Cucumis and Cucurbita spp.
Alternaria Leaf Spot – (fungus – Alternaria cucumerina): Small, circular watersoaked spots first appear on the leaves, later enlarging to one and one-half inch or more in diameter. Definite concentric rings and margins appear giving the diseased area a “bull’s eye” appearance. The fungus overwinters in crop residue and on other cucurbit plants. Numerous air-borne spores are produced on diseased spots. Crop rotation and seed treatment help. A preventative fungicide [see section below] program should be followed when the disease occurs consistently. (See Photo of Alternaria Leaf Spot)
Angular Leaf Spot – (bacterium – Pseudomonas syringae pv. lacrymans): This disease appears on leaves, stems and fruit. Spots are small, angular, straw-colored and watersoaked. Leaf spots often dry and fall out giving the leaf a “shot-hole” appearance, similar to those caused by anthracnose. Spots on fruit are usually smaller and circular in shape. Bacteria overwinter in crop residue and on seed. Hard rains splash the bacteria to stems and leaves. The disease may reach epidemic proportions during periods of heavy rains, particularly if temperatures remain high. Crop rotation with non-cucurbit crops is helpful for control of angular leaf spot. Application of copper fungicide will assist in control. Cucurbits, however, are sensitive to copper when young, and repeated applications of copper may cause yellowing of foliage around the edges of the leaves. See fungicide [see section below]
Anthracnose – (fungus – Colletotrichum obiculare): First symptoms are spots on the foliage that begin as yellowish or watersoaked areas. Spots enlarge and turn brown to black in color. Diseased tissue dries and the center of the spots fall out, giving the leaf a “shot-hole” appearance. Symptoms on fruits consist of circular, black, sunken, cankers varying in size depending on the host plant. When abundant moisture is present, the center of the spot is surrounded by a gelatinous pink mass. The fungus overwinters on old cucurbit vines and residues. It may also be seed-borne. Plants may be infected at any stage of growth. Disease appearance in the field depends mainly on rainy, cool weather for a period of several days. All above ground portions may be infected. Seed treatment, crop rotation and destruction of crop residues are important practices for the control of this disease. Preventative fungicide applications, as recommended for downy mildew control, should be made if weather conditions are favorable for disease development. Preventative fungicide [see section below] applications will protect plants against infection.
Charcoal Rot – (fungus – Macrophomina phaseolina): Symptoms resemble those of gummy-stem blight and other vine diseases of melons. First symptoms are dying of leaves close to the crown. As the disease progresses, entire runners wilt and die. Close observation of the crown will reveal brown cankers on the stems both below and above the soil line. Black, hard bodies produced by the fungus, known as sclerotia, are found on the surface of the cankers. Crop rotation may be of limited use since the fungus affects a large number of hosts, including corn and grain sorghum. Dry conditions are normally associated with charcoal rot.
Choanephora Wet-Rot – (fungus – Choanephora cucurbitarum): This disease occurs almost exclusively on crenshaw, squash and pumpkins. The blossom ends of fruit turn black, watersoaked, and covered by a fungal growth. The fungus is usually confined to the end of the fruit, but if conditions favorable for disease development prevail, the entire fruit may decay. Greatest damage by this disease is caused during prolonged damp weather.
Downy Mildew – (fungus – Pseudoperonospora cubensis): Angular yellow spots appear on the upper surface of the leaf during periods of high humidity. The underside of leaves, opposite the yellow spots, becomes covered with grayish growth which is the spore producing structures of the fungus. This growth is more noticeable early in the morning when heavy dew is present. Spores are easily carried by wind from diseased plants. Cool temperatures along with free moisture are ideal for mildew infection and spread. Hot, dry weather may reduce or stop disease development. Resistant varieties should be used when possible. Preventative fungicide [see section below] applications should begin at bloom and continue until the crop is harvested. (See Photo of Downy Mildew)
Fusarium Rind Rot – (fungus – Fusarium roseum): Rind rot is mainly a cantaloupe disease. Rots appear on the melon rind which at first may go unnoticed. However, after peeling the fruit, large brown decomposed areas may be found in the flesh. Since the disease may go unnoticed in the field, melons may be harvested, packed and shipped without the handlers being aware of the problem.
