B. DESCRIPTION OF THE PROBLEM
1. History of the Disease
2. Geographical Distribution of Watermelon Mosaic
3. Description of Watermelon Mosaic Symptoms
4. Strains of Mosaic and Host Ranges
C. METHODS OF TRANSMISSION
1. Mechanical Transmission
2. Aphid Transmission
3. Seed Non-Transmission
4. Varietal Susceptibility
D. SUGGESTED METHODS FOR CURBING SPREAD OF WM'I
2. Weed Control
3. A-Piid Control
4. Trp Crons
5. Time of Planting
6. Surner Host Frea Period
7. Good Pest Control Program
E. RESEARCH NOW IN PROGRESS AND PLANNED FOR THF FUTURE
Development of watermelon mosaic again this year in south Florida indicates that
this disease has become a serious problem of regular occurrence in watermelon pro-
duction. The seriousness of the problem prompted a concerted effort on the part of
Florida Agricultural Experiment Station and Extension workers to evaluate the situa-
tion to plan stop-gap control measures for use now and needed research for the future.
This is a report of the evaluation made by this group. County Agents, growers, supply-
men and all others interested in watermelon production should familiarize themselves
with what is known about watermelon mosaic and what can be done to lessen mosaic
B. DESCRIPTION OF THE PROBLEM
1. History of the Disease. Watermelon mosaic was first observed at Fort Meade,
Florida, in 1932. Eighteen years later in 1950, it was found again at Felda, Immo-
kalee, and Webster. From then on it has been observed each year in varying degrees.
In 1958 it caused economic losses in scattered fields at Immokalee but it developed
to very serious proportion there and elsewhere again in 1962. Thereafter, watermelon
mosaic has developed regularly each year in varying amounts in some areas of the
State. The 1965 season started with heavy outbreaks in Immokalee with indications of
a repeat of the serious losses caused during the 1962 season.
2. Geographical Distribution of Watermelon Mosaic. Watermelon mosaic has been
found in almost all of the major watermelon producing counties one or more times
during the past four seasons. The tri-county area of Lee, Collier and Hendry was
hit the hardest in 1962. The 1965 season appears to be a repeat of the 1962 season.
There is every reason to believe that watermelons in other areas of the State may be
seriously affected again in 1965.
3. Description of Watermelon Mosaic Symptoms. Diseased plants first show
symptoms toward the tip ends of the vines. Leaves infected with mosaic are mottled
with areas showing lighter green color than the normal color of leaves. Often, the
leaves exhibit a "strap-leaf" appearance where size is much smaller than normal.
Tips of the vines protrude above the vine giving a "petunia-like" appearance. Melons
set after the vine is heavily infected are wanted, distorted in shape, mottled in
color, smaller in size and generally poor in quality. Heavily infected watermelons
are not marketable.
4. Strains of Mosaic and Host Ranges. Two mosaic strains have been identified
from watermelon producing areas of Florida. They are identified as WMV-1 and WMV-2.
These two strains have been found singly in different seasons at Immokalee, but both
in the same season in central Florida. According to our information at the present
time, symptoms of these two virus strains are quite similar.
WMV-1 is restricted to the cucurbit family in its host range. These include
cukes, squash, cantaloupes, watermelons and all wild cucurbits. WMV-2 attacks all of
these plants plus members of at least two or three other families of plants. The
latter includes both cultivated and wild legumes. WMV-2, therefore, may become quite
widespread in any area in Florida.
C. METHODS OF TRANSMISSION
It is important that methods of transmission of watermelon mosaic are understood
in order to avoid use of ineffective control practices. The following are known facts
about how WMV is and is not spread in a watermelon planting.
1. Mechanical Transmission. WMV can be transmitted by mechanical means in the
laboratory. Machinery moving thru the field and workers handling vines and melons
during the growing stages may possibly transmit WMV from plant to plant.
2. Aphid Transmission. FMV is transmitted by winged aphids. No other insects
are presently suspected. However, thirteen species of aphids have been identified
which are capable of transmitting WMV.
