Everything you need to know about controlling diseases of cucumber.
Cucumber crop is attacked by several fungal, bacterial, and viral diseases, which are given below along with their management practices:
1. Downy Mildew [Pseudoperonospora Cubensis (Berk, and Curt.)]:
The disease is prevalent in warm temperate and tropical regions with abundant moisture. In India, it is present all over the country, causing heavy damage to cucumber. The fungus is an obligate parasite, and attacks only leaves. The symptoms appear as spots or mosaic mottling with light and dark green areas on leaf surface.
The spots quickly turn angular, and bounded veins become yellow and necrotic. On lower side of these spots, purplish downy growth appears during periods of high humidity. The pathogen can infect the plants at temperature between 10° and 27°C with optimum day temperature of 25-30°C and night temperature of 15-20°C.
i. Grow the crop in well-drained soil with wider spacing.
ii. Burn the plant debris immediately after harvest.
iii. Spray mancozeb @ 0.25% at 7 days intervals.
iv. Spray mancozeb (0.01%) and metalaxyl (0.05%) in combination at 10 days interval.
v. Use only resistant varieties/hybrids for cultivation.
2. Powdery Mildew (Sphaerotheca Fuliginia):
Several pathotypes like race-1, 2, and 3 have been observed in most of agro-ecological zones of India. Disease cycle is completed within 3-7 days. The disease appears in cucumber crop grown in warm areas with abundant moisture. The fungus perpetuates on left plant debris or wild hosts and spread quickly through wind. High temperature, high humidity, and heavy dew in night favour the spore formation.
Talcum powder like fungal growth in small spots on lower and then on upper surface of the foliage is the initial symptom. Spots are towards rusty brown. The infected leaves become yellow then necrotic and senescence is accelerated. In case of severe infection, complete premature defoliation and death of vines take place.
i. Remove plant debris and wild hosts from the field.
ii. Follow long crop rotation with cereal crops.
iii. Spray systemic fungicide like tridemorph @ 0.1% at 10 days interval.
iv. Spray 0.2% wettable sulfur at appearance of first symptom and repeat at 20 days (2-3 sprays).
v. Follow 3-4 sprays of Dinocap 100 ml in 150 to 200 litre of water or 0.05% chinomethionat (Morestan) at weekly intervals.
3. Fusarium Wilt (Fusarium Oxysporum and Fusarium Solani):
This is seed and soil borne disease that causes complete crop loss and yield loss up to 80% in epidemic areas. In heavily infected soils, the seedlings of any age starting from 1st leaf stage may be killed at low temperature even before emergence. Shrinkage below the collar region, which may be confused with damping off due to other fungi, is the major symptom of this disease.
The symptoms appear prominently during flowering and fruiting stage. Wilting may be sudden or gradual, taking 4-5 days for complete loss. The fungus enters the roots of germinating seeds and later through natural tears or the wounds created by insects and nematodes, especially Meloidogyne incognita. Disease severity increases with higher light intensity, relative humidity, and high evaporation along with high nitrogen and low potassium and calcium levels.
i. Adopt clean cultivation.
ii. Follow long crop rotation with garlic, radish, onion, beetroot, and Lucerne.
iii. Use only resistant varieties/hybrids for cultivation.
iv. Go for soil solarization as pre-irrigated soil covering with transparent 0.025 mm polyethylene sheet.
v. Treat seed with carbendazim or Captan @ 2.5 g/kg of seed.
vi. Graft the cucumber seedling on Cucurbita ficifolia rootstocks.
vii. Introduce plant growth promoting Rhizobacteria like Pseudomonas putida and Serratia marcescens.
viii. Drench the soil with systemic fungicides such as Benomyl, Bavistin or Thiophanate M 0.1- 0.2%.
