Grape powdery mildew is a common fungal disease that threatens yield and fruit quality, requiring effective control strategies. In this article on your website, WORLD OF PLANTS, we review the symptoms of this disease and ways to prevent it to reduce its impact on grapes.
Symptoms of powdery mildew on grapes
- Scientific name : Powdery mildew of grapes
- Type of disease: innate
- The causative fungus :Erysiphe necator
The fungus infects all green grape tissue, and symptoms of the disease begin to appear in the form of small, dusty spots, white or gray in color, and mealy-looking on the upper or lower surface, or on both surfaces of the leaf, but they are more evident on the upper surface.
When appropriate conditions are available, these spots expand and connect with each other to cover most or all of the surface of the leaf. As the infection progresses, the color of the affected tissue turns brown and the leaves end up withering and falling.
Severely infected young shoots may stop growing and dry up if infected early.
Old infections appear on dormant branches in the form of reddish-brown areas.
Infection of flower clusters leads to failure to set
The growth of fruits stops if they are infected at the beginning of their formation. If it is infected at an advanced stage of maturity, it grows irregularly, its surface becomes rough, and it often cracks.
Early infection of fruits leads to small size of the seeds and a decrease in their sugar content.
Causes of powdery mildew on grapes
Powdery mildew grows when there is moisture and a moderate temperature, and the best humidity for its growth is more than 75%, with a temperature between 20 - 28 degrees Celsius. Therefore, its growth begins in the spring and increases in the summer.
Its growth decreases when exposed to direct sunlight, or in the presence of strong lighting, so we see the fungus growing on the shady parts of the tree.
The susceptibility of grape varieties to powdery mildew varies, and in general, European varieties are more susceptible to the disease
Mushroom germination does not require the presence of water or high air humidity, and it can germinate in relatively dry weather
Early-maturing grape varieties usually escape infection. Because of their rapid maturity before weather conditions become suitable for infection to occur, and vice versa in the case of late-maturing varieties, where the damage from infection increases.
Suitable conditions for the spread of powdery mildew on grapes
The most suitable temperature for mushroom development and growth is 20 to 30 degrees Celsius, and it completely stops activity and growth when temperatures rise above 38 degrees.
Dim light.
Powdery mildew development cycle on grapes
The fungus spends the winter in the form of sticky fruits, or dormant gametophytes inside the bud scales, which are responsible for the initial infection.
Ascospores are released in the spring, carried by the wind, and germinate on the surfaces of green tissue to cause the initial infection.
Conidia spores are produced after 6-8 days and are responsible for the spread of the disease during the season and the occurrence of secondary infections.
In the fall, oocysts form again on infected leaves and branches.
Losses of powdery mildew on grapes
Vigorous attacks cause a loss of 50 to 70% of production (loss may be 100% in early infections on sensitive varieties)
When the fruits are infected, they become deformed and cracked before they ripen, and their marketing value decreases
Strategy for controlling powdery mildew on grapes
Preventive measures to prevent the occurrence of the disease
Infestation is monitored by observing the onset of infestation, which usually occurs on shady or densely branched parts of the grape tree.
Immediately after pruning the grapes, you must remove the stem bark and clean the branches well to get rid of the hidden fungus.
Keeping the surroundings near crops clean by eliminating weeds, plant dirt, damaged parts, and unwanted plants.
Exposing the grape tree to ventilation, sunlight, and light: This is done by opening the heart of the tree by redistributing and directing the green branches and cutting the side branches, and by thinning the leaves around the grape cluster after the contracting stage (thinning the leaves of the tree). And through good pruning in winter.
Chemical and organic control recommendations
Organic control: Fogging with fine sand. This process is highly efficient in combating the disease because the dirt contains silicon dioxide, which absorbs moisture from the surfaces of the leaves, branches and fruits of grapes, which affects the humidity level suitable for the growth and development of the disease.
Chemical control: Mix winter oil + a fungicide such as tebuconazole, azoxycitroene, or mancozeb and metalaxyl, with a general insecticide containing cypermethrin or deltamethrin 2.5%.
We recommend using one or two sprays of micronized sulfur alone without mixing.
Copper sulphate may replace the need for many fungicides, and we recommend continuing to spray it immediately after pruning at intervals.
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References
- Gadoury, D. M., Seem, R. C., Pearson, R. C., Wilcox, W. F., & Dunst, R. M. (2001). Effects of powdery mildew on vine growth, yield, and quality of concord grapes. Plant disease, 85(2), 137-140
- Delp, C. J. (1954). Effect of temperature and humidity on the grape powdery mildew fungus. Phytopathology, 44(11).
- Hartman, John, and Julie Beale. “Powdery mildew of grape.” Plant pathology fact sheet PPFS-FR-S-12 13 (2008).
- Effects of Powdery Mildew on Vine Growth, Yield, and Quality of Concord Grapes – The American Phytopathological Society
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