Persian limes with brown bottoms have stylar end rot.
Also known as stylar end breakdown, stylar end rot primarily affects Bearss, Tahitian, and other Persian lime species but can occur in other lime and lemon varieties. Stylar end rot found on guava is a fungal disease.
Stylar end rot is a physiological disease caused by too much heat. Even though limes, lemons, and other citrus have thick, waxy skins to protect themselves from the sun’s heat and drought conditions, sometimes that protection isn’t enough.
Symptoms of stylar end rot
The stylar end of a fruit is the part with the dried-up petals. It is opposite the stem end. Stylar end rot starts as a small, grayish sunken area that slowly becomes firm and leathery. Some fungal diseases exhibit similar symptoms:
Affected areas can spread to cover 1/4 to 1/2 of the fruit. Bacteria and fungi often infect these compromised areas. The tissues inside break down and turn brown or pink. Diseased fruit can go in the compost pile, but only if it is free of other fungi and bacteria. Otherwise, toss it in the trash.
Preventing stylar end rot
You can’t. Stylar end rot occurs when vulnerable citrus fruits experience an accumulated 18 hours of temperatures over 105°F. Think of it as reverse chill hours. Pick the fruit earlier, after particularly hot summers, and hope for cooler weather next year.
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