Although weed control has always been an important component of tomato production, its importance has increased with the introduction of the sweet potato whitefly and development of the associated irregular ripening problem. Increased incidence of several viral disorders of tomatoes also reinforces the need for good weed control. Common weeds, such as the difficult-to-control nightshade, and volunteer tomatoes (considered a weed in this context) are hosts to many tomato pests, including sweet potato whitefly, bacterial spot, and viruses. Control of these pests is often tied, at least in part, to control of weed hosts. Most growers concentrate on weed control in row middles, and peripheral areas of the farm may be neglected. Weed hosts and pests may flourish in these areas and serve as reservoirs for reinfestation of tomatoes by various pests. Thus, it is important for growers to think in terms of weed management on the entire farm, not just the actual crop area.
Total-farm weed management is more complex than row-middle weed control because several different sites and possible herbicide label restrictions are involved. Often, weed species in row middles differ from those on the rest of the farm, and this might dictate different approaches. Sites other than row middles include roadways, fallow fields, equipment parking areas, well and pump areas, fencerows and associated perimeter areas, and ditches.
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. Some form of chemical weed control may have to be used on these areas. Bare ground on the farm can lead to other serious problems, such as soil erosion and sandblasting of plants. 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 fencerows 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. On canals where drift onto the crop is not a problem and weeds are woodier, Rodeo®, a systemic herbicide, could be used. Other herbicide possibilities exist, as listed in Table 1. Growers are cautioned against using Arsenal® on tomato farms because tomatoes are very sensitive to this herbicide. Particular caution should be exercised if Arsenal® is used on seepage-irrigated farms because it has been observed to move in some situations.
Use of rye as a windbreak is a common practice in the spring; however, in some cases, it can have adverse effects. If undesirable insects, such as thrips, build up on the rye, contact herbicide can be applied to kill the rye, eliminating it as a host, while the remaining stubble continues to serve as a windbreak.
The greatest row-middle weed problem confronting the tomato industry today is nightshade. Nightshade has developed varying levels of resistance to some postemergence herbicides in different areas of the state. Best control with postemergence (directed) contact herbicides is obtained when the nightshade is 4–6 in. tall, rapidly growing, and not stressed. Two herbicide applications in about 50 gal. per acre, using a good surfactant, is usually necessary.
With postdirected contact herbicides, several studies have shown that gallonage above 60 gal. per acre actually dilutes the herbicides and therefore reduces efficacy. Good leaf coverage can be obtained with volumes of 50 gal. or less per acre. A good surfactant can do more to improve the wetting capability of a spray than increasing the water volume. Many adjuvants are available commercially. Some adjuvants contain more active ingredient than others, and herbicide labels may specify a minimum active ingredient rate for the adjuvant in the spray mix. Before selecting an adjuvant, refer to the herbicide label to determine the adjuvant specifications.
Postharvest Vine Desiccation
Also important is good field sanitation with regard to crop residue. Rapid and thorough destruction of tomato vines at the end of the season has always been promoted; however, this practice takes on new importance with the sweet potato whitefly. Because of foliar interception of spray droplets, good canopy penetration of pesticidal sprays is difficult with conventional hydraulic sprayers once the tomato plant develops a vigorous bush. In the spring of 1989, the sweet potato whitefly population on commercial farms was observed to begin a dramatic, rapid increase about the time of first harvest. This increase appears to continue until tomato vines are killed. It is believed this increase is due, in part, to coverage and penetration. Thus, it would be wise for growers to continue spraying for whiteflies until the crop is destroyed and to destroy the crop as soon as possible with the fastest means available. Gramoxone® is labeled for postharvest desiccation of tomato vines. Follow the label directions.
The importance of rapid vine destruction cannot be overstressed. Merely turning off the irrigation and allowing the crop to die will not do; application of a desiccant followed by burning is the prudent course.
Pretransplant chemical weed controls in tomato.
Posttransplant herbicides in tomato.