See:
Apple (Malus spp.) - Fruit Rot, Phytophthora
Cause Phytophthora cactorum, a soilborne fungus-like microorganism frequently carried in irrigation water. Fruit rot has caused significant loss of fruit where irrigation water wetted fruit, usually on lower branches or from overhead irrigation. As little as 1 hour of fruit wetting is needed for infection. Can be a problem not only on stone fruit but also pome fruit such as apple and pear.
Symptoms Firm, tan-colored, rotted spots on areas up to more than 1 inch in diameter on the fruit. Older infections can cause the whole fruit to rot on the tree.
Cultural control
- Use low-angle or microsprinkler heads, placed under trees, so water does not wet fruit. Drip irrigation can also be used.
- Reduce the length of irrigation sets.
Chemical control
- Copper metered into water at 1 part per million.
- Zonix at 45 to 76 fl oz/100 gal water. Can be sprayed on crop or injected into irrigation system. Short residual. 4-hr reentry.
Reference Liu, F., Li, B., Lian, S., Dong, X.L., Wang, C.X., Zhang, Z.F., and Liang, W. 2018. Effects of temperature and moisture on the infection and development of apple fruit rot caused by Phytophthora cactorum. Plant Disease 102:1811-1819