PLANT PROTECTION

Garlic is attacked by many diseases and insects pests. Therefore garlic growers need to know the details of symptoms of diseases and insect pests attacking garlic and their control measures for getting healthy crop. Diseases and insects pests, and their control measures attacking garlic crop are described as under :

Diseased crop

DISEASES

Purple blotch

Purple blotch (Alternaria porri) appears on leaves as small whitish sunken lesions with purple centres that rapidly enlarge. The leaves fall over gradually. Spraying of Mancozeb @ 2.5 g/litre of water at 15 days intervals gives good control.

Purple blotch

Stemphylium blight

Stemphylium blight (Stemphylium vesicarium) appears as small, yellow to orange flecks or streaks on leaf. These soon develop into elongated, spindle-shaped to ovate elongate, diffused spots, often reaching the leaf tips. They usually turn gray at the centre, brown to dark olive brown with the development of conidiophores and conidia of the pathogen. The spots frequently coalesce into extended patches blightening the leaves and gradually the entire foliage.

Stemphylium blight

Cercospora leaf blight

Caused by Cercospora duddiae,this disease appears on leaves as small, ash-coloured and irregular shaped spots scattered on leaf lamina. The spots coalesce gradually and results in blightening of foliage. Spraying of ziram or captan @ 2.0 g/litre of water or copper oxychloride @ 3.0 g /litre of water at fortnightly intervals gives good control.

Cercospora leaf blight

Powdery mildew

Powdery mildew is caused by Leveillula taurica. Distinct pale-yellow patches of variable size on abaxial surface of leaf associated with powdery mass are its main characteristics. Sulphur fungicides @ 2.0 g/ litre of water if sprayed at regular intervals of 15 days after disease appearance gives good control.

Powdery mildew



Mosaic disease

Garlic plants infected with mosaic virus show typical symptoms of chlorotic mottling and strips on first emerging leaf followed by pale-yellow broken stripes, resulting in typical mosaic pattern on matured leaves. Yellowish dots on leaves, whitish leaf margin or twisting of leaves are also recorded on a few cultivars. Generally symptoms are mild on younger leaves than on matured leaves. Bulbs harvested from mosaic affected plants after maturity remain smaller in size and cloves are fewer in number. Since virus is transmitted through aphids, spraying of monocrotophos @ 0.5 ml/litre of water or methyl demeton @ 0.75 ml/litre of water is useful.

Mosaic disease

INSECT PESTS

Thrips

Thrips (Thrips tabaci) are perhaps the most important insect pests attacking garlic. They are most common during warm weather. They feed on leaf surfaces, causing them to whiten or silver. They are slender about 1/25" long usually hide in angles of leaves. Spraying of Malathion @ 1 ml/ litre or methyldemeton @ 0.75 ml/ litre of water gives good control if sprayed at frequent intervals. Four sprays of fluvalinate (0.04%) at fortnightly interval at Karnal and malathion (0.1%) at Nasik gave best performance in controlling thrips. Phorate @ 1 kg ai/ha at planting and 30 days after planting controls thrips effectively.

Stem and bulb nematode

Caused by Ditylenchus dipsaci, it is a tiny worm almost invisible to the naked eye which invades the tissues of garlic, and can seriously reduce the yield. The infestation causes the basal portion of mature plants to swell and become spongy and frequently broken by longitudinal splits. The tissue rots, the leaves and stems become rotten and twisted and the plant is stunted. In severely infested plants, basal plants may be so rotted that it pulls away when the plant is lifted, leaving the roots in the ground. The nematodes are, however, not common in garlic in India.

Pre-planting soil fumigation with dichloro-propene-dichloropropane mixture @ 500 kg/ha is recommended. Ethylene dibromide and dibromochloropropane (Nemagon) should not be used as these leave residues in soil.

Mites

The eriophyid mite, Aceria tulipae, a common pest of garlic is so small that its infestations are frequently over-looked. Young garlic plants injured by mites usually have destroyed and twisted leaves with conspicuous yellow or light green streaking. The leaves may not emerge readily from the cloves and leaf blades sepaerate poorly after emergence. Bulbs stored for long period may be severely attacked by mites. The cloves wither and bulbs lose their firmness.

Rhizoglyphus callae and Glycohaqus domesticus

These may attack on bulbs in storage. Fumigation of dry bulbs with methyl bromide after harvesting and before storage control the mites. Application of methyl bromide @ 1kg/1000 cubic feet for 2 hours at 26C is recommended.


PHYSIOLOGICAL DISORDERS

Sprouting of bulbs in the field is noticed sometimes towards the start of maturity stage of bulbs particularly when there are winter rains or excessive soil moisture and nitrogen supply. This disorder is, however, not of permanent nature and varies from variety-to-variety. Early-planting also causes sprouting. Splitting is also noticed sometimes in some varieties, which is due to delayed harvesting or irrigation after long spell of drought. For past few years rubberification problem is noticed in Rajkot area and also Nilgiri hills of Tamil Nadu. It is increasing day-by-day. The actual cause for the disorder was since not identified, a trial was planned in collaboration with TNAU at HRS, Ooty, during 1995-96. The effects of insecticides, fungicides, micronutrients and growth regulator were studied. It was observed that rubberification was totally controlled by application of micronutrients i.e. zinc sulphate and ammonium molybdate. It was also controlled by neem cake insecticides and growth regulator like GA.

Further studies are required to eliminate this problem in different areas as the incidence is noticed variably in different areas.

The aerial bulbil formation is also quite common in garlic where lower temperature prevails for more period or there are more temperature variation. It, however, does not affect much on yield or quality of bulbs and varietal variation is noticed where G-282 is seen more prone for this type of effect.

The rubberification and premature sprouting of bulbs are main physiological disorders in garlic. The reasons for these disorders are summarized as under for hill grown garlic in Tamil Nadu.

  • Rubberification and premature sprouting of bulbs are noticed mostly in fields which are located in low-lying areas of watershed where there is heavy deposition of nutrients alongwith silt during heavy rains.

  • These problems are severe in garlic fields which are more frequently irrigated than the normal requirements of garlic bulbs.

  • With the application of higher levels of nitrogen, there is an increased level of pre-mature sprouting of bulbs which results in splitting and rubberification of bulbs.

  • Rubberification incidence also increases when higher level of nitrogen is applied in the form of urea.

  • Due to increased levels of nitrogen, in addition to the increased production of rubberized, there is an increased level of thrips incidence in leaves during the later stage of crop. The thrips lacerate leaves and cause severe damage to the crop. Through lacerated wounds by thrips, there is a severe incidence of blast disease, caused by Boltrytis allii, when crops is 70-90 days old, which further deteriorates the quality of garlic bulbs.
  • Short duration type of garlic (Mettupalayam type) is more susceptible than long duration type (Singapore type).
  • Premature sprouting of bulbs is more in the crop (April-August) when there is high moisture level in soil at maturity of crop due to heavy rains.
  • Delayed harvest during rainy season has increased premature sprouting and splitting of bulbs.
  • Wider spacing of cloves at the time of planting increases uptake of nitrogen and water by the individual plants which increases premature sprouting and rubbenfication of garlic bulbs.