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Alternaria leaf spot, caused by the fungus Alternaria cucumerina occasionally causes damage to muskmelon, cucumber and watermelon in Kansas, but it is not as common as another foliar disease called anthracnose. The disease tends to be more severe on weakened plants. Symptoms are first apparent on older leaves nearest the center of the hill. Small yellow spots on the leaves enlarge rapidly, turn brown, and often coalesce to kill large portions of the leaf surface. Dark, concentric rings form within the lesions, resulting in a target-board effect to the leaf spot. Defoliation causes a reduction in plant vigor and exposes the fruit to sunscald injury. Alternaria leaf spot symptoms can be confused with those of anthracnose. In general, lesions caused by Alternaria have the typical bull's eye pattern, while those of anthracnose do not.
CONTROLThe Alternaria fungus may survive in crop debris. Therefore, follow a 3 year rotation with a non-curcurbit (melon, squash, pumpkin, etc.) crop. If this is not possible, or if the planting site is restricted (i.e. a home garden) deep plow or till the debris into soil to hasten decomposition. Alternatively for home gardeners, remove and destroy vines after the growing season. Start new plantings with clean seed. Because the fungus tends to attack weakened plants, maintain optimal growing conditions, including balanced fertility, uniform moisture and good soil texture. Fungicides listed for the control of anthracnose (with the exception of benomyl) also will control Alternaria leaf spot. Make applications when vines begin to touch in the rows and continue at 7- to 21-day intervals as needed.
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Fungicide |
Typical Application Interval |
Examples of Trade Names |
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azoxystrobin |
7 to 14 days, |
Quadris |
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chlorothalonil |
7 to 14 days, |
Daconil, Terranil, Echo, Bravo, others Homeowner: Daconil, Fungi-Gard, Liquid Fungicide, Encore, Monterey Bravo |
| copper products | 7 to 14 days, |
Basic Copper Sulfate, copper hydroxide (Kocide 2000 and others), Tenn-Cop 5E, copper resinate, Others Copper products may cause plant injury and reduce yields of watermelon and muskmelon Homeowner: Copper Fungicide, Bordeaux |
| mancozeb | 7 to 14 days |
Dithane, Penncozeb, Manex Homeowner: Mancozeb |
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Web updated 9/01/06 |