LATE-SEASON VINE DECLINES OF MELONS: PATHOLOGICAL, CULTURAL OR BOTH?

There are numerous well-known and characterized soilborne diseases of melons capable of limiting production; however, over the last 20 years or so, a recalcitrant syndrome loosely referred to as ‘vine decline’ has appeared around the world. Watermelon and melon are affected most and, at times, the entire crop may be lost. While specific symptoms often vary from field to field or from place to place, the overall symptom is the same – the vines begin to wilt and collapse late in the season, often only days or a few weeks before harvest. This results in fruits that are of lower quality in size, sugar content, and soluble solids. In addition, they are more susceptible to sunburn and cracking due to the loss of the protective leaf canopy. For many years now, pathologists have investigated the cause of these late-season vine declines. In the last 10 years, numerous organisms have been described as a causal or contributing agent. Most of these have been fungi, but viruses and bacteria also have been implicated. In other cases, no causal agent has been described. While a definitive causal agent for most has not been determined, there is one that has been well characterized – Monosporascus root rot and vine decline (MRR&VD). MRR&VD is caused by the soilborne ascomycete (Pyrenomycete) fungus, Monosporascus cannonballus. 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 around the world. In cases where M. cannonballus has not been found, other fungi have been isolated from root tissue and, in some cases, implicated in the disease. Late-season vine declines have become increasingly more prevalent on melons around the world in the last two decades and their sudden appearance coincides with significant changes in the way melons are produced. Two changes in particular (drip irrigation and transplants) are believed to play an important contributing role in exacerbating the severity of vine declines. Melon seedlings grown in transplant cells and then placed in the field typically do not form well-developed taproots. Consequently, they have poorly developed root systems that cannot adequately supply the plant with its water and nutrient needs late in the season. In addition, drip irrigation results in a constantly wet soil that aids in the colonization of roots by ‘root nibbling’ pathogens that further weaken the plant. The result often is a rapid and complete collapse of the plant late in the season. Techniques that encourage good root development can significantly reduce the severity of many vine declines. These include things such as direct seeding, use of transplants grown in cone cylinders, furrow or overhead irrigation, and the use of dual drip tapes buried at least 12-15 cm below the soil.