The Q&A Archives: Brownish gray mold on summer squash

Question: I'm growing summer squash in tubs on my city patio. The first fruits were fine, but now a brownish gray mold has infected the blossoms, turning the small fruits to mush. What causes this mold, and how do I avoid it? Candy Erhard Alexandria, VA

Answer: Molding on summer squash fruit is a symptom of poor pollination, too much nitrogen fertilizer or Choanephora wet rot, says Greg Welbaum, vegetable crop specialist at Virginia Polytechnic Institute and State University in Blacksburg. Poor pollination is the most likely cause in your case. If squash aren't fully pollinated, the fruit will begin to decay and secondary rot organisms will invade, explains Welbaum. Since squash plants have separate male and female blossoms and need insects to pollinate the blossoms, getting adequate pollination can be tricky. Bees are the main pollinators, and because you're growing the squash in containers on your patio in an urban area, it may be hard for bees to find and pollinate your plants, he notes. The solution is to hand pollinate them yourself. Pick a fully open male blossom (no small fruit behind the blossom) and remove the petals. Immediately to a fully open female blossom (look for a small fruit behind the blossom) and swab the male anthers onto the female stigma (the tope of the tall tubes inside the flower). Be sure to distribute the pollen evenly on the stigma. One male flower can pollinate several female blossoms. If you have been successful, you should see the fruit swelling within five days. Too much nitrogen fertilizer can cause fruit to abort, and then rot organisms can invade, says Welbaum. For squash in containers, apply a water soluble fertilizer at 1/3 strength once a week. Before flowering use 10 10 10 fertilizer. After flowering,switch to a 5 10 10 fertilizer. Chaonephora wet rot is an airborne fungus that infects wilted blossoms after pollination and causes young fruit to rot. It thrives when plants are continually wet. This disease is easy to identify because the fruiting body, which produces the spores, looks like small black headed pins coming from the infected fruits, explains Welbaum. Remove the infected fruits immediately to keep the disease from spreading, he advises.

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