Significance

Monosporascus root rot and vine decline is an emerging disease worldwide, and one that only recently has gained attention among plant pathologists. It is the primary pathogen of late-season vine declines of melons in most melon-growing areas of the world. The causal agent, Monosporascus cannonballus, was described as a genus et species novus by Pollack and Uecker in 1974 based on specimens obtained from necrotic muskmelon roots from Arizona. No pathogenicity trials were conducted at that time. The first confirmed report of pathogenicity was from Israel in 1983 where it was shown to the cause of a mature melon plant collapse, although the pathogen was identified as M. eutypoides. An earlier report from Japan indicated that this fungus was nonpathogenic. Pathogenicity of isolates from the United States was first reported in 1991 by Mertely et al. in Texas, and the disease was named Monosporascus root rot and vine decline (MRR/VD). By 1997 the disease had been reported from Texas, Arizona, and California in the US and from 13 other countries. It has since been reported in four additional countries bringing the total number of countries reporting MRR/VD in 2009 to 18. (Figure 16, Table 1). It is highly probable it will be detected in additional countries in the near future.

Figure 16.

Losses from MRR/VD can be quite extensive. The pathogen appears to be uniformly dispersed throughout fields causing widespread and uniform infection. Damage is done to the root system throughout the growing season and, typically, within a week or two of harvest, the vines wilt or collapse quickly and completely. This results in plants whose fruit are exposed to direct solar radiation and often crack and sunburn. In addition, since much of the sugar content is partitioned into the fruit in the last few weeks of growth, fruit from infected vines are low in sugar and of poor quality and size. In many cases, there can be total economic loss.

Monosporascus root rot and vine decline is an example of a vine decline disease of melon. Vine decline is a generic term used to describe a recalcitrant group of "diseases" with similar symptoms but different putative causal agents. General symptoms of vine declines include yellowing and death of the crown leaves and a gradual decline of the vine as the plant approaches maturity. A rapid collapse of the vine occurs late in the season, typically just before harvest. The primary effect of the infection is destruction of the roots that restricts water uptake and transport late in the season when the plant is experiencing a high water demand due to increased stomatal transpiration and water transport to the fruit resultin in vines that collapse almost "overnight." Vine declines have become increasingly more prominent in the last 25 years.

A common question is: "Why has this disease become so prominent in the last decade?" There is no definitive answer to this, but there are several factors that probably have contributed to it. First, it probably did not appear as a new disease in the 1980s, but rather was already present in a number of places, but either was not identified or was misidentified. Once it was reported as a pathogen of melons, other laboratories perhaps were inclined to look more carefully. Additionally, there are several cultural and cropping conditions in melons that have changed radically in the last 25 years that are coincident with the increase in vine decline diseases: a) the virtually complete shift from open-pollinated melons and watermelons to hybrids, resulting in plants with an earlier and more concentrated fruit set; b) the shift from direct seeding to the use of seedling transplants; and c) the shift from growing plants on bare ground with furrow or overhead irrigation to the use of plastic mulch and drip irrigation. These latter two practices result in plants with poorly developed and shallow root systems that tend to grow parallel beneath the plastic instead of penetrating deep into the soil profile. And lastly, with higher inputs and production costs, many growers have shortened their rotation schemes and are reusing fields sooner or, in some cases, continually cropping melons for two or more successive seasons. This practice significantly adds to the inoculum build-up in the soil.

All of these factors may affect the overall condition of the plant and predispose it to infection. Another factor to consider is that over the last 25 years, more and more non-crop land has been cleared and planted to watermelons and melons. There is some evidence that M. cannonballus is a native or indigenous pathogen. The planting of highly susceptible plants, such as melons, may have selected for or baited the fungus and allowed its rapid increase. Since new melon varieties and cultural practices often are quickly adopted by growers around the world, it is possible that the selection for the pathogen also occurred quickly. While there are no hard data to support this hypothesis, it warrants some consideration and testing.

M. cannonballus is a devastating pathogen of melons wherever it occurs, but it cannot explain or account for all cases of vine declines and collapses seen around the world. In cases where M. cannonballus has not been found, other organisms have been implicated in the disease. Most of these are fungi, but viruses, bacteria and even herbicides also have been implicated. In other cases, no definitive causal agents have been described. A significant contributor to the vine decline syndrome is a poorly developed root system. Transplanted seedlings grown in traditional small cells typically do not develop strong taproots (Figure 17). As the taproot elongates and reaches the bottom of the transplant cell, it curls around the edges, resulting in a taproot that either quits growing or continues in a circular, twisted configuration along the sides of the transplant cell. When transplanted into the field, the taproot does not recover and grow down into the soil profile, but rather produces a plant with a proliferation of lateral roots growing horizontally along the transplant bed following the water stream but having no significant taproot (Figure 17). By the end of the season, when water demand is high, the compromised root system labors to keep up. At this stage, it doesn’t take much additional stress to cause collapse. Cultural methods that improve root development can help alleviate many of the vine decline symptoms seen around the world. Lengthening the transplant container, such as with the use of Cone-tainers™, (Figure 18), typically used for pine seedlings and other trees, will allow the taproot to develop more fully (Figure 19). Additionally, the use of improved irrigation techniques such as dual drip tapes buried approximately 15 to 20 cm below the soil surface, on either side of the plants places the water deeper in the soil and on the outside of the beds, forcing the roots to explore more of the soil profile, ultimately producing a more vigorous root system that can better support the demand of the vines and fruit late in the season and withstand minor stresses (Figure 20). Treatment of soilless potting mixtures with one or more mycorrhizal fungi prior to seeding also may improve root development. Direct seeded plants, however, still typically produce the best roots and vines.

Figure 17.

Figure 18.	Figure 19.

Figure 20.

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by The American Phytopathological Society