or chlorothalonil have proven effective against C. cucumerinum.
- Alabama Cooperative Extension System: Guide to Commercial Pumpkin and Winter Squash Production
- Cornell University Department of Plant Pathology: Fruit Rots of Squash and Pumpkin
- North Dakota State University: Angular Leafspot of Cucumber
- University of Florida IFAS Extension: Management of Gummy Stem Blight (Black Rot) in Curcurbits in Florida
- North Carolina State University Department of Plant Pathology: Curcurbits -- Downy Mildew
- Cornell University Department of Plant Pathology: Scab of Curcurbits