Alternaria Rot

Have you ever tried eating an orange that looks ripe but ended up being very unripe?
​Major pucker factor!

Your orange may have been infected with alternaria rot, a fungal disease that attacks lemons and navel oranges, causing the fruit to change color before they are ripe.

Orange affected by alternaria rot (page 415 of "Citrus fruits;" 1915) Public Domain

Citrus affected by the Alternaria citric fungus may look perfectly ripe and ready on the outside, at first. After fruit is harvested, the fungi continue breeding and feeding on the inside. By the time the infection is fully established, the stylar (flower) end of the fruit may show a dark brown or black area. Alternaria rot is also called black rot when it appears in navel oranges. If you cut infected fruit in half, you will see the rotted area has spread into the core of the fruit.

There are several varieties of the Alternaria citric fungi. Most of them do not produce toxins (but you still won’t want to eat the fruit). So, how can you prevent or treat alternaria rot?

First, healthy plants are better at protecting themselves. Navel orange trees experiencing citrus fruit split and water stress are going to more vulnerable to infection that trees which have received regular irrigation. Fungicides are generally ineffective.

Remove infected or otherwise damaged fruit and delay harvesting for as long as possible. This will allow infected fruits to show themselves and be be disposed of, before harvesting the unaffected fruit.