significant differences in yield were found. This spring, we planted late
(March 17) and virus appeared early, one month after planting. In addition, a large
source of virus had built up two weeks prior to this, in the squash planting east of
our watermelon plots. Again, oil-treated plots reached 50% infection one week later
than plots not receiving oil, but this time, because of the earliness of infection,
all plots will be 100% infected by harvest. As of this writing we have not yet
begun to harvest and compare fruit damage.

 Although a one week delay is not impressive, that we could achieve this in a
small plot situation is significant. Oil-treated plots were constantly exposed to
aphids that picked up virus in adjacent untreated plots. In a large field
situation, when the entire field is being treated, much longer delays should be
possible, especially if oil is applied consistently right from the beginning of the
spring growing season. The two fields we compared in Weirsdale showed vast
differences in the amount of final disease. During the fall, however, there may be
too many sources of virus for oil to be highly effective. The fall situation may be
comparable to the situation in our small plots where virus built up in untreated
plots next to treated plots.

 Endosulfan, although it did control melon aphids in the fall of 1988 (the only
 time we had problems with aphids attacking the crop), had little effect on virus
 spread, and in fact may have slightly increased the rate of spread, perhaps by
 irritating the aphids and causing them to move more rapidly from plant to plant. We
 saw little difference between the check and the insecticide-treated plots this year
 when we reduced the number of endosulfan applications from six to two. Thus, the
 use of insecticides in the spring crop does not seem justified unless insects
 causing direct damage to the plants are present, such as rindworms.

 The rapid growth and large surface area of watermelon plants make it difficult
 to protect them with anything that must be applied to leaves. Even young watermelon
 plants (prior to fruit set) have up to 45 square feet of leaf surface area (one
 surface only). Stylet oil, if used properly however, may delay the onset of virus
 epidemics, particularly in the spring when sources of virus are not as abundant as
 in the fall. The extra cost may be justified only when virus has consistently been
 a problem. We are trying to identify, through our surveys, the characteristics of
 virus-prone areas and are also attempting to correlate the first incidence of
 disease and peak aphid flights with winter and early spring weather conditions.
 This would allow a grower to better assess his or her risks of suffering a serious
 disease problem and thus provide a better basis for making a decision to use a
 costly control measure.



 Note: One recent problem with using stylet oil has been the loss of the EDBC
 fungicides for cucurbits. The remaining broad spectrum fungicide is not compatible
 (although Benlate and Ridomil are) with oil and should not be applied within two
 weeks of an oil treatment. In this part of the state, fungicides are not usually
 needed early in the season when stylet oil would be most useful so there may still
 be a way to obtain the benefits of both if the weather is cooperative.