Their counterparts along the same band of latitude from Southeast Asia to the Mediterranean have the same problem, according to Mary Olsen, University of Arizona Extension plant pathologist at Tucson, who’s been studying the fungus for years.
Olsen talked about it and other melon pathogens during the Desert Crops Workshop, held recently in Yuma under the sponsorship of Western Farm Press, University of Arizona Cooperative Extension, and University of California Cooperative Extension.
So named because its dark spores appear under a microscope as tiny cannonball-shaped structures, it was reported in the Yuma area more than 40 years ago. Pathologists have known about it all along and believe it has become more prominent because of the way melons are farmed, particularly under drip irrigation.
"There are probably several reasons for this," Olsen said, "one being the shorter rotations between melon crops. Sometimes we see three melon crops in a row, and that can cause problems with just about any pest."
Cultural practices -- particularly plastic mulch and buried drip irrigation -- that encourage shallow root structures promote infestations by causing abundant spores to germinate. And, she added, the disease is likely often misidentified or simply overlooked.
Lurks in plant
The spores invade the fine, secondary roots of a plant and the earlier and heavier fruit set of hybrid varieties is more attractive to them. The disease lurks in the plant until late in the season when decline shows.
Another soilborne disease of melons, both on crowns and roots, is Macrophomina phaseolina, or charcoal rot. Her research, she said, shows that it occurs more often with the stress factors of less moisture and high salinity in the upper level of soil.
The crown invasion is more common under drip irrigation than furrow. "In fact, drip irrigation is just perfect for this pathogen. It is more severe in fall melons than spring melons because fall crops are planted right back into soil where the inoculum has built up."
While the crown rot shows in other melons, in watermelons, the very same disease shows up more often as root rot and often is present with Monsporascus.
Rhizoctonia solani has not been customarily a concern in melons, until recently. "We’ve found it with buried drip irrigation and reduced tillage after grain or green manures in situations where the soil is not turned well. The layer of organic matter is very conducive to this disease, and we see it more in the spring than the fall."
Olsen plans further investigation but said she now suspects it too is caused by lack of rotation of watermelon with other crops.
Watermelon growers often ask if Pythium root rot, or damping off, can be economically treated, and Olsen had a word of caution.
"Running Ridomil through buried drip lines seems to be economically feasible, but you have to take care in putting these kinds of fungicides through drip lines."
She pointed to research literature on using small dosages of fungicide on carrots for a number of years. According to those sources, the problem is not Phythium becoming resistant to the fungicide but soil microbes turning to the fungicide as a food source and breaking it down.
Moving to Fusarium wilt, she said conventional wisdom for years was that Arizona soils were not conducive to this pathogen, even though it is common all over the world. "Well, all of a sudden we have Fusarium wilt on lettuce in Yuma. We hope there’s enough resistance in lettuce varieties, but now we just don’t know."
James McCreight, USDA research horticulturist at Salinas, Calif., said he and University of Arizona and University of California specialists have been working on the question of Fusarium resistance on lettuce.
They are screening many varieties for resistance, and he said resistance in Salinas 88 is "holding up well now, but we want to confirm that."
Fusarium appeared in Huron, Calif., in 1990, in Huron and Yuma in 2001, and in Watsonville, Calif., in 2002. Two races have been determined. It has also been found in Japan and Italy.