1. DOWNY MILDEW
This disease was first time reported in Britain (1841), then in USA (1844), whereas in India it was first recorded during 1974-75 from Kashmir valley and appeared destructive in the year 1975-76. Yield losses upto 60-70 per cent have been reported.
Symptoms: Two types i.e. local and systemic infection occur.
- Systemic infection occurs when the plants are raised from diseased bulbs or infected seedlings are used for planting.
- Plants raised from such bulbs remain stunted, become distorted and light green in colour (Plate – 1a).
- In humid weather conditions, sporulation developed on the leaves and covered them with felty whitish to grayish fungal growth.
- Local infection is caused by air borne zoopsores which produce oval to cylindrical spots (Plate–1b).
- In humid weather, the fungus develops as white to purplish downy growth on these spots.
- Affected leaves become pale green, fold over, later collapse and in such cases undersize bulbs are produced.
- On seed stalks, circular to elongate lesions are produced and infected stalks break over with the weight of the seed umbel, thereby causing the seed to shrivel.
Pathogen:
- The disease is caused by Peronospora destructor (Berk.) Casp.
- The mycelium of the fungus is non-septate, intercellular with filamentous haustoria.
- The sporangiophores are aseptate, hyaline swollen at the base and 3 to 4 times dichotomously branched.
- The sterigmata were acute to subacute and bear pyriform to fusiform sporangia.
- Oospores develop late in the season and germinate by germ tube.
Disease cycle and epidemiology:
- The main source of primary infection is mycelium in infected onion bulbs and such bulbs when used for seed production give rise to diseased plant.
- The fungus may also perpetuate as oospore.
- The disease spread by wind borne, and short lived spores.
- The onion seedlings from disease prone areas also play significant role in introduction of disease in new localities.
- The sporulation and infection by P. destructor usually takes place overnight while dissemination of the newly formed conidia occurs during morning hours.
- The temperature i.e 13o C and > 95 per cent relative humidity are optimum for the development of the disease.
- Presence of dew or raindrops on the leaf surface further enhances the chances of infection.
- Closely spaced and densely planted crops develope more disease compared with widely spaced ones while an increase in level of nitrogen fertilization increased disease severity proportionately and potassium decreased it.
- A preliminary model, ONIMIL was developed to forecast primary infection, which is able to determine for each day the probability of P. destructor establishing an infection on onion and its infectivity level.
- MILIONCAST, a model was developed based on the data from the controlled environment studies to predict the rate of sporulation in relation to temperature and relative humidity.
Management:
- Collect and burn the infected plant debris.
- Use healthy seed/ planting material for propagation.
- Onion lines like IC 48045, IC-32149, IC-33617, IC-49371 and DOP-11 have been reported resistant to this disease.
- Spray the crop with metalaxyl + mancozeb (0.25%) immediately with the initiation of the disease followed by sprays of mancozeb (0.25%) or copper oxychloride (0.3%) and repeat at 10-14 days interval.
2. PURPLE BLOTCH
This disease is common in India, both in seed as well as in bulb crop and mostly aggravated by Botrytis infection and thrips attack.
Symptoms:
- The characteristic symptoms of the disease appear as small, water-soaked lesions on the leaves or seed stalks that quickly develop white centres.
- These lesions enlarge, coalese become zonated and brown to purple under favourable conditions and extend both upward and downward (Plate -2).
- In moist weather, the surface of the spot is covered with the brown or almost black sporulation of the fungus.
- Usually the affected leaf or stem falls down and dies within 3 or 4 weeks under favourable environmental conditions.
- Similar lesions may form on seed stalks; as a result, seeds either do not develop or are shriveled.
- In garlic, the disease appears on leaves with the similar symptoms.
Pathogen:
- The fungus responsible for this disease is Alternaria porri (Ellis) Cif.
- Conidiophores arise singly or in groups and are straight or flexuous, often geniculate, septate, pale to mid brown in colour.
- Conidia are muriform and are usually solitary, straight or curved, obclavate and taper to a beak that is commonly about the same length or slightly larger than the body of the conidium.
- Each conidium has 8 to 12 transverse and zero to several longitudinal or oblique septa, the beak is flexuous, pale and tapering and each cell of the conidium is capable of germination by germ tube.
- Existence of chlamydospores in the fungus is also reported.
