Cucurbit growers in Kentucky should scout their crops for powdery mildew this week. Powdery mildew is a common disease of all cucurbits that typically makes its first appearance near the middle of the growing season. The disease occurs in greenhouses and high tunnels as well as in the field. 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 and Disease Cycle
Cucurbit powdery mildew is caused by the fungal pathogen, Podosphaera xanthii. This pathogen overwinters in crop residues and perennial weeds. Powdery mildew is favored by high humidity; however, unlike many fungi that require leaf wetness for infection, moisture on plant surfaces can inhibit powdery mildew pathogens. For this reason, powdery mildew may be more common on the underside of leaves under rainy conditions. In drier weather, it is common on the tops of leaves. Unlike downy mildew, powdery mildew easily spreads over leaf veins.
Once established, powdery mildew can spread rapidly in dense plantings, and in severe cases results in defoliation. Fruit do not become infected with the pathogen, yet this disease will limit yields due to reduced photosynthesis in the leaves and overall poor plant health. Carving pumpkin growers take note: powdery mildew can destroy the pumpkin “handle” if left unmanaged.
Cucurbit powdery mildew begins as a foliar disease. The most noticeable signs are white, powdery spots that appear on upper or lower leaf surfaces (Figure 1). As the disease progresses, the entire leaf surface becomes covered, and leaves may become yellow and necrotic. Over time leaves die and defoliation occurs. Stems and leaf petioles may also become infected. Cucurbits with a longer growing season, such as winter squash and pumpkins, tend to suffer more damage from powdery mildew, as fruit yield and quality may be reduced.
- Utilize disease resistant cultivars (Cucumber, melons, squash, and pumpkin).
- Plant in sunny areas with good airflow.
- Use recommended plant spacing to facilitate air movement and leaf drying.
- Remove and destroy heavily infected plants.
Early in the cucurbit growing season, a preventative fungicide program utilizing mancozeb, chlorothalonil, and copper should suppress early infections by the powdery mildew pathogen (see pg. 115 in ID-36). In week 4 or 5, or at first symptom development (whichever happens first), a systemic fungicide should be used to continue to suppress disease. Fungicides recommended for heavy powdery mildew pressure include Vivando, Torino, Quintec, Pristine, and Rally. Under lower disease pressure, several other fungicides, such as those in FRAC groups 1, 3, 7, and 11, will also be suppressive.
As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals.
- Foliar Diseases of Cucurbits (PPFS-VG-10)
- 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