Everything you need to know about controlling diseases of summer squash.
1. Anthracnose (Colletotrichum Lagenarium (Pass.) Ellis & Halsted):
The disease infects all the above ground parts of the crop as a result yellowish water soaked areas, which later coalesce, enlarge in size and turn brown to black, are appeared on leaves. The necrotic portion dries and shatters.
Elongated water soaked sunken lesions also appear on stem, and light yellow to brown discoloration of the stem lesions take place due to abundant sporulation. Sunken, dark brown to black lesions, which vary in size, depending upon plant age and weather condition, also appear on fruit.
i. Field sanitation by burning of crop debris reduces the primary inoculums.
ii. Always collect seed from healthy fruits and disease free area.
iii. Seeds must be treated with carbendazim or Captan (2.5 g/kg seed).
iv. Foliar spray of fungicide such as Benomyl, Bavistin, and Thiophanate-M (0.1 %) reduces the disease incidence.
v. Foliar spray of carbendazim (0.1%) or Chlorothalonil (0.2%) gives good control of the disease but spray must be started before the infection occurs.
2. Downy Mildew (Pseudoperonospora Cubensis (Berk. & Curt.) Rosto):
It is a serious and most destructive disease of summer squash. Symptoms appear as numerous irregular small yellow areas surrounded by green tissues scattered all over the leaves. In due course of time, lesions grow in size and coalesce with each other. Old lesions become necrotic, and these are clearly demarcated with slight yellow areas. Severely infected leaves roll upward with brownish tinge that produces a blighted appearance. Grayish black downy fungal growth is observed on under surface of leaf in humid weather.
i. Crop should be grown at wider spacing in well-drained soil.
ii. Air circulation and sunlight exposure help in checking the disease initiation and development.
iii. Field sanitation by burning crop debris helps in reducing the inoculums.
iv. Protective spray of mancozeb 0.25% at seven days interval gives good control.
v. In severe cases, foliar spray of Metalaxyl + mancozeb @ 0.2% may be given only once.
The infection appears first on upper side of leaves and stem as scattered white to dull white powdery growth, which quickly covers most of the leaf surface and leads to heavy reduction in photosynthesis area. In due course of time, all above ground parts are infected, and finally, the lesions turn brown and necrotic. The affected leaves become yellowish, dry, and defoliated. The fruits do not develop properly, and sometimes, they are covered with white powdery mass.
i. The fungicidal spray of Penconazole (0.5 g/litre) can give very good control of the disease.
ii. Spray with Karathane (1.0-1.5 ml/litre), carbendazim (0.5-1.0 g/litre), Calixin (1.0 g/litre), or Sulfex (2.5g/l) is recommended by Suhug and Mehta (1982).
iii. Spray Nimrod or Benlate (0.05%) on foliage for very effective control.
4. Angular Leaf Spot (Pseudomonas Lachrymans Carsner):
This is a minor disease of summer squash. Dry grey spots appear on leaves only at initial stage and small water soaked spots on fruits during later stages. On leaves, they enlarge up to about 3 mm in diameter, becoming tan on upper surface and gummy or shiny on lower surface and assuming an angular shape as the lesion is delimited by veins.
The necrotic centers of leaf spots may drop out. On stems, petioles and fruits, the water soaked spots are covered with white crusty bacterial exudates. As the fruits begin to mature, brown lesions in fleshy tissue beneath the rind develop, and discoloration continues along the vascular system, which extends to the seeds.
i. Crop rotation and field sanitation can minimize the level of inoculums.
ii. Seed treatment with hot water at 54°C for 30 minutes or sodium hyperchlorite (1%) + spreader-sticker (1%) for 40 minutes reduces the level of seed transmission.
iii. Foliar spray of Metalaxyl and mancozeb (3 g/litre) is also beneficial in controlling disease.
5. Alternaria Blight and Fruit Rot (Alternaria Cucumerina Elliot):
Initial infection appears on leaves as small spots, which rapidly increase in number and size. Later on, severely infected leaves become blighted.
i. Use disease free seeds.
ii. Follow seed treatment with Captan (2.5 g/kg seed).
iii. Destroy crop debris after cropping period.
iv. Follow crop rotation and proper drainage of field.
v. Spray the crop with Chlorothalonil (0.2%) or Mancozeb (0.25%).
vi. Borax wash (2.5%) at 45°C for 30 second before packing of fruits prevents fruit rot.
6. Fusarium Root Rot of Squash (Fusarium Solani F. Sp. Cucurbitae Snyder & Hansen):
Vascular browning, gummosis, and tyloses in xylem vessels are observed in mature plants, and subsequently, the plants get wilted. The pathogen is both seed and soil borne.
i. Follow long crop rotation including cereal crops.
ii. Follow clean cultivation.
iii. Use only disease free healthy seed.
iv. Grow resistance varieties in disease prone areas.
v. Avoid root injury during intercultural operations.
vi. Application of Trichoderma @ 5 kg/ha during soil preparation is advantageous.
Mosaic is a very common disease of summer squash, which is caused by PRSV-W, WMV-2, and CMV in single or mixed infection. All these three viruses are transmitted by aphid in a non- persistent manner. Bishnoi et al. (1985) reported that the virus infection transmitted by Aphis gossypii, A. malvoides and Myzus persicae in squashes occurs at early stage.
The leaves of disease plants develop vein clearing, vein yellowing, blistering in patches, puckering, and deformation. The internodes of plants are shortened, resulting in crowding of young shoots. Severely infected plants do not bear fruits.
The effective management of viral diseases demands integration of management practices such as avoidance of infection sources, control of virus vectors, modification of cultural practices, and resistance of host plant. Rouging and destroying of virus-infected plants are the effective measure to restrict the spread of disease.
i. Avoidance of Infection Source:
Infected seeds/planting materials, host, adjoining crops, and volunteer points are the main sources of infection, thus, precaution should be taken in selection of seeds/planting materials and also to avoid the sources that cause infection.
ii. Use of Virus Free Seed:
The certified seeds should be used to avoid primary inoculums, and the true seeds if contaminated with any virus may be treated with dry heat at 70-76°C for 1 to 3 days or 10% solution of tri- sodium phosphate for 30 minutes.
iii. Avoidance of Collateral Hosts:
Allied crops and weeds are the important hosts for perpetuation of viruses, thus, removal of weeds in and around main fields reduces the incidence of viral diseases.
iv. Avoidance and Control of Vectors:
In northern India, early sowing of cucurbits in summer reduces the viral disease due to low population of vectors in this season.
v. Use of Reflective Mulches:
Reflective mulches greatly reduced the number of aphids, and thereby prevented virus infestation in summer squash. Toscano et al. (1979) found that the aluminium foil and white plastic gave 96 and 68% reduction in aphid population and reduced virus infection on crop by 85 and 63%, respectively over control.
The results obtained by George and Kring (1971) also showed that aluminium foil mulch effectively delayed virus infection and no plants showed mosaic symptoms 19 days after transplanting. The yield of plants mulched with aluminium foil was more than twice that of un-mulched plants.
vi. Chemical Control of Vectors:
This is most commonly used approach for the management of viral diseases and effective for vectors born viruses, however, the very important thing for chemical control is its timely application. Spraying Dimethoate or Metasystox @ 1.5 ml/litre at 10 days interval controls the vectors effectively. Population of vectors can also be regulated by weekly sprays of mineral oil at 2.0%.