Grey mold caused by the fungal pathogen Botrytis cinerea is a disease that affects many crops. While now well documented, its management is challenging due to host diversity, various modes of action and because it can survive in many different forms.
Understanding the life cycle of B. cinerea in each crop is the key to successful management. Find out everything you need to know about this disease and the best practices to protect your crop from it.
1 - About the disease and sources of inoculum
Grey mold caused by the fungal pathogen Botrytis cinerea, can infect more than 1000 plant species (Elad Y. et al., 2016). B. cinerea can infect most plant parts such as leaves, flowers, stems, petioles, and fruits (Fig.1).
Managing this disease is challenging due to the diversity of hosts, which it attacks using various modes of action and because it can survive as multiple types of inoculums such as mycelia, conidia or sclerotia in soil and in crop debris (Aleid J.D. & Wubben J.P., 2007).
Understanding the life cycle of Botrytis in each crop,
is the key to success in its management
In field crops such as strawberries, raspberries, blueberries, onions, lettuce, and grapes, this disease can significantly reduce yield and postharvest quality.
Sclerotia can germinate to produce spores, which can be splashed by rain or overhead irrigation onto the crops. Spores are also moved by air currents to reach target crops.
Other sources of inoculum are common weed species found in pear orchards, such as African wood sorrel (Oxalis pes-caprae) as well as other spp. of Oxalis, White sweet clover (Melilotus alba Medicus.), Common chickweed (Stellaria media (L.) Vill.) and Dandelion (Taraxacum officinale G. H. Weber ex Wiggers), (Spotts R.A. & Serdani, M., 2006).
Pollen exudates from common weeds in fruit orchards and vineyards, can stimulate the fungal growth and increase the virulence of the pathogen (Fourie J.F., & Holz G. 1998), after rain or irrigation.
2 - Botrytis in field-grown strawberry
Many strawberries varieties grown in cool climates are susceptible to B. cinerea infections when extended leaf wetness periods occur.
Higher humidity conditions can be caused by rainfall as well as by a big variation between day and night temperatures, especially where a water body such as the ocean, a dam, a river, or lake is nearby.
Conidia of B. cinerea can infect open flowers of strawberries (primary infections), whereas conidia from B. cinerea-infected flower parts are major sources of secondary inoculum for the penetration of fruit receptacle tissues (Bristow P. R. et al., 1986). B. cinerea conidia germination can take at least 8 h when a free water film is available on plant surfaces (Jarvis W.R., 1962).
When good cultural practices are applied to reduce the inoculum levels of B. cinerea in strawberry fields, the need for fungicide applications can be reduced significantly.
Managing B. cinerea inoculum levels and infections in strawberry fields, using cultural practices and fungicide application
Remove sources of inoculum, such as infected plant tissue and mummified fruit, during and after the strawberry season.
Avoid direct contact of strawberries with soil, by using straw mulching or covering the planting beds with polyethylene foils.
Most of the inoculum is present on the ground, and in plant residues and soil moisture promotes conidia germination (Daugaard H., 1999).
Crop growth management
Avoid dense canopies, which can create a microclimate that is favorable for B. cinerea infections.
Too much Nitrogen fertilizer can cause dense canopies.
Avoid overhead sprinkler systems, which can splash B. cinerea conidia onto flowers/fruit.
Drip irrigation systems are preferable.
An Integrated Pest Management (IPM) approach, using both chemical and biological fungicides in alternation – from early flowering (10% open flowers) – with a 7-day interval, is most effective for Botrytis grey mold control.
Biological fungicides have the added benefit of a zero-day harvest interval and can therefore be applied also during fruit development.
3 - Botrytis in grapevine
Although B. cinerea spores are airborne and can come from neighboring crops, most of the inoculum that leads to a Botrytis outbreak in a vineyard originates within that vineyard.
For Botrytis bunch rot to establish, there are two infection pathways which can initiate disease in vineyards.
