Powdery mildew is a disease that occurs worldwide and causes serious losses to crops. It is considered to be much more serious than downy mildew. The disease appears in epiphytotic forms and occurs throughout the temperate zones, and it is worst in dry weather, causing great loss to the crop. About 10,000 species of angiosperms belonging to more than 1600 genera are susceptible to mildew fungi.
Powdery mildew rarely kills its host, but it is responsible for water and nutrient loss, impaired growth and development, and can affect photosynthesis, respiration, transpiration, and yield.
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Causal Organism of Powdery Mildew
Several genera can cause the disease in several hosts. It is caused by a group of fungi belonging to the order Erysiphales.
The most common genera are Erysiphe, Leveillua, Podosphaera, Sphaerotheca, and Uncincula. These are the sexual stages and in the asexual stage, members of the genus Oidium are the causal organism.
The fungus is an obligate parasite having septate, hyaline profusely branched superficial mycelium sending finger-shaped haustoria to the host cells. Powdery mildew forms a network of fungal filaments (hyphae) which are securely anchored by absorbing organs called haustoria that obtain nutrients from the host.
These hyphae reproduce sexually or asexually producing cleistothecium or conidia respectively.
Host plants– Powdery mildew can affect a variety of plants belonging to families such as Cucurbitaceae, Solanaceae, Rosaceae, etc.
Symptoms of Powdery Mildew
The symptoms may vary depending on the host, pathogen species, environmental conditions, and plant tissue during the primary infection. The general symptoms are as follows-
White powdery coating– First symptoms appear on the leaves in the form of white floury patches on both sides of leaves.
Spread of powdery The disease spreads to the other green parts of the plant such as tendrils, pods, stems, etc.
Leaf discoloration patches on the leaves originate in the form of minute discolored specks from which a powdery mass radiates on all sides. In the advanced stages of the disease, large areas of the host get covered with white floury patches.
Leaf distortion– Affected leaves may become curled or distorted in some species. This is because the fungus affects the cellular activities in plants reducing the ability of photosynthesis. Infected plants impart a dirty appearance. In extremely severe infections, the infected leaves are shed leaving the stem devoid of the leaves.
The specific symptoms according to some common plant categories are as follows-
In vegetables, white patches appear on the upper surface along with that, leaves may turn yellow or brown at the edges and necrotic patches may develop during severe infection.
In fruits, it generally affects both the upper and lower surfaces of the leaves and the entire leaves may curl with a purple-bronze appearance. The disease causes a rough brown texture on the fruit skin.
In ornamental plants, infected buds do not open properly resulting in deformed flowers. In the case of cereals, the fungus affects the leaves and stems which leads to a decrease in yield and poor grain quality.
Stages of Symptoms
Early stage- In this stage, small, white, powdery spots appear on the leaves and other plant parts.
Mid stage- In this stage, the powdery patches expand covering large areas of the plant. Leaves may become yellow or brown and plants show stunted growth.
Late stage- In this stage leaves and stems become distorted and premature defoliation occurs, yield and quality are decreased.
Disease cycle of Powdery Mildew
The disease cycle involves the following phases-
Overwintering- During unfavorable conditions, the fungus survives through the resting structures called dormant mycelium or ascospores living on the host or in plant debris.
Primary infection– The primary infection takes place through ascospores released in the soil as a result of the disintegration of the wall of cleistothecium.
Under favorable conditions, the ascospores come in contact with the lowermost leaves of the host, germinate by germ tube, and cause primary infection. From the lowermost leaf, the infection spreads to the other leaves. Later the symptoms in the form of a white powdery mass appear. These patches contain conidia and conidiophores. The primary infection may also come from the conidia produced on other hosts as the causal organism.
