Like all food crops, winter squash are susceptible to various fungal diseases. Your strongest weapons against these are best planting practices, which help prevent disease in the first place.

This is especially important, as there are no fungicides approved for home use for many diseases.

These best practices are aimed at producing strong, healthy plants that can withstand disease, and at avoiding situations that contribute to the development of disease. They involve keeping plants clean, dry, and undamaged.

Watering: Water your winter squash deeply about once a week (check the soil for dryness). Do not over-water. Soggy soil invites disease. Focus on watering the plant, not the vines or the fruit. Try not to handle wet vines; that’s a good way to spread fungal diseases. Destroy any infected plant matter; do not compost it.

Mulch: Mulch can help with water retention—but be vigilant and check for insect or fungal activity.

Other best practices include:

• Buy healthy, disease-free seeds from reputable sources.
• Plant your winter squash in full sun.
• Plant in sites with good drainage; if planting in open ground, choose a higher spot for better drainage.
• Check plants regularly for signs of disease.

Common winter squash diseases

Here are some of the usual culprits that might infest your squash plants. Remember, it’s important to remove flowers and leaves to prevent the spread of disease once it’s found its way onto your plant.

Phytophthora Blight

Cause: Fungus (Phytophthora capsic)

Symptoms:

• undersized and distorted fruit
• white mold develops on skin
• yeast-like growth after white mold

How it spreads:

• lives in the soil for years
• spores spread through soil on equipment and rain splashing on fruits

Treatment:

• remove all infected parts
• phosphite (chemical treatment)

Prevention:

• grow a mustard biofumigant cover crop in the spring before planting squash—Kodiak, Pacific Gold, Caliente 199 are examples of mustards used for this purpose
• avoid overhead watering
• rotate crops
• plant in a well-drained area
• choose resistant varieties

Sclerotinia White Mold

Cause: Fungus (Sclerotinia sclerotiorum)

Symptoms:

• watery, odorless rot
• white cottony mold with black balls of fungus

How it spreads:

• lives in the soil for years
• spreads from fruit to fruit
• in storage

Treatment:

• remove all infected parts
• let soil dry between irrigation

Prevention:

• long-term rotation with corn or small grains
• don’t rotate with beans or cabbage which can carry and spread the disease
• avoid planting near other crops affected by Sclerotinia White Mold
• avoid overhead watering
• plant in a well-drained area
• choose resistant varieties

Anthracnose

Cause: Fungus

Symptoms:

• leaves turn yellow at the tips, eventually turning brown all around
• browned, dying leaves
• dark, sunken lesions on fruit and/or stems

How it spreads:

• spores spread by rain and splashing water
• wind

Treatment:

• remove all infected parts
• clear ground of leaf and twig litter
• apply liquid copper fungicide to reduce recurrence

Prevention:

• prune plant to provide good air circulation and access to sunlight
• provide proper water and fertilizer

Downy Mildew

Cause: Fungus

Symptoms:

• pale spots or long pale patches on leaves
• gray-purple fuzzy growth on leaf surface
• leaves turn pale, then yellow
• leaf tips collapse

How it spreads:

• splashing water (rain, irrigation) disturbs spores
• cool temperatures favor growth

Treatment:

• destroy infected plants
• apply foliar fungicides

Prevention:

• purchase disease-free plants
• rotate crops with non-allium species for three to four years
• plant in well-draining areas
• homemade fungicides
• choose planting sites with good air movement and no shade

Gummy Stem Blight

Cause: Fungus

Symptoms:

• sudden death of a vine or the entire plant
• oozing cankers on the vines
• long, water-soaked areas on the stems
• brown or black spots on older leaves

How it spreads:

• spores spread by rain and splashing water
• movement through garden
• wind

Treatment:

• use drip irrigation rather than overhead watering to keep leaves from getting overly wet
• liquid copper fungicide, if infection is severe
• remove and discard infected parts

Prevention:

• always water the soil around the plant; avoid wetting leaves
• ensure plant is getting adequate water, light, and fertilizer
• liquid copper fungicide
• crop rotation: do not plant cucurbits in the same area for at least two years

Powdery Mildew

Cause: Fungus overwinters on infected plant debris and emerges in warm, humid weather

Symptoms:

• white, powdery patches on leaves
• leaves may curl and turn upward
• new shoots appear stunted
• older infections look like a tan or reddish-brown felt covering; these contain spores
• heavy mildew infections can stunt a plant’s growth

How it spreads:

• fungus overwinters in infected plant debris
• spores do not need moisture to germinate; often called a “dry weather disease”
• wind, insects, and birds

Treatment:

• in the spring, prune any shoots that have a white coating (spores)
• removed and destroy infected areas

Prevention:

• liquid copper fungicide
• if necessary, sulfur-based fungicide
• plant disease-resistant varieties
• remove plant debris

Below is a sampling of other diseases that may plague your squash crop. The key to prevention is to maintain good air flow in your plants and to keep the leaves from getting overly wet. Check with your local extension center for diagnosis and advice on controlling outbreaks.

• Sclerotinia White Mold
• Alternaria Leaf Spot
• Bacterial Wilt
• Scab

Which diseases have you had to treat on your winter squash? Please tell us how you prevent and handle diseases. If you spot other symptoms on your winter squash that are not mentioned here, contact your local extension center or garden center for a consult—and please let us know what you discover.