This spring, Southwest Florida watermelon growers have been hammered by a disorder currently being called watermelon vine decline.  The problem was first seen in the spring of 2003 in southwest Florida and fall of 2003 in west central Florida, when a number of growers reported problems with watermelon vine collapsing as the crop approached harvest or soon after first harvest.  Initial symptoms appear as a slight yellowing or weak area in the field, followed by wilting of the vines, scorched and brown leaves, defoliation, and rapid mature vine collapse.  In most cases progression of symptoms is quite rapid with a week to 10 days between the onset of symptoms and destruction of an entire field.  In a high proportion of affected fields harvested fruit displayed a greasy necrosis (brown) on the interior portion of the rind that rendered the fruit non-marketable.  In almost all cases fruit quality was greatly reduced.

It is estimated of that approximately 60% of the watermelon acreage in the Immokalee area was affected this spring with losses ranging from 30% to near total depending on the field.  Estimates of losses to growers range from $25 to over $50 million dollars. 

The disease is particularly vicious in that it manifests itself after growers have made their maximum investment in bringing the crop to maturity.  In cases where fruit necrosis is present the disease is even more devastating as a grower loses additional money in the form of harvest and transport cost to market as well in addition to loss of confidence of the buyers who have had to turn back loads.

Under the leadership of Dr. Pam Roberts Pathologist at UF/IFAS-SWFREC, in Immokalee, investigations on the disease and its occurrence are under way thanks to an emergency grant from the Assistant Vice-President and the Deans of IFAS.  With grant funds Dr. Roberts has recruited three plant pathologists who are worldwide-recognized specialists in melon declines and other disorders to help look into the problem.

Since the disease first appeared in 2003 numerous samples and field visits have been made.  To date, results obtained so far are inconclusive but indicate that there are nutritional deficiencies in most fields tested, for both plant tissue and soil nutrients.   The most frequent soil deficiencies on the samples we collected are K and S, with some fields having B, Mn and Fe deficiencies as well.  A number of the tissue samples were deficient in K and S most frequently, but some showed deficiencies of N, P, Mg, Cu, and Mn. 

Although nutrient deficiencies have been detected, it is believed that they simply may have contributed to weakening the plants, thus increasing susceptibility to disease and allowing for some pathogenic organism to invade the plant. 

There seems to be no consistency regarding soil moisture as it varies from very wet (> 20%) to very low (< 3%) in the various sampled fields.  It does appear that wetter areas are affected first, but then the decline spreads over drier areas of the field. 

Microorganisms including fungi and bacteria were isolated from symptomatic tissue.  The predominant fungus recovered was Fusarium spp.; however, Rhizoctonia and Pythium spp. and additional isolates were recovered using selective and semi-selective culture media for commonly occurring root pathogenic fungi and bacteria.  Further isolations are now being made from special media.  Symptomatic fruit tissue is also being tested for recovery of pathogens.  Isolates and bacterial cultures are being maintained for pathogenicity screening in the next few months.  The ability to cause similar symptoms by planting in soil from infested fields will be investigated in greenhouse studies.

Samples are also being examined for transmissible agents by electron microscopy and recovery of double-stranded RNA and indicator plant inoculations for further investigation of viruses/viroids, especially those that could be transmitted by insects.  Screening of plant tissue for known viruses by serological testing has not yielded any viruses other than those commonly found in watermelon fields in South Florida.

In summary, considerable effort and resources are being directed towards finding the cause of the problem and avoiding it in the future.  However to date all studies are inconclusive, and no firm conclusions can be made.  Recently the Florida Watermelon Association offered to contribute additional funds to help identify the causal agent and possible remedies.