Fighting Botrytis in ornamental crops

Botrytis cinerea (grey mould) is one of the most widespread and destructive fungal diseases of ornamental crops. It infects over 200 plant species worldwide causing annual losses of $10 billion to $100 billion. Botrytis cinerea is responsible for the grey mould disease in a number of ornamental crops including roses, carnations, violet, begonia, chrysanthemum, gerbera, dahlia and geranium reducing both quality and yield. In ornamental crops, yield loss can be total because infected cut flowers are rejected at auctions.

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The majority of losses caused by B. cinerea in cut flowers occur in the packing house and during transport to the market. Botrytis cinerea is aggravated by latency of the petal infection, which may not present visible symptoms at the time of flower harvest.  Although flowers are handled at temperatures between 2 and 10°C after harvest, higher incidence of B. cinerea has been observed, mainly during transport and storage.

Botrytis cinerea is evident on plant surfaces as grey fluffy mycelium. Initially, the symptoms appear on infected flower petals as restricted lesions. Subsequently, these lesions become necrotic and spread to the whole petals and the receptacle, finally resulting in collapse of the flower head and petal drop reducing the market value of the flowers and reducing the vase life. Disease control can be difficult because the pathogen can infect several plant parts including flowers, stems, leaves and buds at pre and post-harvest stages.

Infections initiated by conidia can be completed in about eight hours, if high humidity and low temperature are available. A higher incidence of B. cinerea is usually observed during rainy seasons. This happen perhaps because temperature is normally low and relative humidity is high in this period of the year.

Botrytis cinerea can persist in greenhouses year round as mycelium, conidia or sclerotia on living or dead tissue and as sclerotia or conidia in infested soil. Infected plants or infected and decaying plant parts in the greenhouse form a source of inoculum. B. cinerea can thrive under a range of temperatures between 2 and 30^oC. The optimum temperatures for the different growth phases range from 12–30^oC. B. cinerea will therefore always be a potential threat in greenhouse crops. Spore dispersal is stimulated by rising or falling humidity. Temperature and relative humidity (RH) both have a direct effect on B. cinerea epidemics, but in greenhouses the effect of climate generally constitutes a combined effect of the two.

Management of Botrytis epidemics in greenhouse crops

Due to favorable weather conditions all year round, inadequate incorporation of integrated control measures, planting of highly susceptible varieties and increasing customer demand for good quality cut flowers among other factors can lead to serious economic losses. The problem with several ornamental crops is that no disease is found in the greenhouses, but symptoms become visible in the post-harvest phase due to more conducive conditions for B. cinerea during cold-chain marketing.

Controlling B. cinerea is commonly achieved with a combination of pesticide treatments, biological control agents and agronomic practices. Although very helpful, agronomic practices alone cannot prevent the disease in greenhouse crops, so chemical treatments and biological control agents should normally be used.

Sanitation

In all greenhouse crops, it is advised to remove old blooms, canes showing dieback and other B. cinerea-infected plants or plant debris immediately from the greenhouse within plastic bags, to prevent further progression of the fungus into the plant and the build-up of inoculum. Debris left in the greenhouse can be a potential source of initial inoculum the following season, thus emphasizing the importance of sanitation inside and outside greenhouses.

Greenhouse climate

In the greenhouse, humidity is controlled by heating, ventilation and irrigation practices. It is particularly important to avoid periods of free water on plant surfaces, because this allows B. cinerea spores to germinate. Periods of high humidity coincident with suitable temperatures favors the development of gray mold outbreaks in flower crops. Windows on the sides of the greenhouse can be opened during the production phase to control RH and provide good ventilation to prevent botrytis development. Canopy Wetness of > 4 h should be avoided.

Cultivar

In practice growers choose cultivars for their market value and end-user preference and not for their decreased susceptibility to diseases.  However, where possible, select cultivars which are known to be less susceptible than others to B. cinerea and are less problematic at the consumer level.  For roses, for instance, the susceptibility of a cultivar and the differences between different cultivars can vary due to different plant architecture and canopy density.

Plant spacing

Plant spacing influences the microclimate in the crop and the extent of direct contact between plants. In densely planted crop, grey mould incidences can be higher compared to more widely spaced plants. Always consider the breeder’s recommended plant spacing for optimal plant growth and disease management.

Cropping method

Pruning makes the microclimate in the canopy less conducive to B. cinerea, but causes wounds which may be infected by B. cinerea. Pruning wounds are less likely to become infected by B. cinerea when they are cut close to the stem instead of leaving a small fragment of the petiole on the stem.

Fertilizer

Fertilizer rates in soil-grown crops and the composition of the nutrient solution used in substrate-grown crops both influence host susceptibility to grey mould. High nitrogen rates enhance plant growth and foliage density. In general, increasing the nitrogen level is believed to result in increased susceptibility to B. cinerea.

Calcium is generally associated with reduced susceptibility to B. cinerea because it plays a role in the integrity of the cell wall and increases resistance to enzymatic degradation by B. cinerea. In roses grown in nutrient solutions to which Ca²⁺ was added, the Ca²⁺ content in the plant was raised and post-harvest botrytis blight severity was strongly reduced (Baas et al. 2000).

Chemical control

Chemical control remains the main way to reduce the incidence of B. cinerea in ornamental crops, but control necessitates preventive applications of chemical fungicides in order to achieve good efficacy and to minimize the risk of resistance development in B. cinerea populations.

Effective options for B. cinerea control are available either as preharvest sprays in the greenhouse or as postharvest dips of flowers to prevent disease development. Optimize spray volumes to achieve a sufficient spray deposition of susceptible tissue for good efficacy.

Several families of fungicides are available and can be classified according to their biochemical modes of action e.g. fungicides affecting fungal respiration; anti-microtubule toxicants; compounds affecting osmoregulation; fungicides whose toxicity is reversed by amino acids; and sterol biosynthesis inhibitors. Alternation of various groups of fungicides is recommended to avoid resistance development.

One effective chemical option against B. cinerea is Switch, which contains a combination of the anilinopyrimidine, cyprodinil, and the phenylpyrrole, fludioxonil. Treatments with Switch have demonstrated good efficacy and consistent control of both preharvest and postharvest botrytis in cut flowers.