Downy Mildew on Butternut Squash — How to Identify and Treat It

Downy mildew is frequently confused with powdery mildew, but the two diseases look different, behave differently, and require different treatment approaches. Knowing which one you are dealing with is the first step toward managing it effectively and saving your harvest.

How to Identify Downy Mildew

Downy mildew causes angular yellow or pale green patches on the upper surface of leaves, bounded by the leaf veins. When you turn the affected leaf over, you will find a grey, purple-grey, or brownish downy growth on the underside — this is the spore-producing structure of the pathogen. The yellow patches on the top and the downy growth beneath together are the defining characteristic. Unlike powdery mildew, which appears on the top of the leaf, downy mildew spores are produced under the leaf, away from direct sunlight.

Conditions That Favour Downy Mildew

Downy mildew is caused by Pseudoperonospora cubensis, an oomycete pathogen that thrives in cool, wet conditions. It is most severe when nights are cool and damp, when dew forms on leaves overnight, and when plants are growing closely together with restricted airflow. The spores travel on wind and water splash, so a single infected plant in the garden or nearby allotment can spread the disease rapidly across a whole bed during wet weather.

Treatment Options

Once downy mildew appears, removing affected leaves promptly reduces the spore load and slows the spread. Copper-based fungicides — approved for organic use — are effective against downy mildew when applied preventatively or at the very first sign of infection. Apply to both the upper and lower leaf surfaces thoroughly. Reapply after rain. Synthetic fungicides containing metalaxyl are highly effective but are reserved for commercial use in many countries.

Preventing Spread

Improving airflow by spacing plants correctly and removing excess foliage reduces the humid microclimate that downy mildew requires. Water at the base of plants rather than from overhead to keep leaf surfaces dry. Avoid working among wet plants, as hands and tools can transfer spores from plant to plant. Remove and dispose of all infected plant debris at the end of the season — do not compost it, as the pathogen can persist in plant material.

Distinguishing From Powdery Mildew at a Glance

The simple test: look under the leaf. Powdery mildew is a white coating on the top surface only. Downy mildew produces angular yellow patches on top with a fuzzy, grey growth underneath. The two can occasionally occur together in the same plant in late season, making the distinction important for choosing the right treatment.