- Sexual stage of the fungus is unknown.
Disease cycle and epidemiology:
- The fungus survives from one season to other in infected plant debris as dormant mycelium.
- The fungus can also survive in diseased onion leaf and seed stalk debris for 12 months buried at 5 and 7.5 cm depth.
- Wherever the chlamydospores of the fungus are formed they can also serve as source of perination.
- Persistent dew for extended periods or high relative humidity (>85%) are essential for infection and sporulation.
- The fungus can grow over a wide range of temperature (6-34o C) with optimum at 25oC.
Management:
- Collect and destroy the infected plant debris. Follow crop rotation with unrelated crops.
- Cvs./ lines viz. Pusa Red, IIHR-56-1, IC 48059, IC 48179, IC 39887, IC 48025, ALR and Ro-1 have been reported as resistant to this disease.
- Treat the seed with captan (0.3%) or thiram (0.3%).
- With the initiation of the disease, spray the crop with mancozeb (0.25%) or copper oxychloride (0.3%) or hexaconazole (0.05%) and repeat at 10-14 days interval.
3. STEMPHYLIUM BLIGHT
Symptoms:
- The first symptoms of the disease appear on the radical leaves as small, yellow to pale orange flecks or streaks in the middle of leaf, which soon become elongated, spindle shaped to ovate spots, surrounded by characteristic pinkish margin (Plate – 3).
- These spots turn grey at the centre and then become brown to dark olive brown with the development of conidiophores and conidia of the pathogen. Similar symptoms appear on the inflorescence stalks.
- In garlic crop, the symptoms of disease appear on the inner side of the leaf only.
Pathogen:
- The disease is caused by Stemphylium vesicarium (Wallr.) E. Simmons (Tel. : Pleospora allii (Rabenh) Ces. & de Not).
- Conidiophores are straight to variously curved, 1-4 septa, simple or occasionally one branched, cylindrical but enlarging apically to the site of the conidium production, light yellow brown to medium golden brown in the swollen apex.
- Conidia are oblong or broadly oval and sometimes in equilateral one to six transverse septa and one to three longitudinal septa and constricted at the major transverse septa.
- Conidia are light to medium golden brown to olive brown in colour and each has a conspicuous basal scar like zone.
- Perithecia mature within 3 to 6 months.
- Asci are cylindrical to clavate in shape. Ascospores are ellipsoidal and the upper half is narrowly tapered.
Disease cycle and epidemiology:
- The fungus perpetuates in the infected plant debris.
- Moderate temperature and humidity coupled with precipitation favour the disease initiation and development.
Management:
- Cultural practices like collection and destruction of plant debris, crop rotation and summer ploughings are recommended for the control of this disease.
- Sprays of mancozeb (0.25%) alongwith sticker Triton or Sandovit have been found to be very effective for the control of this disease.
- Initiate sprays in the first week of February and repeat at 10-14 days interval.
4. FUSARIUM BASAL ROT
Symptoms:
- The main symptoms of the disease are wilting and rapid dying back of leaves from the tips as the plant approach maturity.
- Infected garlic plant shows reddish or reddish purple discolouration on stems and bulbs early in the season with some discolouration on bulb sheath at harvest, the bulbs become soft and when cut a watering decay is noticed (Plate – 4)
- The damping-off symptoms of seedlings are also noticed.
Pathogen:
- The disease is caused by Fusarium oxysporum Schlechtend ex Fr. f.sp. cepae (Hans.) Snyd. & Hans. and F. solani (Mart.) App. & Wollenw.
- Both species of Fusarium produces microconidia, macroconidia and chlamydospores.
- Macroconidia are uniformly curved and have 3-4 septa.
Disease cycle and epidemiology:
- Both pathogens perpetuate in soil through chlamydospores.
- The pathogen (s) may have been disseminated widely by infected onion sets and garlic cloves.
- The disease generally appears when soil temperature is 25-28o C along with high soil moisture.
Management:
- Follow long crop rotation because both pathogens are persistent soil inhabitants.
- Cultivar like IIHR Yellow, breeding line SI.29 and Hybrid 1 have some level of resistance to this disease.
- Dipping of seedlings before transplanting in the suspension of carbendazim (0.1%) is recommended.
- Pre-harvest sprays of carbendazim (0.1%) also reduce the post harvest decay in storage.
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