Vegetables - Disease Management Pat Donald, MU (573) 882-2716 Om Sehgal, MU (573) 882-7060 KEY 1 = Symptoms 2 = Control CUCURBITS (Cucumber, Melons, Pumpkin, Squash) Seed decay, damping off (several fungi) 1. Pre-emergence death ofseeds or seedlings. Post-emergence death of seedlings. 2. Rotate crops at least two years of unrelated crops will help. Angular leaf spot (bacteria) Pseudomonas lachrymans or syringae 1. Water-soaked, angular leaf spots up to 1/8 inch on cucumber. Later dry up and drop out. On squash, yellow halos surround spots. On watermelon, small circular spots are dark-colored with yellow halos. Fruit spots are circular and smaller than leaf spots. 2. Crop rotation. Spray at 5-7 day intervals in early season. Fixed copper (50 percent WP) or Kocide DF, plus an insecticide. Exercise care in using coppers before plants begin to vine. Then change to chorothalonil or other labeled fungicide mixed with copper fungicide. Avoid excessive nitrogen and sprinkler irrigation. Avoid working among plants under wet conditions. Select tolerance or resistant varieties. Anthracnose of cucumbers and melons (fungus) Colletotrichum orbiculare or C. lagenarium 1. Leaf spots begin as yellowish water-soaked spots. Develop into brown spots, 1/4-1/2 inch, on affected leaves. Death of leaves may follow. Elongated, sunken cankers on stems. Sunken, circular cankers with pink centers and brown margins on fruit. 2. Select resistant varieties of cucumbers and watermelons. Crop rotation. Spray at 7-14 day intervals starting when plants are in the 2-leaf stage. Be sure to get good coverage on lower and upper leaf surfaces. Use benomyl (Benlate 50 WP 4-8 oz/A), chlorothalonil (Bravo 500 2 1/4-2 3/4 oz./A, 720 1 1/2-2 pt./A or 90 DG 1 1/4-1 1/2 lb./A) or thiophanate-methyl (Topsin M 85 WDG .2 lb./A), Dithane DF 2- 3 lb./A (some melon varieties sensitive), Dithane F-45 2-3 lb./A (some melon varieties sensitive), Dithane M-45, Penncozeb 1 1/2-3 lb/A, Penncozeb DF 1 1/2-3 lb./A, Manzate 200 DF 2-3 lb./A, Maneb 80 1 1/2 lb./A, or Maneb + Zinc F4 1.2 qt./A, Maneb 75 1.5 lb./A. Cantaloupe, muskmelon and honeydew - Bravo W75 1 1/2-2 lb./A. Bacterial wilt of all cucurbits (bacteria) Erwinia tracheiphila 1. Rapid wilting and dying of plants. White sticky ooze from ends of stems when cut. Cucumbers, muskmelons and pumpkins and squash are most susceptible. 2. Use approved insecticide to control cucumber beetles that transmit the bacteria. Begin spraying as soon as plants begin to emerge and continue through season while beetles are active. Rogue and remove diseased plants. Black rot and Gummy stem blight of pumpkin and squash (fungus) Didymella bryoniae also called Mycosphaerella melonis 1. Gray to dark brown dead spots on leaves. Water-soaked gray spots on fruit later turn black. Stems may be girdled & vines die beyond girdle. Also a storage rot. 2. Use resistant varieties of cucumbers, muskmelons and watermelons. Spray foliar fungicide at 7-14 day intervals throughout season after runners are formed. Use benomyl (Benlate 50 WP 8 oz./A) or chlorothalonil (Bravo 500 2 3/4-4 1/4 pt./A, 720 or 90 DG 1 1/2-2 1/2 lb./A) or fixed copper or thiophanate-methyl (Topsin M), Dithane DF 2-3 lb./A (some melon varieties sensitive), Dithane F-45 1.6-2.4 qt./A. Carefully handle to prevent injury to rind during harvest. Use cultural practices that provide good air drainage and rapid drying. Downy mildew of all cucurbits (fungus) Pseudoperonospora cubensis 1. Moist conditions favor disease development. Yellowish angular spots on upper leaf surfaces. White to purplish downy growth on lower leaf surfaces. Leaves may curl, turn brown, and die. 2. Apply fungicide at 7-day intervals throughout season after runners are formed. Use chlorothalonil (Bravo 500 2 1/4-2 3/4 pt./A, 720 1 1/2-2 pt./A or 90 DG 1 1/4-1 1/2 