3. Seed Non-Transmission. WMV is not transmitted thru seed. After many tests,
seed has never been shown to carry WMV.
4. Varietal Susceptibility. All varieties of watermelon are susceptible. Pre-
liminary reports by growers indicate some slight differences in susceptibility. It
is still too early to state whether or not the varieties differ sufficiently in
resistance to WMV to suggest a change in varieties.
D. SUGGESTED METHODS FOR CURBING SPREAD OF WMV
There is no complete control of WMV presently. The best that can be expected is
a delay in incidence or a reduction in severity of WMV injury to watermelon crops.
The following are some possible means of delaying or reducing WMV.
1. Sanitation. Destroy all crop residue after harvest is completed. This is
especially true of vine crops and any volunteers that may emerge subsequently. Since
the aphid is attracted to yellow objects, it is suggested that sunflowers and other
yellow-flowered crops be destroyed before they start producing flowers.
In addition, it may be wise not to cull melons. Any other operation, such
as vine-turning, etc., should be eliminated, if possible. Use of machinery should
be kept to a minimum and then used with care so as not to speed transmission from
diseased to healthy plants.
2. Weed Control. Host plants in the surrounding area and on ditch banks of a
field should be destroyed to prevent aphid build-up in those areas. Host plants
were discussed under "Host range."
3. Aphid Control. It has been shown experimentally that spraying weeds and other
host plants in the border areas around a field is very important in virus control.
By this approach aphids are killed before they reach the crop. Spraying the water-
melon crop itself is strongly recommended even though a winged aphid may infect a
watermelon plant before it can be killed.
4. Trap Crops. Research work with other crops has indicated use of trap crops
in areas bordering watermelon fields. These trap crops should not be host plants as
described under "Mosaic Strains." Rather, trap crops, such as sweetcorn, pepper,
potatoes, tomatoes, eggplants, etc., which are attractive to aphids, should be planted.
Trap crops are supposed to trap the winged aphids where a thorough aphid-
control program would tend to eliminate them. Trap crops offer the possibility of
using approved systemic aphicides in the control program.
5. Time of Planting. Delay in time of planting, especially in south Florida,
is strongly recommended. The shorter period of time a Watermelon crop remains in the
field, the less danger there is from infection by mosaic. Furthermore, this practice
would tend to introduce a break in the cycle of mosaic spread, possibly, by having a
vine-crop free period following late November or early December frosts or freezes which
would kill out fall vine-crops.
6. Summer Host Free Period. A period of one to three months where no vine-
crops are permitted to grow in one area would cause a definite break in the cycle of
the WMV-1 strain. This would appear to be worthwhile in the Immokalee area.
Growers, as a group, should destroy all vine-crop residues after harvest is
completed and all volunteers that emerge thereafter until the fall planting of vine
crops is started.
7. Good Pest Control Program. Growers are urged to carry out an effective pest
control program. Mosaic may not be completely controlled, but the odds are that the
severity of the disease can be lessened by such a program.
E. RESEARCH NOW IN PROGRESS AND PLANNED FOR THE FUTURE
The Florida Agricultural Experiment Stations recognize the seriousness of water-
melon mosaic virus. Research work to solve the problem has been in progress for two
Plans are now being made to widen the scope of research. The following are
problems related to WMV that are being studied by workers of the Experiment Stations
at Immokalee, Bradenton, Leesburg and Gainesville.
1. Breeding. Selecting and breeding for a variety resistant to both WMV strains.
2. Insects. Determining species of aphids and other insects transmitting virus
and studying the life history of each in hopes of finding methods for control.
3. Hosts. Identification of host range of each strain of the virus.
4. Controls. Testing various methods which offer some promise for control of
the disease. These include trap crops, host plant control, chemicals, etc.
5. Fundamentals. Studying the basic aspects of all phases of the problem in
hopes of developing new leads which may eventually result in solving the problem,
This material was assembled by James Montelaro, Associate Vegetable Crops Specialis
from information obtained in the literature from the staff of the Florida Agricultural
Experiment Stations, Florida Agricultural Extension Service, and the United States
Department of Agriculture. The list of contributors is too long to list here.