I. Introduction: Rhizopus rot is a soft
fungal rot of harvested or over-ripe stone fruits. Fungal growth and fruit decay are
greatly retarded in cold storage but advance rapidly at warm temperatures, allowing loss
of many fruit within the shipping container.
II. Symptoms: Dime-sized lesions are cinnamon or
chocolate-colored and may be difficult to distinguish from early brown rot lesions. A
distinguishing characteristic is that Rhizopus rot causes the skin to slip readily from
the decaying flesh underneath, while brown rot does not. At warm temperatures, the fungus
rapidly advances through the entire fruit. The early appearance of the fungal mycelium is
as a fluffy white mass. This later turns dark gray to black as the fungus begins to
sporulate (photo 2-65).
III. Disease Cycle: The fungus survives unfavorable
environmental conditions as dark colored zygospores in the remnants of rotted fruit in
bins or on the orchard floor. Fruit in the early stages of ripening require injury from
factors such as insects, hail or cracking for infection to occur. At harvest the skin is
often injured at the stem. Rotted fruit on the orchard floor allow for inoculum buildup as
the harvest season progresses. After harvest, Rhizopus rot can spread from fruit to fruit
without injury at the point of contact. Rot progression is temperature related, with rapid
fungal growth at the optimum temperature of 81 F (27 C), but no spore germination or
growth at 40 F (4 C).
IV. Monitoring: Be aware of conditions (insects,
hail, birds) that injure fruit in the preharvest period and during the harvest operation.
Check for Rhizopus rot (photo 2-65) buildup while monitoring for brown rot throughout the
preharvest period. Monitor ten fruit per sample tree for disease incidence. Greater than
two infected fruit per ten acres (eight trees sampled) represents a high risk at this
time. Rhizopus rot is more likely to be a problem where fruits are allowed to fully ripen
on the tree. Monitor sanitary conditions of field bins or crates, the hydrocooler, and the
packinghouse to reduce the incidence of this disease.
V. Management: Since the fungus
attacks fruit mainly after harvest, storage at 39 F will control the disease becasue the
fungus does no grow at temperatures below 40 F. To minimize the incidence of
Rhizopus rot, handle fruit carefully to avoid wounds, keep storage containers and
warehouses clean, and keep hydrocooling water clean. Preharvest fungicides,
postharvest fungicide dips and/or sprays, or impregnated fruit wrapping papers help
prevent the disease, also.
Chemical control -
Chemical control - home
orchardists (pdf file - Acrobat Reader required)
Text prepared by K.S. Yoder and K.D. Hickey
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