White Mould

(Question)

Today I discovered white mould on my butternut squash leaves. They are next to our tomato plants. I live in Toronto and the garden gets full sun most of the day. How do I get rid of the mould? Tear all of the plants out or is there another way to get rid the mould? Will it spread to the tomato plants?

Thanks for your help. I have lots of squash on the plants so I would hate to pull them all out. Is there a way to prevent the mould next year?

(Answer)

The white mould on your butternut squash leaves is powdery mildew, a fungal disorder that is common on squash plants at this time of the summer (August) when the days are hot and dry, and the nights are cool with heavy dew. The fungal spores develop on plant debris (not in the soil) and are easily transferred in the air to plants; the spores will germinate on the host plants when conditions are right. Powdery mildew fungi are host specific (different fungi infect different plants) so it will not spread to your tomato plants.

Although powdery mildew is unsightly, it is generally not fatal to the plant. At this point, your best solution is to remove the affected leaves and stems and put them in the garbage.You don’t need to tear out your plants.  As long as the plants are still alive, the fruit should be left to mature. The fungus does not grow on the fruit itself. After the fruit has been harvested, it is important that you thoroughly clean up the plant debris and dispose of it in the garbage.

To prevent this disease in future, make sure that plants are planted in full sun (you already have full sun), are not crowded and have plenty of air circulation around them. Also, when buying seed or plants, look for mildew resistant varieties. You can find a list of some here.

You may wish to refer to the following websites for additional information.

http://www.omafra.gov.on.ca/english/crops/facts/00-031.htm#powdery

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7406.html

https://extension.unh.edu/resources/files/Resource000617_Rep639.pdf

All the best for a good crop of butternut squash.