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Botrytis Blight (Grey Mold) of Strawberries Hosts and symptoms Botrytis blight, commonly referred to as gray mold and often caused by the fungus in the Sclerotiniaceae family, Botrytis cinerea, is one of the most common greenhouse crop diseases that can cause extreme losses of crops (9). Losses may be more than 50% of the total strawberry crop in the field during flowering and harvest (1). Appearance of a fuzzy gray mold and rotting fruit can occur at any stage of the strawberry fruit’s life cycle. Brown lesions may develop around the site of infection on the plant. These can occur anywhere on the mature strawberry with stomata or a wound, but especially near the stem as the flowers are often infected. The mature strawberries do not become mushy, but do become dry and hard once they are completely rotted. Lesions on the immature strawberry may cause the fruit to be deformed or even die before reaching maturity. The name “gray mold” comes from the sign that is the appearance of fuzzy gray spores and mycelium on the infected strawberry tissues (7). Disease cycle Botrytis blight of strawberries follows a polycyclic disease cycle. This fungus can present itself in a strawberry field in a few ways, one of these being due to the transfer of conidia (asexual spores) by wind or water from nearby plants/fields. It can also occur by overwintering in the soil or on dead plant tissues in the field as sclerotia, which are small, dark, circular structures (6). Infection can occur at any time, but typically occurs in injured, new, or aging tissues (10). Diving into more detail, we can start the cycle here with conidia being dispersed by the wind. This can either lead to a direct infection of a mature strawberry fruit or it may lead to infection of a flower occurring during a period of high moisture. From this flower, the infection can spread to the green strawberry fruit, leading to the mature strawberry being covered in fuzzy masses of conidia, which is what we also get after direct infection of a mature strawberry. Botrytis may then colonize the fruit and overwinter in strawberry plant debris as sclerotia before the conidiophore containing its conidia becomes moist in the spring and initiates sporulation and growth (3). Management Prevention, as always, is one of the best ways to manage botrytis blight of strawberries. Because this fungus thrives in wet conditions, it is important to try to minimize these conditions as much as possible. This can be done by being sure to properly space the plants for good air flow and to protect from wounding them in these wet conditions (5). It is also important to prune or stake the strawberry plants to really ensure proper air flow, sterilizing the pruning shears after each cut in bleach and water (4). It is also important to remove any areas of the plant infected with blight from the field/growing area, but to be sure not to attempt this in wet conditions. Botrytis sclerotia overwinters in the dead plant tissues, so be sure to remove all previous plant debris before replanting (5). When watering the strawberry plants, use a soaker hose to apply directly to the roots and avoid contact with the leaves to prevent moisture on them. You could also water in the morning to allow for the sun to be able to dry the leaves. It is also beneficial to infuse the soil with mycorrhizal fungi and bacteria for prevention of Botrytis blight as well as increasing the efficiency of the plant’s roots taking up proper nutrition. Application of neem oil fungicide may be an effective prevention method through regular use as well (4). Other preventative measures that apply specifically to greenhouse growth of strawberries include using fans and vents to ensure proper ventilation and to keep down the humidity. Furthermore, do not harvest any berries directly after pruning, but rather wait a few hours because spores may be lingering in the air (4). When prevention fails, fungicides can also provide a way of treatment for Botrytis blight of strawberries. Two organic treatments include use of liquid copper fungicides and potassium bicarbonate. To prevent the strawberry plants from spore infestation, apply liquid copper fungicides. To reduce growth of Botrytis blight, apply potassium bicarbonate by spraying the infected plant regularly (4). A study produced on Sweet Charlie strawberries applying fenhexamid throughout certain intervals after the plant had flowered found that the flowers of a strawberry plant are more likely to be affected by B. cinerea than the green fruit, which suggests the stamens to be the main source for infection. Therefore, to achieve optimal success when applying the fungicide, do so about every seven days during peak blooming periods (8). Unfortunately, B. cinerea is able to adapt to fungicides and form immunity to common methods, so it is important to switch up the treatment method you use, such as by using a mix of organic and microbial methods (4). Furthermore, another study tested and found antagonistic bacterial strains with the ability to kill Botrytis blight by inoculating strawberries with B. cinerea and treating with different bacteria to find these strains to have inhibited the growth of B. cinerea. These strains include Bacillus lentimorbus, B. megaterium, B. pumilis, B. subtilis, Enterobacter intermedius, Kurthis sibirica, Paenibacillus polymyxa, and Pantoea agglomerans. This method may also be able to provide a sustainable production of strawberries (2).

