Watermelon Crops At Risk

Watermelon Vine Decline and Fruit Rot - Update
by
Pam Roberts, Rosa M. Muchovej, Gene McAvoy and Phyllis Gilreath

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 million dollars 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.