Weed control in tomatoes is always important, but when you consider some common weeds are hosts of pests and disease, it is even more critical. Weeds such as nightshade and voluntary tomatoes are hosts to threats like sweet potato whitefly, bacterial spot, and other viruses.
According to a report by William M. Stall, UF/IFAS horticulturist, control of these weeds is necessary to help control such pests. He suggests thinking of weed control in terms of the entire farm, not just actual crop areas such as row middles. Weed hosts can flourish in ditches, fence rows, and fallow fields. These weeds can serve as a reservoir for reinfestations of crops.
Disking is probably the least expensive weed control procedure for fallow fields. Where weed growth is mostly grasses, clean cultivation is not as important as in fields infested with nightshade and other disease and insect hosts. In the latter situation, weed growth should be kept to a minimum throughout the year. If cover crops are planted, they should be plants that do not serve as hosts for tomato diseases and insects. Some perimeter areas are easily disked, but berms and field ditches are not, so some form of chemical weed control may have to be used in these areas.
“We are not advocating bare ground on the farm as this can lead to other serious problems, such as soil erosion and sand blasting of plants,” states Stall. “However, where undesirable plants exist, some control should be practiced, if practical, and replacement of undesirable species with less troublesome ones, such as bahiagrass, might be worthwhile.
“Certainly fence rows and areas around buildings and pumps should be kept weed free, if for no other reason than safety. Herbicides can be applied in these situations, provided care is exercised to keep them from drifting onto the tomato crop. Field ditches and canals present special considerations because many herbicides are not labeled for use on aquatic sites. Where herbicidal spray may contact water and be in close proximity to tomato plants, for all practical purposes, growers probably would be wise to use diquat only.”
In his report, Stall notes that nightshade has become one of the most problematic weeds facing Florida tomato growers. It has developed varying levels of resistance to some post-emergent herbicides in different areas of the state.
Best control with post-emergence (directed) contact herbicides is obtained when the nightshade is 4 to 6 inches tall, rapidly growing, and not stressed. Two applications of approximately 50 gallons per acre using a good surfactant are usually necessary.
With post-directed contact herbicides, several studies have shown that more than 60 gallons per acre will actually dilute the herbicides and therefore reduce efficacy. Good leaf coverage can be obtained with volumes of 50 gallons or less per acre. A good surfactant can do more to improve the wetting capability of a spray than can increasing the water volume.
It is all about good field sanitation in tomato production, which starts with fast and complete dessication of tomato vines after harvest. In season, the large canopy of tomato plants makes it more difficult for pesticide penetration, thus lending itself to whitefly survival.
Sweet potato whitefly populations will continue to grow until tomato vines are killed. It is advised that growers continue spraying for whiteflies until the crop is destroyed. Gramoxone Inteon (paraquat, Syngenta Crop Protection) and Firestorm (paraquat dichloride, Chemtura Corp.) are labeled for postharvest desiccation of tomato vines. Follow the label directions.
Stall concludes that turning off irrigation and letting vines die is a poor choice over rapid dessication.