White blotches on zucchini leaves may be normal or may be fungus

Question:

My zucchini plants are large, but now they have large white spots on their leaves and stems. It looks like they have grains of white sand on the top surface, but there is much more on the lower surface of many of the leaves. What is this and should I be worried?

B.G.
Alamogordo

Answer:

Zucchini squash plants will produce large white blotches on their leaves naturally as part of the leaf coloration. Small hairs on the surface of the leaf may give the appearance of sand as the light hits them, but it will be obvious that the main part of the white color is the leaf surface. These blotches are part of the leaf and will not rub off.

Powdery mildew fungus can also cause the appearance of white coloration on the leaves of squash and many other plants. In this case, the white coloration due to the "fruiting structures" of the fungus as they produce spores will look more like a dusting of flour on the leaf. It is common at this time of the year because the night temperatures are appropriate for growth and spore production in powdery mildew. To minimize powdery mildew problems, irrigate your garden early in the morning on every second or third day. The object is to allow the air around the leaves to dry by nightfall. If you water in the evening, the humidity around the plants will favor the growth of the fungus. If you water daily, the soil may remain moist enough to humidify the air, so water as infrequently as possible and early in the morning.

At this late time of the season, frost will soon kill the squash plants in much of New Mexico (if it has not happened already). That will solve the problem for this year. However, removing the plant debris from the garden will reduce the number of spores to cause problems next year (many will remain, so watering timing remains a consideration next year). You can compost this plant debris, but it is important that it is exposed to high composting temperatures that develop in the center of the compost pile to kill disease spores on the plant material. You can also put the plant debris into tightly closed plastic bags exposed to the sun for several days (so that the temperature inside the bag exceeds 130 degrees F.). This treatment will also kill the fungus and then you can compost the treated plant material without concern for spreading diseases next year.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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