Works Cited 1.	Bulger, M. A., Ellis, M. A., & Madden, L. V. (1987, February). Influence of Temperature and Wetness Duration on Infection of Strawberry Flowers by Botrytis cinereal and Disease Incidence of Fruit Originating from Infected Flowers. The Ohio State University, Department of Plant Pathology. Columbus, OH. Retrieved November 27, 2019, from https://www.apsnet.org/publications/phytopathology/backissues/Documents/1987Articles/Phyto77n08_1225.pdf.

2.	Donmez, M. F., Esitken, A., Yildiz, H., & Ercisli, S. (2011). BIOCONTROL OF BOTRYTIS CINEREA ON STRAWBERRY FRUIT BY PLANT GROWTH PROMOTING BACTERIA. The Journal of Animal & Plant Sciences, (24), 758-763. doi: https://www.researchgate.net/profile/Sezai_Ercisli2/publication/266340597_Biocontrol_of_Botrytis_Cinerea_on_strawberry_fruit_by_plant_growth_promoting_bacteria/links/551fd17f0cf2a2d9e1409e41.pdf

3.	Ellis, M. A. (2016, April 15). Botrytis Fruit Rot “Gray Mold” of Strawberry, Raspberry, and Blackberry. Retrieved October 20, 2019, from https://ohioline.osu.edu/factsheet/plpath-fru-36.

4.	Espiritu, K. (2019, February 1). Botrytis Cinerea: Controlling This Devastating Plant Disease. Retrieved November 27, 2019, from https://www.epicgardening.com/botrytis-cinerea/.

5.	J., S. L. (2018, February). Botrytis Blight: Botrytis cinerea; Botrytis spp. Cornell University College of Agriculture and Life Sciences. Ithaca, NY. Retrieved October 18, 2019, from http://plantclinic.cornell.edu/factsheets/botrytisblight.pdf.

6.	Koike, S. T., & Bolda, M. (2016, July). Botrytis Fruit Rot of Strawberry. UC Cooperative Extension. Monterey County, Santa Cruz County. Retrieved October 20, 2019, from https://ucanr.edu/blogs/strawberries_caneberries/blogfiles/37846.pdf.

7.	Louws, F. (2018, April 12). Botrytis Fruit Rot / Gray Mold on Strawberry. Retrieved October 15, 2019, from https://content.ces.ncsu.edu/botrytis-cinerea-botrytis-fruit-rot-and-blight-on-strawberry.

8.	Mertely, J. C., Mackenzie, S. J., & Legard, D. E. (2002). Timing of Fungicide Applications for Botrytis cinerea Based on Development Stage of Strawberry Flowers and Fruit. Plant Disease, 86(9), 1019-1024. doi: 10.1094/pdis.2002.86.9.1019.

9.	Schraufnagel, J., & Hudelson, B. (2010, May 11). Gray Mold (Botrytis Blight). Retrieved October 15, 2019, from https://hort.extension.wisc.edu/articles/gray-mold-botrytis-blight/.

10.	Wick, R. L., & Madeiras, A. (2019, June 5). Botrytis Blight of Greenhouse Crops. Retrieved October 15, 2019, from https://ag.umass.edu/greenhouse-floriculture/fact-sheets/botrytis-blight-blight-of-greenhouse-crops.