Gray mould is so called because of the layer of gray conidia and conidiophores (fruiting structures) that develop on infected tissue. In general, Botrytis appears most often as blossom blight or rot on fruit. It's common manifestations in horticulture, however, are stem cankers and damping-off (where stems of the plant collapse at the soil line). Cankers usually begin at the base of the petioles of infected leaves. They can range between dark brown and black in colour, and have a water-soaked appearance. As the fungus develops, cankers become covered with fuzzy, greenish-tinted gray spores. Black sclerotia may also be present, causing the infected tissue to look like it has been dusted with pepper. Leaves may also have irregularly-shaped brown spots which start along their edges. Botrytis can also manifest as brown spots on white flowers, or white spots on coloured blooms. Tubers or bulbs may develop storage rot, which is characterised by deeply sunken, water-soaked tissue. Gray spores may or may not be present in these areas as well.
Botrytis primarily affects outdoor plants, particularly in areas with high humidity. Untreated wounds can make a plant even more vulnerable. Crowding and overhead watering also contribute, as can improper fertilisation practices. Like many Fungi, Botrytis is insidious in that it can spread not only by moving water, but also by even subtle air movement.
Upon identifying a Botrytis infection, immediately cut out and destroy affected tissues, in an area well away from other plants. Wounds should then be treated with a fungicide. Avoid benzimidazole (thiophanatemethyl) and dicarboximide (ipridione and vinclozin), as Botrytis has shown resistance to these products. Humidity is not only a causal factor, it can also have a decided impact on the severity of an outbreak. If it is at all possible to reduce humidity, do so.
Sanitation is also extremely important in controlling and preventing Botrytis. This fungus can survive on, and spread from, old pots, tools, gloves-even the floor. If you suspect Botrytis, sterilise EVERYTHING thoroughly with a disinfectant or 10:1 bleach solution. Protecting plants from exposure to dew and rain, especially in cooler temperatures, can help prevent a infestations in outdoor plants. For indoor plants, proper air circulation is a crucial preventative.
COLLETOTRICHUM SPP. (Gleosporum)
These species of Fungi are characterised by spots of light brown rot which increase in size and eventually turn black. Infected tissue may also show pinkish or orange coloured spores. It most frequently enters through wounds, and young plants are especially vulnerable. Again, this fungus is encouraged by high humidity, as well as high temperatures. Reducing moisture (humidity and watering) will help, as will a reduction in temperature, if possible. If plants are not too far gone, treat with a fungicidal drench. The Cacti most vulnerable to Colletotrichum are Cereus, Echinocactus and Mammillaria. Amongst the other succulents, Crassula, Sedum and Sempervivum are the most susceptible.
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