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 for summer fruit cooling. As little as 1 hour of fruit wetting is needed for infection. Apple susceptibility increases 10 days before harvest. Can be a problem not only on pome fruit but also stone fruit such as peach.
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. Can be confused with fire blight on immature pears but is lighter in color and soft to the touch. Some years, disease also may spread to fruit pedicels and a little way into first-year wood, causing a dieback that resembles fire blight.
- 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.
- Copper metered into water at 1 part per million.
- Higher concentrations will cause fruit russeting in 'Anjou' pears.
- 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 Yamak, F., Peever, T.L., Grove, G.G., and Boal, R.J. 2002. Occurrence and identification of Phytophthora spp. pathogenic to pear fruit in irrigation water in the Wenatchee River Valley of Washington state. Phytopathology 92: 1210-1217.
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