"Ag Today" Newspaper Column

 

Michael R. Milam

Agronomy Specialist and County Program Director

For Daily Dunklin Democrat-Ag Today

June 25, 2007

Disease and spider mites in watermelons and cantaloupes

                        We have been fortunate in some aspects this year in that the dry conditions have not favored the buildup of foliar disease in watermelons and cantaloupes. Since we have not filled the horticulture specialist vacancy in Dunklin County, I have had the opportunity to visit with melon growers to see if we could make an assessment as to why plants were wilting and dying.

                        Early in the season just as the cantaloupes were just starting to form, I like at a field that had plants wilting. When I look at cantaloupes, the first disease that I want to rule out is bacterial wilt. This disease caused by the organism, Erwinia tracheiphila, does not affect watermelon. Usually, when the bacterial wilt is found, there is a random pattern. If field borders are affected and not other areas of the field, the culprit is likely to be herbicide drift. So a random pattern is a partial indicator of this disease.

                        Usually the plant will wilt suddenly. The first symptoms are the flagging or wilting of leaves on one or more vines. As the disease progresses, the plants will wilt and die rapidly. So how can this disease be diagnosed? A rather simple test is to cut the wilted vine near the main stem, rejoining the cut surfaces, and then slowly drawing the sections apart. Bacterial wilt can be identified positively, it a thin strand of slime extends between the two sections. This is also referred to as “stringing out” the sap. As these bacteria multiply, this slime will cause the vascular tissue to be clogged up. When this occurs, the plant can’t take up nutrients or water and they wilt and die.

                        This disease is spread by cucumber beetles so it is important to keep this pest under control. Producers should scout their cantaloupe fields for the presence of these beetles. Fields should be monitored several times a week and insecticides applied. Admire 2F and Admire PRO can be applied preplant, as a post plant drench, or through trickle irrigation water. Admire has a 21 day pre-harvest interval. Furadan can also be applied, as well as many products during the season. Read the label directions and make sure to follow all directions including pre-harvest interval and rates. It is best to apply the insecticides and miscodes during the evening hours to keep from killing the bees.

                        Another problem that I have seen several times this year is that of root-knot nematodes in cantaloupe. If there are any nematodes in a cantaloupe field, it won’t be long until symptoms are present. Nematodes are microscopic worms that inhabit sandy soils and can parasitize the roots of most vegetable crops. As you might expect, nematodes are usually more severe during hot dry summers. Again, the nematodes multiply and destroy the vascular system of the plant preventing the uptake of nutrients and water causing the plants to wilt. The roots will develop galls or knots on the roots which make it easy to diagnose. In addition, the nematodes injure the roots allowing disease organisms to enter the plant. Fusarium wilt is associated with the root-knot nematode.

                        Once the plants show symptoms, there is nothing that can be applied to control the nematodes or disease; however, irrigation will often keep the plants growing longer to produce a few more cantaloupes before the plants die.

                        It is easier to control the nematodes with methyl bromide or sodium methyl dithiocarbamate if the levels are high. Vydate can be used to control low levels of nematodes if the product is put under the plastic.

                        I have already seen evidence of spider mites in watermelons and I have heard many reports of spraying for mites in both melons and cotton. Spider mites are also a hot dry weather problem and will often disappear after a rain.

                        I was visiting with a colleague in Mississippi County last week and we saw evidence of spider mite feeding. The leaves were mottled and were turning yellow. At that time, we could find no spider mites because the rainfall had knocked them off of the plants. The mites feed on the underside of the leaves. They suck on the sap which removes plant moisture and causes the mottled leaf appearance. Mites can be seen by shaking them on white paper or they can be viewed with a 10X hand lens.

                        I have been able to identify spider mite infestations driving down the highway at sixty miles an hour. When I see the yellow and brown patterns in the field, I know that the infestation has already gotten out of hand and some yield loss has occurred. However, spraying can prevent them from spreading to other areas of the field. They typically will be more of a problem along dusty roads or field borders before spreading.

                        Kelthane has been the primary miticide used in melons, but Acramite, Agri-Mec, Capture, Danitol, Diazinon, and Malathion are listed in the Midwest Vegetable production Guide for Commercial Growers-2007.

                        So while we have had somewhat of a reprieve this year from foliar disease this year, spider mites and vascular disease have been more of a problem on melons and cantaloupes.

                        Producers should continue monitor their fields to make sure that insects, mites, or disease do not build up and limit the yield of the crop.

 

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