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Significance

Monosporascus root rot and vine decline is an emerging disease worldwide, and one that only recently has gained attention among plant pathologists. Two good reviews on this disease have been published recently. 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. Both a new genus and new specific epithet had to be created for this fungus. Its name describes the unusual trait of producing a single ascospore that looks like a cannonball. 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: Mexico, Guatemala, Honduras, Spain, Israel, Iran, Libya, Tunisia, Pakistan, India, Saudi Arabia, Japan, and Taiwan (figure 15).

Figure 15.

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 group of diseases with similar symptoms but different causal agents. General symptoms of this group of diseases 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 prevents water uptake and transport late in the season when the plant is experiencing a high water demand due to increased transpiration rates and water transport to the fruit. In addition, tyloses form in many of the remaining roots (R. Cohen, personal communication) choking off additional water transport and probably serve as the "last straw," resulting in vines that collapse almost "overnight." Vine declines have become increasingly more prominent in the last 15 years.

A common question is: "Why has this disease become so prominent in the last few years?" 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 in Israel, 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 15 to 20 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 conditions 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 20 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.