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Causal agent

Choanephora cucurbitarum (Berk. & Ravenel) Thaxt.

Symptoms

Affected fruit are watery and soft.  Hairy, white fungal growth appears on the rotted area with purple-black fruiting bodies forming as the disease progresses.  The fungus growth resemble small black-headed pins stuck into the fruit, sometimes called "whiskers."  Disease development is promoted by high moisture conditions.

Sometimes misdiagnosed or confused with

Rhizopus soft rot caused by Rhizopus stolonifer.  The two diseases are identical to the naked eye, but can be easily distinguished by observing the branching pattern of the fungus.  Also, Choanephora thrives in a hot, humid climate and grows well at 25 C or above.  Rhizopus also thrives under high humidity, but has a much lower optimum growth temperature (15C); it's pathogenicity declines above 20C and is not viable above 37 C.

Diagnostic methods

In the field.  Look for fast-growing, whisker-like fungal growth on the surface of the fruit.  With some practice, the branching pattern of the spore-bearing tips of the fungus can be resolved using a 20X handlens.

In the lab.  Few fungi grow as fast as Choanephora or are as distinctive in there appearance, both macro- and micro-scopically.  It is very easy to confuse Choanephora with Rhizopus if a closer look is not taken.  However, under higher magnification it is easy to detect the radiating appendages at the tips of Choanephora sporangiophores which are missing in Rhizopus.

Occurrence & geographic distribution

Choanephora rot is uncommon on cantaloupe postharvest.  The disease is much more common on squashes.  The pathogen is distributed worldwide.

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Phytosanitary risk & quarantine issues

None

Disease cycle & epidemiology

The fungus survives as a saprophyte or as chlamydospores and as zygospores.  Numerous spores are produced and are disseminated by wind, insects and water.  Periods of high humidity and high temperatures favor disease development.  Squash flowers are particularly susceptible to colonization by this fungus and it is not uncommon to see more that 90% of squash blossoms blighted when conditions are favorable.  The same is not true of cantaloupe blossoms.

Management

No practical control measures have been developed in the field.  Control using fungicides is generally not practical because new blossoms open daily and conditions favoring disease development come and go rapidly.  Refrigerated storage and careful handling will minimize decay postharvest.

Other notes