Secondary infection– The secondary infection of the host takes place by conidia which are produced in large numbers and disseminated by wind. Conidia are formed in chains on specialized structures called conidiophores. The white powdery appearance on leaves is mainly due to conidia. Upon reaching the host, the conidia germinate to develop new mycelium and haustoria and cause infection. During the growing season, more than one secondary infection may occur because the establishment of infection and conidia production is a quick process. This results in severe large-scale infection of the crops.
Cleistothecia formation– Towards the end of the growing season, the fungus reproduces sexually by forming cleistothecia. These are hard spherical fruiting bodies that contain ascospores. Cleistothecia appear as black dots on infected leaves and stems. The cleistothecia withstand unfavorable conditions and remain dormant which later germinates during favorable conditions.
Factors affecting the disease cycle
Temperature- The optimum temperature of about 15-27 C is required for the growth of the pathogen. The temperature above or below this optimum temperature is unfavorable for the growth of the pathogen.
Humidity– The pathogen requires high humidity to infect the plant. Dry conditions may suppress the growth of fungus.
Air circulation– Poor aeration due to shaded areas, overcrowding, etc. are favorable for fungal growth.
Prevention and Control of Powdery Mildew
Preventive measures to control powdery mildew are as follows-
Cultural practices– Adequate space should be maintained between the plants which improves air circulation, reduces humidity, and makes it harder for the fungus to establish and spread. Regular pruning should be done to remove excess foliage and thinning should be done for more light to reach inner leaves. Plants should be grown in areas that receive abundant sunlight.
Watering- Watering plants during the morning allows foliage to dry during the day thus reducing humidity. Avoid the wetting of the foliage during watering.
Removal of infected plants– Infected plants should be removed to prevent infection in other plants. Proper disposal of plant debris should be done to eliminate overwintering spores.
Fertilization– Excessive nitrogen fertilizers should be avoided and adequate potassium and phosphorus should be provided to plants.
Resistant varieties– Many species have been bred for resistant varieties such as Marketmore varieties of cucurbits, and Knock Out varieties of roses have been developed to overcome the disease.
Fungicides- Sulphur-based fungicides should be applied for the effective treatment of powdery mildew. Proper dusting and spraying of the fungicides should be done. Organic fungicides such as neem oil can be used to prevent powdery mildew.
Bacillus subtilis and Streptomyces lydicus are considered bio-fungicides that colonize the plant surface and prevent fungal spores from germinating.
Beneficial insects such as ladybugs prey on fungal spores thus preventing the infections.
Regular inspection of plants should be done to recognize early signs of infections which prevent disease from spreading.
References
- Powdery mildew | Description, Symptoms, & Control. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/powdery-mildew
- Colorado State University Extension. (2016, February 22). Powdery Mildews – 2.902 – extension. Extension. https://extension.colostate.edu/topic-areas/yard-garden/powdery-mildews-2-902/
- Hagen, L. (2022, August 24). Powdery Mildew Treatment & Prevention (A How-To Guide). GardenDesign.com. https://www.gardendesign.com/how-to/powdery-mildew.html
- Powdery mildew in the flower garden. (n.d.). UMN Extension. https://extension.umn.edu/plant-diseases/powdery-mildew-flower-garden
- Vasistha, B. R., Sinha, A. K., & Kumar, A. (2016).Fungi. S. Chand & Company Ltd.
- Gupta, V. K., & Sharma, S. K. (2011). Plant Pathology. Kalyani Publishers.
- Powdery mildew. (n.d.). CT.gov – Connecticut’s Official State Website. https://portal.ct.gov/caes/fact-sheets/plant-pathology/powdery-mildew
- Powdery mildew. (n.d.). https://extension.psu.edu/powdery-mildew
- Powdery mildew. (2019, July 13). [Slide show]. SlideShare. https://www.slideshare.net/SAISIKANPATRA/powdery-mildew-155298549
- Identification of powdery mildew fungi. (n.d.). Identification of Powdery Mildew Fungi. https://www.apsnet.org/edcenter/disandpath/fungalasco/labexercises/Pages/PowderyMildew.aspx
