Blumeria graminis f. sp. tritici
|Scientific Name||Blumeria graminis (DC.) E.O. Speer f. sp. tritici Em. Marchal|
|Common Names||Powdery mildew of wheat|
The white, fluffy, later grey-tan colonies of powdery mildew are easily detected on leaves stems and ears, although the upper surface of leaves are most commonly infected. Before the development of white patches producing masses of conidia and causing the powdery appearance chlorotic flecks on the plant tissue appear as initial symptoms. Cleistothecia may be formed occurring late in the season and found embedded in the mildew colonies as dark-coloured dots. Under favourable conditions, sporulation starts within 7-8 days after infection. The disease affects flowering, fruiting, seedling and vegetative growing stages of the plants. Conidia are dispersed within the crop by the wind. Epidemics will tend to occur during conditions of alternating wet and dry weather.
Powdery mildew is caused by Blumeria graminis, an ascomycete belonging to the Erysiphales. It is an obligately parasite specific to wheat and overwinters mainly as mycelium on autumn-sown cereals. Although cleistothecia releasing ascospores produced during the late summer are resistant to cold and drying out, they appear to be of secondary importance in overwintering and as a source of inoculum in the spring. With rising temperatures in spring, conidia are produced rapidly on dormant mycelium beginning with growth. Conidia usually germinate over a wide range of temperatures from about 5 ° - 30 °C, although 15 ° - 20 °C is probably optimal, together with a relative humidity of about 95 %. Free water inhibits conidial germination.
The mycelium is hyaline, branched and conidia are borne in chains on bulbus swelling conidiophores arising from the mycelium. Conidia are hyaline, oblong to ovate, 25-40 x 8-10 µm. After germination the germ tube forms an appressorium on the host cuticle. A hyphal peg penetrates the cuticle and the cell wall and forms a digitate haustorium in the epidermal cell. Mycelium is developed only on the surface of the plant. The globoid cleistothecia are immersed in the mycelium. They are 135-250 µm in diameter with simple appendages, and develop 8 - 25 asci, which are usually 8-spored and 70-108 x 25-40 µm. Ascospores are elliptic, hyaline to pale brown, and 20-23 x 10-13 µm. Multiple races of the fungus exist and new ones continue to be formed as a result of genetic recombination. Besides the sexual cycle contributing to diversity, the disease is polycyclic and well adapted for aerial dispersal and long distance transport
Integrated Crop Management
Powdery mildew is one of the most common and destructive diseases of cereals widely distributed in wheat growing areas throughout the world. Losses up to 45 % have been documented. Actual losses depend on the time of disease epidemic onset and its severity resulting in the reduction of head numbers and kernel weight. The disease is encouraged by excessive use of nitrogen fertilizer and by very early autumn sowing. In the autumn, heavily infected plants may be less resistant to winter frosts. Host-plant resistance is important in the control of powdery mildew on cereals.
Isolation of autumn-sown and spring-sown cereals will reduce the risk of infection of the autumn-sown crop spreading to the spring-sown crop. Computer-based decision support systems have been developed to aid fungicide spray recommendations for the main diseases on wheat including powdery mildew. Alternation of different types of fungicides or combined applications can reduce the risk of fungicide resistance development
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