How to Recognize Powdery Mildew in Strawberry and Control it Efficiently

Strawberries (Fragaria * ananassa) are unique, delicious, nutritious, high-value crops that bear their seeds on the outside surface of the fruit-berry. Nowadays, this beautiful fruit is cultivated worldwide in different climactic conditions, both on open fields and inside greenhouses. This has been possible due to its genetic diversity and adaptation across different environmental conditions. Today, China is the leading strawberry producer globally (1).

Like in other crops, different pests and diseases can put the quality and yield of strawberries at risk and usually makes the application of management measures necessary. Powdery mildew is one of the most common and distructive diseases in strawberry cultivation that has become a significant problem for farmers globally. To decrease the damage caused by this pathogen, it is essential that farmers can identify the disease (symptoms) on time and proceed to all necessary control measures.

Powdery mildews is a disease caused by Podosphaera aphanis fungus, an obligatory biotroph. This means that the fungus can survive only on the living tissue of a host plant. The pathogen is more common in warm and dry climates with high humidity. They are more frequent in strawberry crops growing in greenhouses and plastic tunnels.

Disease Cycle 

Under favorable conditions, the fungus continues to grow and propagate (developing spores). Temperatures between 15-27°C, dry leaf surfaces, moderate to high humidity, and windy conditions favor disease development. It usually takes 7-14 days for the germination of the conidiophore for spore production. The biotroph pathogen enters a new field from a previous source of inoculum that has either overwintered or from already infected leaves or transplants. It can survive and overwinter as cleistothecia (sexual fruiting bodies) on crop residues. In this form, it can survive low temperatures. On maturation with the rise of temperature in the spring, it starts releasing ascospores (which could be observed as black patches on the lower leaf surface), forming a major infection source in the coming season (Figure 1C). Life cycle completion depends on temperature, humidity, and spore production.

Common Symptoms of Powdery Mildew in Strawberries

There are several symptoms that a grower can observe and may reveal that his/her strawberry plants have been infected by Powdery mildew. Powdery mildew symptoms are easily recognizable by the presence of white powdery dust on aerial plant tissues such as leaf surfaces, stems, flowers, and fruits. Younger leaves and fruit tissue are more susceptible to infection.

Infected Strawberry Leaves by powdery mildew

Healthy strawberry leaves are flat. Initially, white powdery-like small colonies are seen on the underside of leaves. With time and under favorable conditions, these small colonies spread, covering the entire underside of a leaf. Upon severe infections, the edges of the leaves curl (Figure 1A), exposing white mycelium. In Figure 1B, typical fungal mycelium can be seen on lower leaf surface. However, the curling of leaves should not be confused with water stress symptoms. Diseased leaves later start turning purple and reddish. The fungus can be seen on the upper leaf surface also.

Severe infections by powdery mildew (Figures 1C and 1D) reduce photosynthesis and ultimately affect plant vigor, yield, and fruit quality (2).

Flowers infected by powdery mildew

Infestation in the floral parts results in deformed flowers and reduction or even absence of fruits. White mycelium is usually visible to the naked eye, especially when the disease has progressed.

Powdery Mildew Symptoms in Strawberry Fruits 

On fruits, the growth of Powdery Mildew is favored by warm temperatures and high humidity levels. White mycelial growth could be seen on the seeds, thus producing poor-quality berries, affecting marketability and decreasing the product’s commercial value (Figure 2A and 2B).

Upon severe infections, powdery mildew can reduce the yield and quality of berries. This happens by decreasing the fruit set, causing inadequate ripening, cracking, and deformation of fruits that obtain a poor flavor and extensive post-harvest losses (3).

How does powdery mildew spread? 

Powdery mildew spores, called conidia, are spread by wind. In new fields-areas, they are spread through planting infected material (transplants). Human activity inside greenhouses (tools, etc.) could also help distribute spores.