4. Anthracnose (Colletotrichum Lagenarium Berk. & Mont.):
Cucumber is worst affected crop in all humid regions of the world. Small water soaked or yellowish spots that appear on aerial parts of the plant become brown and angular or roughly circular on enlarging rapidly. Sometimes dry leaf spots are formed, resulting in shriveling and death of leaf. The fungus also attacks the fruit as a result the small fruits may drop, and appear on mature fruits as numerous small slightly sunken, circular and water soaked spots with dark border.
Red gummy exudates may also appear on lesions. High humidity and high temperature are very conducive to favour the spread of disease. Etiologically race 1, 2, and 3 are morphologically and culturally indistinguishable. Race 1 and 2 were virulent on all cucumber varieties. The pathogen perpetuates in the infected plant debris in soil and superficially on seed. It may survive on weeds also. Germination and growth of fungus occur best between 22° and 27°C.
i. Adopt long crop rotation with cereal crops.
ii. Provide proper drainage.
iii. Destruct cucurbitaceous weeds in crop vicinity.
iv. Treat the seed with Thiram or carbendazim 2 g/kg of seed.
v. Spray carbendazim (0.1%) or chlorothalnil (0.2%) at 7 days interval.
5. Gummy Stem Blight or Black Rot (Didymellia Bryoniae and Plioma Cucurbitacearum):
Gummy stem blight and black rot both damage the cucumber crop in the tropics and subtropics during rainy season. Striped cucumber beetle is the main vector that transmits the disease. The fungus remains in and on the seed, and in soil both serving as primary source of infection.
Seed rotting, seedling death, leaf spots, cankers on stem, petiole and fruit stalk, stem decay, wilt and fruit rot are some common symptoms that appear during infection. Watery scoop exudates, which form a yellowish brown gum after drying, ooze out from the nodal portion of the stem.
i. Use healthy and disease free seed.
ii. Follow long crop rotation with cereal crops.
iii. Follow deep ploughing during hot summer months.
iv. Destroy/decompose the plant refuse quickly.
v. Provide extra quantity of potash during cucumber fruit development.
vi. Spray the crop with mancozeb @ 0.25% or carbendazim @ 0.1%.
vii. Handle the fruits carefully to avoid wounding responsible for risk of postharvest fruit rot.
viii. Use only resistant cucumber varieties for cultivation.
6. Seed Rot and Damping Off (Pythium and Fusarium Species):
Seed rot and damping off are caused by Pythium aphanidermatum (Edson), Pythium irregulare (Buisman,), Pythium spinosum (Sawada), Pythium ultimum (Traw) and Fusarium oxysporum f. sp. cucumerinum (Owen). This pathogen frequently causes seed rot, damping off, seedling blight and root rot in tropical and subtropical zones.
All the pathogens are affected by soil temperature and particularly by moisture as well as rainfall during seedling stage. In glass house cucumbers, severity of infection increases with temperature and frequent watering. The aerial transmission of Pythium aphanidermatum fungus normally occur by the shore flies (Scatella stangnalis), which feed on cucumber.
i. Expose the soil to sun by repeated deep ploughing.
ii. Follow soil solarization practice.
iii. Treat the seed with Thiram or Captan @ 2.0 g/kg of seed.
iv. Spray Mycostop powder formulation prepared from Pseudomonas fluorescence or Streptomyces sp. prior and after planting.
7. Fruit Rot (Pythium and Phytophtora Species):
In cucumber, fruit rot is most common fungal disease caused by Pythium aphanidermatum, Pythium debaryanum, Pythium butlere, Pythium torulosum, Phytophtora capsici, Phytophtora drechslere and Phytophtora nicotianae. Other pathogens like R. solani, F. oxysporum and F. solani present in soil also cause similar symptoms individually or in association. The disease forms a luxuriant weedy mycelial web on the affected fruits contacting soil.
i. Adopt clean cultivation by removing disease debris.
ii. Follow long crop rotation.
iii. Treat the seed with Captan @ 2.5 g/kg of seeds.
iv. Avoid direct contact of mature cucumber fruits by wrapping in cucumber leaves.
