Mildew: What it is and how to prevent it?

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December 11, 2023
min read
Mildew is recognised as one of the biggest issues for both greenhouses as well as vertical indoor farm. Why is this such a big problem, what plants does it affect, and how can you prevent it from becoming a problem in the first place?

Where it all begins

Mildew, primarily the powdery and downy varieties, finds its origin in the microclimates of greenhouses. These microscopic menaces thrive in warm and humid conditions, making greenhouses an ideal habitat. They take root as spores, often carried in on contaminated plant material, tools, or even the wind. This is why deep-cleaning your greenhouse and greenhouse tools after every harvest season is of vital importance for plant health. And of course you should try to limit exposure from the outside world, i.e. people visiting your greenhouse should take the necessary cleanliness precautions.

Mildew, primarily the powdery and downy varieties, originates in the microclimates of greenhouses, thriving in warm and humid conditions. These microscopic threats take root as spores, which can enter through various pathways such as contaminated plant material, tools, or airborne transport. Common entry points include contaminated tools, infected plant material, and even spores carried by wind, insects, or clothing. Ensuring proper cleanliness precautions during greenhouse activities and implementing thorough cleaning practices after each harvest season is crucial for maintaining plant health. Additionally, limiting exposure from the outside world by taking necessary cleanliness precautions can further prevent mildew infestations.

Powdery Mildew
Powdery Mildew

What is the difference between powdery and downy mildew?

Two distinct fungal pathogens, powdery mildew and downy mildew, present distinctive challenges to plant health. The key divergence lies in morphological attributes and optimal environmental conditions. Powdery mildew manifests as circular white spots or patches, progressing into a dusty white or grey coating on leaves, stems, and flowers. It flourishes in warm and arid conditions, and exhibits a predilection for moisture, shade, and dense plant environments. The etiological agent, classified within the family Erysiphaceae, undergoes asexual reproduction through airborne conidia. Mitigating powdery mildew involves the application of chemical treatments, surgical removal of affected plant components, and optimisation of air circulation.

Conversely, downy mildew presents initially deceptive symptoms, characterised by leaf yellowing or speckling, with faint gray fuzz emerging on the undersides. Flourishing in high humidity, cool temperatures, and crowded plant conditions, it specifically targets botanical entities like basils, impatiens, melons, and viburnum. Classified within the family Peronosporaceae, downy mildew thrives in cooler and more humid environments, engaging in both sexual and asexual reproduction, facilitated by structures like oospores. Strategies for combating downy mildew encompass fungicidal application, utilisation of copper-based sprays, and the adoption of resistant cultivars.

While powdery mildew presents as circular white powdery spots, in stark contrast, downy mildew manifests through yellow to brown lesions. Powdery mildew exhibits a preference for warm and arid conditions and propagates predominantly through asexual means. In contrast, downy mildew thrives in cooler, more humid settings and employs a combination of sexual and asexual reproduction. A precise comprehension of these differences equips cultivators with targeted strategies to counteract these fungal adversaries and fortify the resilience of their crops.

Powdery- & Downy mildew
The difference between Downy- and Powdery Mildew

Major Crops Affected

Powdery and downy mildews can affect a wide range of crops such as: Grapes, Apples, Peaches, Cherries, Strawberries, Melons, Cucumbers, Squash, Lettuce, Spinach, Potatoes, Onions, Roses  

Some ways to prevent mildew

Cultivar Selection: Choose mildew-resistant cultivars and rootstocks, and maintain a 7-14 day spray interval if you focus on biological control. Examples are Geronimo F1 (tomato) and Florida Beauty (strawberry);

Proper Ventilation: Ensure good airflow within the greenhouse. Adequate ventilation helps reduce humidity levels, creating an environment less favourable for mildew growth;

Regular Cleaning: Clean and sanitise the greenhouse, including all tools, equipment, and surfaces, after each growing season. This minimises the chances of spores lingering and causing infections;

Plant Inspection: Thoroughly inspect new plants before introducing them to the greenhouse. Avoid bringing in infected plant material, which can carry mildew spores;

Quarantine: If possible, quarantine new plants for a short period before introducing them to the main growing area. This allows time to identify and address potential issues;

Appropriate Watering: Use watering methods that minimise water splashing. Drip irrigation or soaker hoses directly target the root zone, reducing the spread of waterborne spores;

Optimal Spacing: Properly space plants to allow for good air circulation. This helps prevent the development of stagnant air pockets that can contribute to mildew growth;

Biological Control: Explore biological agents like beneficial insects and microorganisms to combat mildew. Examples are Ladybugs, Green Lacewings (specifically the larvae), and mites (Phytoseiulus persimilis);

Temperature and Humidity Control: Maintain optimal temperature and humidity levels within the greenhouse. This can be achieved through proper climate control systems;

Monitor Vigilantly: Routine inspection is your first line of defence. Implement scouting programs to catch mildew in its earliest stages.

Remember, a comprehensive approach is key. The battle against mildew is never-ending, and arming yourself with knowledge is the first step to victory!

Written by:
Dr. Mohanna Mollavali

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