💥 The first is a latent infection starting in early flowering, followed by grape berries becoming infected early in the season (Fig.2) (Beresford R.M. & Hill G.N. 2008). A common site for infection occurs where the flower cap has detached, leaving a wound. Subsequent, young developing berries produce natural anti-fungal chemicals, therefore limiting the spreading of B. cinerea. Anti-fungal chemicals disappear however as berries ripen, and the latent infections can develop into bunch rots that spread from berry to berry (Beresford R.M. & Hill G.N. 2008).
💥 As soon as flower caps, aborted berries and other debris become colonized by B. cinerea in the early season, it provides the second infection pathway, when it is caught within the bunches. This becomes a major source of inoculum for berry infection (Fig. 2) when the anti-fungal chemicals disappear during ripening.
Factors influencing grapevine susceptibility to Botrytis infection
Botrytis cinerea is a cool climate disease. Temperatures between 15°C and 23°C are optimum for a Botrytis. Temperatures >23°C, are less favorable for this disease.
Rainfall during early spring, just before and after bud burst, is a key factor in the germination of Botrytis sclerotia which overwintered on grape canes.
This is enough to provide inoculum for both the first and the second infection pathways.
Rainfall alone is however not enough to predict the Botrytis risk further in the growing season. Leaf wetness, due to condensation overnight, must clearly also be considered to determine the risk of a Botrytis infection.
When a vineyard is located close to water (river, dam, lake) often warm day temperatures and lower night temperatures will cause sufficient condensation to promote Botrytis infection.
The longer wet conditions persist, the greater the probability of infection.
Densely grown canopies can create a microclimate in vines which are very favorable for Botrytis infection, due to moisture retention in the canopy for longer periods. At the same time, such closed canopies will reduce spray application penetration.
Grapevine growth stage
During flowering, specifically, just after flower hood drop, a wound is created where the flower hood has abscised, creating a point of entry for the fungus. This creates the first susceptible period in grapevine.
The second most susceptible period is on berries during and after ripening (veraison). Berries soften and sugar levels increase, which allows infection.
Any wounded green tissue is susceptible to Botrytis.
Wounding is commonly caused by:
- vineyard practices such as machine operations
- berry splitting after rain
- feeding by birds or insects and hail or frost.
Injured tissue is most susceptible in wet weather when spores landing on damaged tissue experience conditions suitable for germination and infection.
Managing Botrytis inoculum levels and infections in vineyards, using cultural practices and fungicide applications
Avoid densely grown canopies due to high Nitrogen fertilization programs. Less dense canopies will promote good air circulation and light penetration. This will also allow faster drying of plant parts and therefore reduce the risk of disease.
Remove leaves around clusters before bunch closure to improve air circulation and avoid moisture retention while at the same time improving spray penetration.
Prevent wounding by controlling insects, birds, and other grape diseases.
Protect wounds which were caused by leaf-cutting machines, by applying microbial biofungicides which can colonize and protect the wound sites immediately.
Monitor climatic conditions such as rainfall and leaf wetness periods.
Also monitor tissue damage through green practices, to be able to protect damaged berries.
Use effective fungicides in an IPM program, alternating chemical and biological fungicides to minimize chemical residues.
Plan IPM program based on rainfall & leaf wetness period, temperature, and susceptible growth stages of the crop.
4 - Botrytis in greenhouse crops
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Botrytis grey mold can cause severe damage to greenhouse crops such as fruiting vegetables like tomatoes & cucumbers, soft fruits like strawberries & blueberries and finally ornamental flowers like cyclamens, roses, chrysanthemums & carnations.
The most characteristic symptom is a grey furry mold covering the infected area. Stem infections on greenhouse vegetables can be severe and cause wilting of the plants around the infected area (Fig. 3). Damage and losses in greenhouse vegetable crops are mainly due to plant death after stem infections.
Botrytis spores can enter the greenhouse space through open windows. It is also possible that cuttings and seedlings can already arrive contaminated with this pathogen (Elad, Y. & Yunis, H. 1993). Inside the greenhouse, B. cinerea survives on cut plant surfaces, dead plant material, still attached to the plant, plant debris and in soil/growing media. In tomato plants flowers, fruits, leaves and stems can be infected by this fungus (Aleid J. & Wubben Jos. P. 2007).
