Rhizopus rot pops up after ripe peaches have been harvested. When it comes to this postharvest disease, some years appear to be worse than others.
Rhizopus rot favors harvested or over-ripe stone fruit. Early symptoms, which are small brown lesions, can be indistinguishable from early brown rot infection. A diagnostic characteristic of Rhizopus rot is the fruit skin will slip readily from the decaying flesh underneath; this does not happen for brown rot. Warm temperatures encourage rapid fungal growth, which is indicative of a fluffy white mass that later turns dark gray to black as the fungus sporulates. Overnight, one can be startled by fruit sporting "beards." Since Rhizopus rots sporulate heavily, it is very likely for rot to spread from fruit to fruit at point of contact.
The fungus survives on the orchard floor, especially on decaying fruit. Consequently, spore buildup can occur as the season progresses and the airborne spores can contaminate fruit on the tree. Unripe fruit are not susceptible to Rhizopus rot. Fruit in the early stages of ripening are vulnerable when injury occurs due to insects, hail, or cracking. Rhizopus rot progression is temperature related, with optimum temperatures around 80°F. At ideal temperatures, the fungus will move rapidly through the fruit.
Managing Rhizopus Rot
Since this fungus mainly attacks fruit after harvest, the last fungicide spray prior to harvest should be one that includes Rhizopus rot on the label for stone fruit. During our brown rot trial this year, we noticed when Indar (12 fl oz/A) is used as the last spray, it is weak managing Rhizopus rot during postharvest. However, two options are available labelled for Rhizopus rot:
- Merivon (FRAC Group 7 + 11; 0 day PHI)
- Luna Sensation (FRAC Group 7 + 11; 1 day PHI)
Another very important management tool--keep the fruit cold
When mature fruit is stored at 39°F, Rhizopus rot will be suppressed since the fungus will not grow or produce spores at temperatures below 40°F. In addition, handle fruit carefully to minimize wounds and bruising as these are entry points for the fungus, as well as keep storage bins and warehouses clean.
When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. Before chemical products are applied, be sure to be in compliance by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.