1. Introduction
Diseases and pests represent a major threat to the commercial production of grapes in the world.
Climatic conditions are conducive to the development of several major grape diseases, including black rot, downy and powdery mildew.
Each of these diseases has the potential to destroy the entire crop.
Most diseases occur simultaneously within the same vineyard during the growing season.
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2. Downy Mildew Causal organism Scientific Name: Plasmopara viticola
Symptoms
Leaves
Roughly circular yellowish discolourations called “oil spots”.
White fluffy growth primarily on the lower leaf surface.
As lesions age, they turn brown from the centre outward.
Severely infected leaves may drop.
Small, circular lesions on upper side of leaf
Downey mildew growth on under side of leaf
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3. Downey mildew growth on berries Severely infected berries
Shoots
Infected shoot tips curl and covered with white fluffy sporulation.
Berries
White fluffy sporulation on small berries.
May shrivel and drop off.
Stay hard when healthy berries start to soften at veraison.
Downey mildew growth on berries
Severely infected berries
Courtesy:
www. Ohioline. osu. edu
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4. Biology
The causal fungus, Plasmopara viticola overwinters as thick-walled oospores in infected tissues and in the soil.
A minimum of 10 mm of rain is required for oospores to produce sporangia that are dispersed by rain splash to young green tissue.
The sporangia release motile zoospores that swim to the stomates, germinate and infect tissues.
Disease development is most rapid between 20oC and 25oC with frequent rain or dew.
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5. Powdery mildew Causal organism
Scientific Name : Erysiphe necator (previously Uncinula necator)
Losses in yield of fruits may be upto 40-60%. In addition to loss of yield, infected berries tend to be higher in acid content than healthy fruits and are unsuitable for wine making.
Symptoms: On all tissues, powdery mildew looks like a greyish-white powder.
Leaves
Frequently first found on the undersides of leaves.
Lesions become apparent on the upper sides of leaves as well.
On rapidly growing leaves, infections on the underside may cause the leaves to appear puckered on top.
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6. Severely infected leaves may become brittle and drop off.
Starting as early as late July, very small orange then brown and eventually black spherical structures cleistothecia develop on the upper and lower surfaces of infected leaves.
Shoots
Initially greyish-white, develop into brown irregular blotches.
Indistinct margins and remain visible after shoot hardening.
Berries
Become covered in conidia.
An initial floury appearance that later becomes dark and grey.
Dry out and may drop off.
Later infections (3-4 weeks post bloom) will have superficial greyish scarring.
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7. Powdery mildew on upper side of leaves
Powdery mildew on under side of leaves
Severe powdery mildew on cane
Severe powdery mildew on berries
Courtesy:
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8. Black rot Causal organism Scientific Name: Guignardia bidwellii
Symptoms
Leaves
Relatively small, brown circular lesions develop on infected leaves and within a few days tiny black spherical fruiting bodies (pycnidia) protrude from them.
Berries:
Infected berries first appear light or chocolate brown but quickly turn darker brown, with masses of black pycnidia developing on the surface.
Finally, infected berries shrivel and turn into hard black raisin-like bodies that are called mummies.
Shoots
under high disease pressure brown to black elongated lesions develop.
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9. Black rot lesions on leaves Tiny black pycnidia on leaf lesion
Elongated lesions on petiole
Infected berry with black pycnidia
Black rot berries in cluster
Mummified berries
Courtesy:
PHOTO BY R. PEARSON
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10. Anthracnose/Birds eye disease
Causal organism
Scientific Name: Elsinoe ampelina
It is especially serious on new sprouts during rainy season.
Among various foliar diseases of grapevine in India, anthracnose has longest spell spread over the period from June to October
Symptoms
Leaves
Initially pin-prick dark spots.
Developing into small round spots with brown or black margins with greyish-white centres which drop out causing a ‘shot-hole’ appearance.
Severe infections cause malformed leaves that shrivel up and drop.
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11. Deep elongated cankers
Shoots
Deep elongated cankers
Greyish in the centre with a raised dark reddish brown to violet-black edge
Infected shoots crack and become brittle
Berries
Reddish circular spots becoming violet with a greyish centre and a raised black edge “bird’s eye rot”
Lesions can extend into the pulp, causing cracking and opening the berry to secondary infections
Severely infected berries dry up and drop prematurely.
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12. Sunken lesions on shoots Anthracnose on berry
Courtesy:
Birds eye rot symptoms on berry
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13. Fusarium Wilt Causal organism Scientific Name: Fusarium oxysporum
Symptoms:
Shoots and leaves wilt and die in mid-season, with characteristic brown-red discolouration of the vascular tissue of the affected areas.
Leaves drop prematurely and clusters fail to develop and shrivel to mummies. Symptoms range from one or two shoots on a vine to whole plant collapse.
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14. Management Collect and burn fallen leaves and twigs.
Sanitation of the orchard is highly essential.
Clean cultivation of vines or removal and destruction of all diseased parts.
Vine should be kept high above ground to allow circulation of air by proper Spacing
Grow resistant varieties like Chholth Red, Chholth white, Skibba Red (Powdery mildew), Amber Queen,Dogridge, Red Sultana (Downey mildew) and Banglore blue, Golden muscat, Golden queen (Anthracnose).
Prophylactic spray with B.M. 1% or Lime sulphur at dormant stage delays development of disease by decrease PM initial inoculum.
Spraying Bordeaux mixture 1% or 0.2% or
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15. Lets sum up
Downey mildew pathogen causes irregular, yellowish, translucent spots on the upper surface of the leaves and white fluffy sporulation on small berries.
On all tissues, powdery mildew looks like a greyish-white powder
Due to black rot, infected berries shrivel and turn into hard black raisin-like bodies that are called mummies.
Anthracnose affected berries shows ‘shot-hole’ appearance.
Pruning practices, trellis designs and vineyard lay outs that maximise air movement and light penetration within vine canopies discourage disease development. However, on susceptible varieties, fungicide spray programmes are required in most seasons to prevent crop loss.
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