Management methods of powdery mildew – How to control powdery mildew in strawberries

If not identified and efficiently controlled on time, powdery mildew can infect fruits and reduce strawberry yield and quality by 30%. So, it is essential to correctly identify and control the disease progression at the early stage of the infection. The most common management options are the application of fungicides and the use of resistant varieties. However, the development of resistance of the fungus to fungicides is an increasing problem. And more importantly, the residues of such agrochemicals in the fruits are becoming a bigger legal issue putting human health at risk. Due to this, maximum acceptable-permited legal limits have been implemented for pesticide residues on food in most countries (3). To decrease the use of fungicides, strawberry growers are advised to adopt IPM (integrated pest management) methods that can effectively control powdery mildew. One thing to consider here is controlling powdery mildew during the fall, which could help in reducing disease development in the following spring (4).

Cultural Control Measures for Powdery Mildew in Strawberries

Farmers and growers can do the following to reduce the inoculum load and spread of the powdery mildew spores.

1. Field monitoring to check for initial symptoms, especially during the mid-spring or early season. It could help decide which management options should be used based on severity. Initial visual symptoms include discoloration and distortion of leaves (2).
2. It is advised to avoid overhead irrigation as it creates a favorable environment for disease development (good moisture and relative humidity).
3. It is better to avoid excess nitrogen fertilization.
4. Use of tolerant or resistant cultivars
5. When choosing a new location for strawberry cultivation, choose places with good sun exposure and minimal shade.
6. Use of silicon-based fertigation or sprays could reduce susceptibility to infection.
7. Maintaining good crop hygiene – It is best to destroy crop residues after the harvest or at the end of the season. This will reduce the inoculum of the next season by reducing the overwintering of pathogens.

Biological Control of Powdery Mildew in Strawberries

The use of Biological Control Agents (BCAs) such as Trichoderma harzianum and Bacillus subtilis are effective when applied at an early stage of infection. However, they are not so effective when applied alone and when the infection has progressed (3). At the same time, sulfur solutions could also be used as a preventive measure (5). One thing growers should always keep in mind is that disease management is easy when the treatments are applied at an early stage, and the amount of inoculum is still limited.

Chemical Control of Powdery Mildew

Fungicides are advised to be used after carefully reading the instructions and with the supervision of an experienced, licensed agronomist. Spraying during windy conditions should be avoided. The efficiency of fungicides increases when applied during the initial stage of disease development. Later, systemic fungicides like Tetraconazole, propiconazole, Azoxystrobin, myclobutanil, etc. could be used alternatively so that the pathogen does not develop resistance against a specific active compound (6). Pre-season spray can help reduce the risk of developing epidemics in the peak season. A model was developed to reduce the number of sprays and, at the same time, increase the efficacy of fungicide application. This model helps predict the period to use fungicides based on favorable environmental conditions. It considers the incidence of powdery mildew, the environmental risk of infection, and the growth stage of strawberries. Different fungicides were used for effective control of powdery mildew. Readers are directed to read this article for details about the model for effective application of fungicides (7).

Note: All the pictures were taken by the Author himself at Luke, Finland during 2021 season.

References 

1. Li, Y. et al. Detection of powdery mildew on strawberry leaves based on DAC-YOLOv4 model. Comput. Electron. Agric. 202, 107418 (2022).
2. Powdery Mildew of Strawberry | NC State Extension Publications. https://content.ces.ncsu.edu/powdery-mildew-of-strawberry.
3. Pertot, I. et al. Integrating biocontrol agents in strawberry powdery mildew control strategies in high tunnel growing systems. Crop Prot. 27, 622–631 (2008).
4. Powdery Mildew / Strawberry / Agriculture: Pest Management Guidelines / UC Statewide IPM Program (UC IPM). https://ipm.ucanr.edu/agriculture/strawberry/powdery-mildew/.
5. GmbH, P. Powdery Mildew of Strawberry | Pests & Diseases. Plantix https://plantix.net/en/library/plant-diseases/100027/powdery-mildew-of-strawberry/.
6. PP-208/PP129: Powdery Mildew of Strawberry. https://edis.ifas.ufl.edu/publication/PP129.
7. How to control strawberry powdery mildew | AHDB. https://archive.ahdb.org.uk/knowledgelibrary/how-to-control-strawberry-powdery-mildew.