lb./A) or fixed copper, Dithane DF 2-3 lb./A (some varieties sensitive), Maneb 80 1 1/2 lb./A, Maneb + Zinc F4 1/2 qt./A, Penncozeb, Penncozeb DF 1 1/2-3 lb./A, or Manzate 200 DF 2-3 lb./A, Aliette 2-5 lb./A, Maneb 75 1/5 lb./A, Dithane 1.6-2.4 qt./A, Ridomil Copper 70W 1.5 lb./A, Ridomil MZ 58 1 1/2 lb./A, Ridomil/Bravo 80W 1 1/2 lb./A. Alternaria leaf spot (fungus) Alternaria cucumerina 1. Occurs primarily on muskmelons, but also on cucumbers and other cucurbits under certain conditions. Leaf spots are small, cicular and water soaked at first. They expand up to 1/2 inch in diameter with dark concentric rings within spots. 2. Use Dithane DF 2-3 lb./A (some melon varieties sensitive), Penncozeb or Penncozeb DF 1 1/2-3 lb./A. Gummy stem blight (fungus) (same as black rot) 1. Mainly cucumber, muskmelons and watermelons. Begins as pale brown or gray spots on leaves, petioles and stems. Stem spots appear first at the nodes and elongates into stem streaks. A gummy exudate often appears near the streaks. Plants can be killed. 2. Use same fungicide treatments as for Black rot. benomyl (Benlate 50 WP 8 oz./A) or thiophanate-methyl (Topsin M) with systemic properties will be most effective. Fusarium wilt of melons (fungus) Fusarium oxysporum f. sp. niveum or melonis 1. Mainly on muskmelons and watermelons. Vines turn yellow and wilt around fruiting time. When stem is cut, vascular tissue is discolored. Pink mold may develop on dead stems. 2. Commercial producers may wish to pasteurize (180 degrees F for one hour) or fumigate soil for seed beds. Vapam can be used to fumigate infested soil. In garden, use 3-5 yr crop rotation. Plant resistant varieties. Use disease-free seed. Grower should consult seed catalogs for current information on resistant varieties. Powdery mildew of all cucurbits (fungus) Erisiphe cichoracearum or Sphaerotheca fuliginea 1. Promoted by heavy dews and high humidity. White powdery mildew growth on leaves and stems. Premature defoliation can result. Loss of leaves can result in sunburn of fruit. 2. Apply fungicide spray at 7-14 day intervals when symptoms appear. Use benomyl (Benlate 50 WP 8 oz./A) or thiophanate-methyl (Topsin M 85WDG .2 lb./A), Bayleton 50DF 2-4 oz./A, Microthiol special 2-4 lb./A or Thiolux 4 lb./A. Root know nematode Meloidogyne spp. 1. Roots knotted with small to large galls. Top of plants may wilt on sunny days with adequate moisture. Plants may be stunted. 2. Homewoners may rotate to non-host crop, relocated garden or solarize soil. Commercial growers can use Mocap 10G (20 lb./acre for cucumbers), Mocap EC (1 1/3 qt./acre for cucumbers), Telone C-17 (see label). Scab of cucumbers and muskmelon (fungus) Cladosporium cucumerinum 1. Largely a fruit infection. Sunken spots and mold. Fruit rot. Dry corky spots, up to 1/2 inch in diameter, develop on cucumber and green muskmelon fruits. The fungus over winters on seed and in crop residues from diseased plants. 2. Use resistant cucumber varieties when available. Apply fungicide at 7-10 day intervals. Use chlorothalonil (Bravo 500 2 3/4-4 1/4 pt./A, 720 2-3 pt./A or 90 DG 1 1/2-2 1/2 lb./A) or fixed copper or Kocide DF. Intervals may need to be shortened if rainy, humid weather persists, use Manzate 200 DF 2-3 lb./A. Virus Diseases (Cucumber Mosaic Virus - CMV) (Watermelon Mosaic Virus WM-2) (Zucchini Yellow Mosaic Virus) 1. Stunted plants with yellow & green mottling. Leaves distorted. Fruit mottled, misshapen and warty. 2. Use approved insecticides to control aphid vectors. Wash hands before handling healthy plants. Eradicate weed hosts around patch. Plant resistant varieties. Grower should consult seed catalogs for current information on resistant varieties. Consult the Plant Disease Clinic for more information on serological testing for virus identification provided by Dr. Om Sehgal.