In cold/non-heated greenhouses such as high and low tunnels, infection periods correspond with favorable climatic conditions such as cool temperatures (10 – 23°C) and rainfall followed by high humidity periods. It is therefore reasonably easy to predict the risk period for B. cinerea. Cultural practices, such as dense canopy growth, can further increase the risk of infection, due to a microclimate in which airflow is limited and water is trapped.
In hot/heated greenhouses, B. cinerea infection can be managed quite well by maintaining lower humidity conditions. However, other environmental conditions still have a role to play in the Botrytis disease risk. An invisible microfilm of water on plant surfaces, for 6 – 8 days, is the only requirement for spore germination. It is therefore important to note that water deposition occurs when the plant is cooler than the surrounding air (Oke, T.R. 1978). This means that even in heated greenhouses there will always be the risk of B. cinerea infection and preventative steps are required.
Managing B. cinerea inoculum levels and infection risk in greenhouse crops
Check propagation material for infection and discard any infected seedlings before entering the greenhouse.
Cut out stem infections
before the whole stem is damaged to save the plant from dying.
inflicted by lower leaf removal with an effective bio-fungicide, to colonize the wound site.
Avoid having any organic matter
Avoid having any organic matter such as plant debris in the greenhouse, which can be a source of inoculum for the next crop.
Dispose of plant residues
Dispose of plant residues by burning or burying them away from the greenhouse property.
Good air circulation and ventilation
Maintaining good air circulation and ventilation will reduce the chances of condensation forming. It is possible to maintain a constant plant and air temperature, by moving air through the crop.
Crop growth management
Avoid the overuse of Nitrogen fertilizers, to prevent a dense canopy which will create a favorable microclimate and reduce the susceptibility of the crop to Botrytis infection (Elad, Y. & Yunis, H. 1993).
Start fungicide applications as soon as the disease is first noticed. In greenhouse conditions where the climatic conditions are well managed, biological fungicides are often the preferred method to control Botrytis, as the inoculum levels are lower. Apply fungicides during high-risk periods for grey mold disease and continue until the risk period ends.
5 - References
1] Aleid J.D. & Wubben J.P (2007): EPIDEMIOLOGY OF BOTRYTIS CINEREA DISEASES IN GREENHOUSES. Elad Y et al (eds.) Botrytis: Biology, Pathology and Control, 319-333.
2] Beresford, R.M. & Hill, G.N. (2008): Botrytis control without fungicide residues – is it just a load of rot? New Zealand Winegrower 12 (2): 104-106.
3] Bristow, P.R., McNicol, R.J. & Williamson, B. (1986): Infection of strawberry flowers by Botrytis cinerea and its relevance to grey mold development. Ann. Appl. Biol. 109(3), 545–554.
4] Daugaard, H. (1999): Cultural methods for controlling Botrytis cinerea pers. in strawberry. Biol. Agric. Hortic. 16(4), 351–361.
5] Elad, Y. & Yunis, H. (1993): Effect of Microclimate and Nutrients on Development of Cucumber Gray Mold (Botrytis cinerea). Phytoparasitica 21(3), 257-268.
6] Elad, Y., Pertot, I., Cotes Prado, A.M. & Stewart, A. (2016): Plant Hosts of Botrytis spp. In: Botrytis – The Fungus, the Pathogen and Its Management in Agricultural Systems, (Fillinger, S. and Elad, Y., eds), 413–486.
7] Fourie, J.F. & Holz, G. (1998): Effects of Fruit and Pollen Exudates on Growth of Botrytis cinerea and Infection of Plum and Nectarine Fruit. Plant Disease 82(2) 165-170.
8] Jarvis, W.R. (1962): The infection of strawberry and raspberry fruits by Botrytis cinerea Fr. Ann. Appl. Biol. 50(3), 569–575.
9] Oke, T.R. (1978): Boundary Layer Climates. Methuen and Co. Ltd. London
10] Spotts, R. A., & Serdani, M. (2006): Inoculum sources of Botrytis cinerea important to pear orchards in Oregon. Plant Dis. 90, 750-754.
11] Williamson, B., Tudzynski, B., Tudzynski P. & van Kan J. A. L. (2007): Molecular Plant Pathology 8 (5), 561 – 580.
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