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Nightshade Competition in Watermelon - Preliminary Trials

Watermelons were planted on 25,000 acres in Florida in each of the 2001-02 and 2002-03 crop years (Fla. Agr. Statistics, 2004). This was down from 45,000 acres in the 1998-99 crop season. Total worth of the watermelon production in the last two years averaged $62 million per year.

Watermelons are very sensitive to interference from weeds. Six smooth amaranth weeds in the row between melon plants will reduce yield 100% (Terry, et al.) Control is critical early in the season. Yield is significantly reduced when competition occurs over 1 week after emergence.

Buker, et al. (2003) found that 2 yellow nustsedge plants/m2 would reduce watermelon yield 10% while 40 nutsedges/m2 would reduce watermelon yield 80%.

Large crabgrass densities of 250 plants/m2 were shown to reduce watermelon yield 90% (Monks and Schultheis, 1998). Also, Wallender and Talbert (1983) found that only 2.67 goosegrass plants/m2 reduced watermelon yield.

There are very few herbicides labeled for preemergence control of weeds in watermelons. Bensulide (Prefar) and naptalam (Alanap) are recommended for tank mix applications. The pre-mix of ethalfluralin and clomozone (Strategy) is also labeled. Strategy cannot be used under mulch, nor applied for transplanted melons. None of the above herbicides will control nightshade which is a major weed in south Florida. Southern counties account for 43 percent of the total watermelon acreage in the state.

A section 18 (emergency use) petition was sent to EPA in the fall of 2003 for the use of terbacil (Sinbar) on watermelon for the control of nightshade. The section 18 was denied due to lack of data on the competition of nightshade in watermelon.

There were no studies on nightshade interference with watermelon. Studies in tomato indicated that total fruit yield was reduced 29% and extra large fruit yield reduced 59% when 1 nightshade plant grew in the plant hole.

Nightshade competition in pepper studies carried out in Florida showed that 1 nightshade per m2 caused 20% pepper yield loss. Four nightshade/m2 reduced yield 75% (Roos, 1998).

The rejection of the emergency use petition from Florida for the use of terbacil (Sinbar) for the control of nightshade and other broadleaf weeds indicated that data was needed on the competitive ability of nightshade in watermelon.
These studies, were initiated in 2004 with funding from FFVA.

1. Additive Studies

Non-mulched/seeded watermelon were be grown with increasing numbers of nightshade plants/m2. Competition was season long. Nightshade densities were 0, 2, 4, 6, plants/m2. Yield from the competition treatments was regressed to establish the percent yield reduction from these densities. All other weeds were eliminated from the experiments.

2. Competition duration

Mulched-transplanted watermelons were grown with 1 nightshade plant per plant hole. The nightshade plants would be allowed to grow and compete with the watermelon for a period of time and then removed. The removal times were 0 (weed free season long) at 1 week, 2 weeks, 4 weeks, 6 weeks and season long. Results would establish early competitive nature of nightshade with watermelon and establish how long nightshade can grow with watermelon before yield restriction occurs.

Results

1. Additive

The watermelon were harvested twice. Both number and weight per melon were taken. Total yield was regressed as percent of the check. Figure 1 demonstrates the % yield as a function of the number of nightshade competing with watermelon.

When 2nightshade per meter competed with the seeded watermelon, yield was reduced 31%. Yield was reduced 53% and 73% when 4 and 6 nightshade competed season long. Higher numbers of nightshade/meter were not used in this trial due to the low germination on the nightshade seed available. Higher numbers/meter should be tried.

2. Competition duration

One nightshade was planted per plant hole with the transplanted watermelons at the times indicated. There was no reduction in watermelon yield even in the season long competition. No interference data could be obtained from this trial.

Comments

These trials indicate that transplanted - mulched watermelon may be more competitive to nightshade competition than seeded - non mulched melons. Studies should be carried out to confirm that fact and to establish competitive effects of nightshade on transplanted watermelon.

Literature Cited

Buckelew, J.K., D.W. Monks, A.W. MacRae. 2004. Interferences of eastern black nightshade (Solanum ptycanthum) with tomato (Lycopersicon esculentum) in a plasticulture production system. WSSA Abstracts 44:49.

Buker, R.S., W.M. Stall, S.M. Olson and D.G. Shilling. 2003. Season long interference of yellow nutsedge (Cyperus esculentus) with direct-seeded and transplanted watermelon (Citrullus lonatus) Weed Tech. 17(4):751-754.

Florida Agricultural Statistics. 2004. Vegetables. http://nass.usda.gov/fl.

Roos, D.L. 1999. American black nightshade (Solanum americanum) interference in bell pepper (Capsicum annuum). MS Thesis, University of Florida, Gainesville, Fla. 90 pp.

Terry, E.R., W.M. Stall, D.G. Shilling, T.A. Bewick and S.R. Kostewicz. 1997. Smooth amaranth interference with watermelon and muskmelon production. HortSciences 32(4):630-632.

Wallender, C.J., and R.E. Talbert. 1983. Goosegrass interference with watermelon growth. Proc. Southern Weed Sci. Soc. 36:158 (abstract).

(Bill Stall, professor - Vegetarian 04-11)