8. Angular Leaf Spot (Pseudomonas Syringae Pv. Lachrymans Smith & Bryan):
Angular leaf spot is a serious disease of cucumber and gherkins. Infested soil and contaminated seeds transmit the disease rapidly from one generation to another. Angular leaf spot causes small angular water soaked or tan coloured spots on leaves, stems, and fruits. The organism over winters on plant refuses, soil and seeds, and is favoured by warm wet weather conditions.
The pathogen can survive for two years in soil or debris from diseased plants. The bacteria can spread through irrigation water and rain splashes. The bacteria enter through the wounds and stomata under a condition of dew or rain. A temperature range of 20-24°C is optimum for disease development but it may continue up to 37°C.
i. Adopt field sanitation by removing disease debris.
ii. Follow long crop rotation as a protective measure.
iii. Treat the seed with hot water at 54°C for 30 minute.
iv. Soak the seeds in mercuric chloride solution (1:1000) for 5-10 minute.
v. Spray Streptomycin at 400 ppm or Bordeaux mixture at 1%.
vi. Grow only resistant/tolerant cultivars.
9. Bacterial Leaf Spot (Xanthomonas Cucurbitae Ex. Bryan):
In India, bacterial leaf spot is quite common disease of cucumber and gherkins. The disease produced symptoms as spots (water soaked areas enlarge and turn brown with age surrounded by a yellow halo) only on leaf rarely on stems and petioles. Causal organism survives ip soil, on plant debris and in seed between testa and integument. Cotyledons are immediately infected as the seed germinates. Optimum temperature for pathogen survival is around 25-30°C.
i. Soak seeds for 60 minutes in 1 : 20 dilution of commercial HCl containing 1% surfactant.
ii. Treat the seed with hot water (54-56°C) for 30 minutes or with 1% sodium hypochloride + 1% surfactant for 40 minutes.
iii. Grow resistant cultivar like Japanese Long Green.
10. Scab (Clcidosporium Cucumerinun):
The disease appears as dark brown sunken spots on stems and fruits. In moist weather, these spots are covered with a greenish mould. Sometimes, a gum like substance oozes from the affected leaves and fruits. The pathogen over-winters in old plant refuses and on seeds.
i. Adopt clean cultivation and long crop rotation.
ii. Use resistant/tolerant varieties for cultivation.
iii. Spray Streptomycin at 400 ppm in standing crop.
11. Cucumber Mosaic Virus:
Cucumber mosaic also referred to as white pickle mosaic is a viral disease, which causes 10-20% yield loss in cucumber. This mosaic cucumo-virus described as “narrow leaf of tomato in New Zealand”, cucumber green mottle mosaic Tobamovirus (CGMMV) in England and cucumber necrosis, cucumber mosaic, cucumber vein yellowing and cucumber (wild) mosaic and squirting cucumber mosaic in other parts of the world.
If the plants are infected with cucumber mosaic virus at cotyledonary stage, the yield is reduced by 89 and 96% in summer and in rainy season, respectively. It is characterized by stunting of plants, mottling, yellowing, and wrinkling of leaves and warting and mottling of fruits.
In cucumber, its transmission through seed was reported 37%. The symptoms produced by cucumber mosaic viruses are dry, brittle, and necrotic lesions along the leaf veins, malformation and internodes shortening, vein clearing, wilting, and drying, particularly in cool weather.
The hosts of cucumber green mottle mosaic virus are Cucumis sativus L., Cucumis melo, Citrullus lanatus, Datura stramonium. Petunia hybrida, Chenopodium hybridum, Phaseolus vulgaris, and Vigna sinensis. The organism also attacks several wild plants, including pokeweed, milkweed, catnip, and ground cherry. It over-winters on roots and seeds, and is spread by aphids and striped cucumber beetles. It can also be spread by personnel working among infected plants and then carrying it to new fields.
i. Use disease free seed.
ii. Collect and destroy the diseased debris.
iii. Eradicate the wild host plants.
iv. Spray Malathion @ 2 ml/l of water at 5-7 days interval.