Fusarium Wilt – (fungus – Fusarium oxysporum f. sp. melonis): Fungus infects roots causing damping-off of seedlings or wilting and plant death of older plants. Brown streaks may be visible on runners near the crown. Internally, roots have a honey brown discoloration, especially in the vascular elements. Resistant varieties and 4-5 year rotations are recommended for control.
Gummy Stem Blight – (fungus – Didymella bryoniae): This fungus infects leaves and stems, and is more prominent in the crown at soil level. Leaf symptoms begin with irregular spots that dry and drop out giving the lesion a ragged appearance. The most conspicuous phase of the disease is the brown exudation in the crown of infected plants. Vine cankers are found near the soil line, producing a gummy brown ooze. The fungus overwinters in the soil and on crop residue. The fungus is seed-borne. Once the fungus becomes established, millions of spores are produced which can be readily disseminated to other plants by rain, wind or mechanical equipment. Crop rotation should be practiced. Spraying with fungicide [see section below] has also shown to be effective.
Monosporasus Root Rot – (fungus – Monosporascus cannonballus): Foliage begins to yellow as plants near maturity. Leaves dieback progressively from the crown outward and plants die prematurely. The taproot is usually necrotic with brownish discoloration in the vascular tissues. The fungus reproduces on small secondary roots. Perithecia (small black spherical structures) of the fungus can be seen on secondary host if plants are carefully dug from soil. Rotation and soil fumigation are effective controls. No resistant varieties are known. Monosporasus root rot is primarily a disease on cantaloupe.
Powdery Mildew – (fungus – Erysiphe cichoracearum and Sphaerotheca fuliginea): Disease appears on leaves as a white powdery mass composed of the spore-bearing structures of the fungus. The disease can affect entire fields. Severely infected leaves shrivel and die. The fungus can also grow on petioles and young stems. Infected plants are yellow, stunted, and may die. Fruits are not attacked, but are usually small and deformed. Powdery mildew is favored by cool, dry weather. Spores are air-borne. For most cucurbits there are resistant varieties. Preventative fungicide [see section below] applications will effectively control powdery mildew. New races of the fungus have appeared which are resistant to some fungicides. Several applications may be necessary for optimum control. (See Photo of Powdery Mildew)
Root Knot Nematodes – (nematode – Meloidogyne spp.): Root knot nematodes are microscopic sized worms that infect roots, causing a cancer-like growth. Knots or galls on the roots prevent normal nutrient and water uptake. (See Photo of Root Knot Nematodes) Controls include chemical and cultural practices. Resistant varieties are not available. See Section on Nematodes – Root Knot
Fungicides – for use on Cucumis and Cucurbita species.
|Angular leaf spot||copper hydroxide, mancozeb.|
|Anthracnose||copper hydroxide, mancozeb, chlorothalonil, thiophanate methyl, benomyl.|
|Downy mildew||chlorothalonil, metalaxyl, copper hydroxide, mancozeb.|
|Gummy stem blight||mancozeb, chlorothalonil, thiophanate, methyl, benomyl.|
|Powdery mildew||copper hydroxide, chlorothalonil, triadimefon, thiophanate methyl, benomyl.|
Virus Diseases – Viruses cause mosaic patterns (light green, dark green) in leaves, puckering, leaf distortion (See Photo), stunting, shortened internodes and misshapened fruit. (See Photo of Symptoms on Yellow Squash). Infected yellow squash and pumpkin produce fruit with green blotches of varying degrees. It is impossible to diagnose specific viruses based on symptoms. Often two or more viruses are detected in a single plant. Cucurbit viruses generally overwinter in weed hosts. Avoid late season planting and control weeds around fields. Insect control has not proven effective. Development of resistant varieties using biotechnology techniques is underway.
|Major Cucurbit Viruses in Texas|
|Papaya ringspot||No||aphid||only cucurbits|
|Zucchini Yellow Mosaic||No||aphid||wide|
|Tobacco Ringspot||Yes||nematode (Xiphinema), thrips, beetles, other||wide|
|Squash leaf curl**||No||whitefly||limited|
*Not important on watermelon **Primarily infects squash and watermelon