Grey mould caused by the fungus, Botrytis cinerea, is a chronically occurring disease that affects greenhouse crops worldwide. Fortunately, growers have access to a variety of tools for managing this commonly occurring disease. Such tools include chemicals and a variety of cultural controls where most emphasis should be placed because grey mould is best managed in a prophylactic or preventive manner. Understanding how these tools should be used and applying this knowledge during crop production provide the best defence against this disease.

Chemical Controls

Products registered in Canada for use on one or more greenhouse vegetable crop against B. cinerea include Rovral®(iprodione), Decree®(fenhexamid), Pristine®(pyraclostrobin + boscalid) and Prestop®(Gliocladium catenulatum).

Rovral

This is a contact fungicide with locally systemic or translaminar properties. This means that the fungicide penetrates the leaf tissue and moves from the treated surface to the opposite untreated surface. It has both protectant and eradicant properties and affects all phases of development in the life cycle of the fungus. This fungicide is active against both spores and mycelium (cottony mass that spreads and produces spores) of the fungus. However, it is more active against growth of the mycelium than germination of spores. Although this fungicide does have post infection activity and can be applied to infected tissue still in the early stages of development, it is best applied preventatively to protect uninfected wounds.

Decree

This is primarily a protectant fungicide with locally systemic activity. It prevents development of the fungus by inhibiting spore germination, germ tube elongation, and growth of the mycelium. Decree will also provide post-infection activity if applied very early in the disease cycle of the fungus. This fungicide is most effective when applied before infection by B. cinerea occurs.

Pristine

Pristine also has translaminar properties and provides a long period of residual protection. This product works on the plant surface to prevent spore germination and penetration, and within the tissue to prevent growth of mycelium. Pristine combines the activity of two fungicides, pyraclostrobin and boscalid which act by disrupting different parts of the energy producing system of the fungus. Together, they serve to inhibit fungal growth by depriving the cells of an energy source and of building blocks required for essential cell components.

Prestop

As indicated in an earlier article in which Prestop was discussed in relation to Pythium, the active ingredient in Prestop is a fungus, Gliocladium catenulatum strain J1446. Gliocladium catenulatum works by suppressing other fungal pathogens by two main mechanisms, competition and parasitism. Gliocladium catenulatum competes with other fungi for nutrients and living space, thereby preventing them from successfully establishing on the plant. It is also able to parasitize competing fungi, resulting in breakdown of their cell walls. This product is best used preventively.

Cultural Controls

Such measures are based on prevention and early detection. Careful monitoring of the crop is one exercise that should be practised at least weekly to facilitate early responses. In addition to monitoring, and perhaps even more importantly, are preventive measures which include the following:

Practising good sanitation throughout crop production

Note that all plant debris left lying around serve as very good breeding sites for B. cinerea. All infected sporulating tissues are a source of invisible "spore clouds" that are efficiently spread throughout the greenhouse by any air movements, particularly during workers' activities. A thorough cleanup at the end of the crop cycle is also necessary because B. cinerea produces survival structures called sclerotia (see Fig 1 and 2) that can live up to about 20 years in dried plant material.

Fig 1: Black, hardy sclerotia of Botrytis cinerea embedded within tissue of tomato stem

Fig 2: Botrytis cinerea sporulating on infected tomato stem

Managing the environment to maintain optimum vapour pressure deficits

Leaf wetness for 2 to 3 hr is all that's needed for infection by B. cinerea. The transition from night to day temperatures is particularly crucial because of the high risk of dew formation. To avoid this, it is best to attain day temperatures about 3 hr prior to sunrise to ensure that plants are warmed up prior to sunrise, at which time air temperatures will begin to increase. Because warm air holds more moisture than cool air, the extra moisture in the air will condense on plants that are at a lower temperature, thereby providing perfect surfaces for infection. Generally, the VPDs or vapour pressure deficit values should be closely monitored to ensure that they are not too low because this will be an indication of high humidity conditions. Good air-circulation around the base of the plants also helps to keep those areas dry.

Avoiding lushness by appropriate management of nutrition and defoliation

Nitrogen and calcium levels are particularly important. Growers should provide nitrogen levels that are sufficient for healthy growth without causing excessively soft growth. Adequate calcium is necessary for strong cell wall formation and therefore for general tissue strength to better resist infections. Timely defoliation also helps to avoid having too dense a canopy and its accompanying high humidity in the immediate environment around the stems and maturing fruits.

Timely completion of any activities that cause wounding

It is best to complete desuckering and defoliation activities sufficiently early in the day to facilitate drying of the wounds before sunset. If wounding occurs late in the day, the moist cut surfaces can collect spores which will be sucked into the stem when transpiration resumes the next day. Such spores can remain hidden without causing infection until after about 10 to 12 weeks when conditions in the plant favour their development.

Appropriate use of fungicides according to their properties

Most fungicides are best used early because once the fungus is deep in the tissues, particularly stem tissues, it is difficult to reach with fungicides. Deeply infected tissues are best removed and properly disposed of prior to treatment with a fungicide. Rotation of fungicides is also recommended to minimize or delay the development of resistance in the pathogen. As with use of all pesticides, labels should be carefully read prior to use for information on rates and use patterns.

Acknowledgements

Assistance in the review of this article by Mark McLear of Arysta LifeScience, Andrew Dornan of Bayer CropScience, and Scott MacDonald of BASF is gratefully acknowledged.