Diseases of fruit plantation medicinal and aromatic crops (2+1)

Downy mildew of grapes

Causal Organism: Plasmopara viticola (Berk. & Curt.) de Bary .


• Before 1870 this disease was endemic to USA. It was first reported from Europe in 1878, and within short period of time, the disease spread like wild fire in France posing threat to the vine industry .It was during this period that Prof. P.M.A. Millardett discovered Bordeaux Mixture against downy mildew. He observed that the vines near the road sprayed with lime and copper sulphate mixture to avoid pilferage by trespassers, were quite healthy as compared to the vines in the interior of unsprayed vineyards (Millardet, 1885). Introduction of downy mildew to various countries was probably through infected nursery/propagating stocks.
• Economic Importance
• The disease results in cluster destruction and loss of vine foliage of the photosynthetic area. Under epidemic situations, vines may be defoliated which results in naked canes and exposure of fruits to sunburn. Vine vigour and crop potential may be reduced in the next crop.

Symptoms


• Symptoms appear on all the aerial parts.
• disease is initially observed as small, translucent, pale yellow spots with indefinite borders on the upper surface of leaves.
• Whitish downy growth on the lowering surfaces comprises of tufts of mycelia, sporangiphores and sporangia of the fungus. On the corresponding upper surface, small, round to angular, light green/chlorotic patches is formed.
• On the under surface of leaves and directly under the spots, a downy growth of fungus appears. The tissue in the spot is traversed by reddish lines.
• Later, the infected areas get killed and turn brown .The growth on the lower surface becomes dirty grey.
• Tender vines are also affected. Infected leaves turn yellow, brown and wither. Flowers die and drop. Fruits become greyish, with hardened skin and shrivels forming mummified berries.
• The necrotic lesions are irregular in outline which enlarge and coalesce to form larger necrotic areas on the leaves, frequently resulting in defoliation.
• Diseased shoots remain stunted. Infected leaves, shoots and tendrils are covered with whitish growth of the fungus.
• Flowers and berries are also affected. Flowers may blight or rot.
• During blossom or early fruiting stage, entire cluster or part of them may be attacked and become quickly covered with downy growth and die.
• If infection occurs at half-grown stage of berries, the fungus grows mostly internally.
• The berries become leathery, wrinkled and develop reddish to brown coloration. The fruits shed when attack is very severe.
• The juice quality of fruit is reduced. Infection of green young shoots, tendrils, stems and fruit stalks results in stunting, distortion and thickening of the tissues which turn brown and die.




Etiology


• Mycelium is coenocytic, thin walled, hyaline and intercellular with spherical haustoria.
• Sporangiophores arise from hyphae in the sub-stomatal spaces and sometimes directly through the cuticle.
• On young berries they emerge through the lenticels.
• Sporangiophore branching is almost at right angles to the main axis and at regular intervals. Secondary branches arise from lower branches.
• From the apex of each branch, 2-3 sterigmata arise and bear sporangia singly.
• The sporangia are thin walled, oval or lemon-shaped.
• Asexual spores: Zoospores are pear shaped, biflagellate and borne in sporangia
• Sexual spores: Oospores thick walled, diploid, developed through gametangial contact (union of dissimilar gamets, Oogonium and Antheridium) and are also called as dormant spores.

Mode of spread and survival


• The pathogen survives on infected leaves and vines as oospores and dormant mycelium.
• The secondary spread is through wind- borne sporangia and zoospores which are found on the new flush.

Epidemiology


• The most favourable temperature for germination of sporangia is between 10-230C.
• Disease development is favoured during rainy season when there is heavy dew, relative humidity above 80% and temperature between 23 and 270C.

Life cycle


Management


• Regulatory measures
• Restriction on the movement of planting materials at regional, national or international level should be imposed as the pathogen spreads through dormant cuttings (planting materials).

Cultural practices

• All infected plant materials and pruned parts must be removed and burnt before bud sprouting so as to reduce primary inoculum.
• Even during growing season plant debris must be avoided in and around the field.(clean cultivation)
• While planting attention should be paid to spacing of vines, row direction and placement of wind breaks, which will ensure maximum aeration, drainage and minimum leaf wetness period.
• To encourage air movement within the plant canopy, practices like removal of leaves around berry clusters, trellis designs and pruning systems which allow more air movement be followed during early development stage of vines.
• Careful disbudding and training of vines should be practiced so as to maintain between soil and foliage.

Chemical control

• After pruning, the vines should be sprayed with Bordeaux Mixture (1.0%) or Difolatan (0.2%) or Copper oxychloride (0.3%) or Chlorothalonil (0.2%).
• When the flushes are formed, spraying with Difolatan (0.2%) or Chlorothalonil (0.2%) or Metalaxyl (0.2%) or Copper oxychloride (%) are effective.
• Spray may be repeated at weekly or fortnightly intervals depending on disease severity and weather conditions.
• When non-systemic fungicide is sprayed during humid and rainy period, spraying should be repeated after every two or three days.

Biological control

• Erwinia herbicola, a saprophytic bacterium, used as liquid culture and sprayed on vines has been found to inhibit P. viticola upto 75%.