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Two root-rotting fungi cause melon crop loss

May 02, 2002

As temperatures increase and melons approach maturity, there is a risk of complete vine collapse and crop loss due to either of two fungi that attack the roots. These two fungi are Monosporascus cannonballus, which causes a disease called "ine decline" and Pythium spp., which cause the disease "sudden wilt."

These fungal pathogens are common in Imperial Valley soils. Both of these fungi form resting structures capable of surviving in the soil for many years.

Plants affected by either pathogen have a reduced ability to supply water to the above-ground portion of the plant. Plant stress due to a heavy fruit load, late-season water deficit stress or stress due to extremely high temperatures can result in collapse of affected plants.

Several species of Pythium are responsible for causing sudden wilt. In the low desert the species that most commonly are responsible for sudden wilt are P. myriotylum and P. aphanadermatum. Both of these species thrive at high temperatures.


Pythium spp. begins by rotting the small feeder roots. As the disease progresses, orange-to-brown water-soaked lesions appear on the laterals and eventually the taproot is affected.

Sudden wilt disease development is favored by saturated soil conditions, so avoid waterlogged soil conditions to prevent diseases caused by Pythium spp. Planting on raised beds to allow for maximum drainage, using alternate furrow irrigations and avoiding saturating beds will reduce the likelihood that this disease will occur.

M. cannonballus growth is favored by high temperatures. A susceptible crop planted in infested soil will be infected earlier when soil temperatures are higher. In addition, the crop is most likely to collapse while temperatures are high, so late- planted spring melons are more likely to suffer losses due to this disease than those planted earlier in the year.

Affected plants will have a reduced number of fine feeder roots is and the secondary roots will have tan to red-orange lesions with well-defined margins. Late in the season, frequently after harvest, the fungus produces spores capable of surviving in the soil without a host for many years. This fungus may produce up to 400,000 spores per plant.

In response to the presence of the fungus on the roots and an additional stress factor, the plant produces blockages in the water conducting vessels. As a result, the above-ground portion of the plant cannot get the water it needs and the plant collapses.

There are several practices that can aid in avoiding damage to melon crops. Avoid planting susceptible crops such as watermelons, cantaloupes, honeydews or other mixed melons in the same field year-after-year.

There is a chemical that can be applied before planting that will kill the spores. However, the cost of this material may make this practice prohibitively expensive. In addition, a new technique has been developed in which the crop is uprooted or a chemical is applied immediately after harvest before the spores are formed. Before spore formation, the fungus can be killed by drying it in the sun or through the application of less expensive materials.

Distinguishing "sudden wilt" from "vine decline" is possible at late stages of disease development. Early symptoms of both diseases appear similar and both fungi may attack the same plant. To determine if M. cannonballus is present, inspect dead roots for small black round structures that protrude from the dead tissue. These structures, called perithecia, can be seen with the naked eye, and are diagnostic for this disease. The structures produced by Pythium spp. are not visible to the naked eye. Laboratory tests are required to confirm the presence of Pythium spp.

Both of these diseases are potential problems for home gardeners as well as commercial farmers. Monosporascus vine decline will only affect melons and their close relatives. This fungus was present on native relatives of melons in this area long before the first melons were grown here commercially, and these spores have been found in soils where commercial melons have never been grown. In addition, Pythium species are common in soils and can cause root rots on many different types of plants.

>> Tom Turini is the plant pathology farm adviser at the University of California-Imperial County Cooperative Extension.

>> The Cooperative Extension serves all residents of the Imperial Valley.

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