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Fungus protects strawberries against grey rot and does away with the need for chemicals

Summer time is strawberry time: strawberries are grown worldwide on an area of 300,000 hectares and are harvested in early summer. Fungicides that are used to treat grey rot might actually be the only thing that diminishes the pure pleasure of eating juicy, tasty strawberries. The Konstanz-based company bio-ferm Research GmbH has developed a fungicide that protects strawberries against Botrytis cinerea infections, one of the most common fruit infections. The fungicide does not lead to the development of resistances and does away with the need for chemicals.

Stefan Kunz, Director R&D of bio-ferm Research GmbH, explains how the new fungicide works. © Stefan Kunz

Humid air, rain and temperatures between 15 and 20 degrees are ideal conditions for Botrytis cinerea, a fungus that causes grey rot in many plants. The fungus belongs to the ascomycetes family of mushrooms and causes infections in more than 235 plants, including many economically important plants such as berries, pomaceous fruit, wine grapes, lettuce, carrots, celery and cabbage. The fungus grows by forming a web of microscopically small threads (mycelium), which form a white, fluffy layer on the surface of fruit and berries. The fungus forms conidia, asexual spores that are dispersed by wind and rainwater and cause further infections. 

“Botrytis cinerea can attack many different types of plant but it must have nutrients before it invades a plant,” said Dr. Stefan Kunz, Director R&D of bio-ferm. Nutrients from pollen, blossom nectar, leaking from wounded plant parts or from dying tissue such as old flower petals provide the required food. Freshly infected, unripe strawberries turn brownish and later whitish-grey. “Infected fruit has to be removed as quickly as possible to prevent the fungus from spreading to other fruit,” said Kunz explaining that if the fungus is not removed, it will destroy huge parts of the harvest.

Resistance formation and toxicity – two risks associated with standard fungicides

Several types of botryticides against Botrytis cinerea are commercially available. These preventive agents need to be applied to the fruit before they come into contact with the pathogen and application is recommended several times a year as the plants are continually growing: vines are usually treated two or three times a year, strawberry plants up to five times. More than ten different botryticides consisting of at least five different groups of active substances are currently commercially available. Each active substance group targets a specific part of the fungal metabolism,” Kunz explained. “However, the fungi can develop resistances to certain groups of active substances and render them ineffective.” 

Botrytis cinerea infections proceed in several steps: shortly after infection, the fruit turn a light brownish colour (left); fruit in the final stage of infection are covered completely with conidia (centre). The photo on the right shows healthy, uninfected fruit. © Stefan Kunz

Modern botryticides therefore tend to contain several groups of active substances. However, little is yet known relating to the effect these combinations have on human health. In Germany, all fungicides need to undergo thorough testing for potential harmful effects on human health and the environment before they are given marketing authorisation. In order to prevent harmful effects, maximum residue limits are kept so low that even the consumption of large quantities of strawberries (and other fruit) leads to no  harmful effects in risk groups (e.g. children). However, these limits only apply to individual substances, which is why it remains unclear whether the residues of several substances in combination have a negative effect on human health. 

The bio-ferm product works through natural competition for space and nutrients

The scanning microscope image shows how the new substance protects the fruit against pathogens: Aureobasidium pullulans (orange) colonises a tiny crack, Botrytis cinera (blue) conidia cannot enter the fine cracks. © bio-ferm GmbH, Photo: Mendgen

bio-ferm Research GmbH’s “Boni Protect forte” contains Aureobasidium pullulans blastospore (a special type of conidia) granules, which block disease pathogens. The A. pullulans strains were originally isolated from the leaves of an apple tree, and subsequently assessed for their suitability in protecting plants against Botrytis cinera infections. A biotechnological production method similar to the production of baker’s yeast was developed: the blastospores are produced in a fermenter and are then concentrated and turned into granules in a fluid bed dryer. They are resuspended in water and applied to the plants. Strawberries are treated with the resuspended A. pollulans blastospores four times at one-week intervals, starting with the appearance of the first blossoms. 

A. pollulans spreads by forming mycelia and buds. Budding involves the formation of new daughter cells that are separated from the mother cell. One cell division process can give rise to several daughter cells. This enables Aureobasidium to spread quickly; under optimal conditions the fungus population can double within three hours. A. pollulans feeds on blossom nectar and exudates from tiny wounds on the surface of leaves and fruit. In fact, it feeds on basically the same nutrients as Botrytis cinerea, with the result that the two species are in natural competition with each other for nutrients (and space). The beneficial fungus consumes the nutrients, depriving the pathogen of the nutrients it needs to propagate and spread.

Compatibility is guaranteed

Problems like toxicity and resistance formation, which usually occur as a result of applying commonly used plant protectants, do not arise with the bio-ferm product. Moreover, Aureobasidium pullulans does not just occur on plants. “Aureobasidium pullulans is omnipresent and occurs everywhere in nature. Therefore, humans are constantly exposed to this fungus,” Kunz explained. Scientific investigations have confirmed the compatibility of the fungal species used with humans. In addition, Kunz believes that Botrytis cinerea is highly unlikely to become resistant to A. pullulans. “Boni Protect forte does not attack the metabolism of the fungal pathogens, which is why there is no risk of resistance even with frequent applications. Boni Protect forte works competitively, which means that A. pullulans and the pathogens compete for space and nutrients, with the result that the pathogen is deprived of vital nutrients and prevented from propagating.” Kunz explained that Botrytis cinerea could only become resistant to the A. pullulans if it changed its lifestyle, which would require a long-term evolutionary process.

The efficiency of the bio-ferm product has been tested in comprehensive field tests. “The product is applied to different plots of land and its effect compared to plots that are untreated or treated with commonly used fungicides,” Kunz said. In order for the product to obtain marketing authorisation, the plots treated with the new product must reveal significant differences to untreated plots and lead to results that are equal or better to those achieved with already commercialized products. The new product is suitable for treating strawberry plantations, other berry cultures and wine grapes. A. pullulans is also contained in plant protectants used for treating fruit rot or fire blight of pomaceous fruit like apples and pears. “Due to the product’s numerous advantages, we believe that it will soon become established as an effective means to ward off Botrytis cinerea infections,” said Kunz with a certain degree of optimism.

Further information:
bio-ferm Research GmbH
Dr. Stefan Kunz
Director R&D
Lohnerhofstraße 7
78467 Konstanz, Germany
Tel.: +49 (0)7531/ 690 661
Fax: +49 (0)7531/ 690 660
E-mail: stefan.kunz(at)bio-ferm.com

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/fungus-protects-strawberries-against-grey-rot-and-does-away-with-the-need-for-chemicals