Cassava Mosaic Virus - Environment

Environment

The severity of cassava mosaic disease is impacted by environmental factors such as light intensity, wind, rainfall, plant density and temperature. Given that the viruses are transmitted by whitefly, the spread of the virus is going to depend largely on the vector. Temperature is the most important environmental factor controlling the size of the vector population. In the literature vector preferred temperature estimates vary from 20 to 30°C to 27 to 32°C but generally high temperatures associated with high fecundity, rapid development, and greater longevity in whitefly. Increased light intensity has also been shown to increase activity of the whitefly vector.

Whitefly can fly at speeds up to 0.2 mph, but in high wind conditions they can move much greater distances in a shorter period of time thus increasing rate of virus spread. This wind dependent spread is reflected in the location of the whitefly in cassava fields, with populations greatest in upwind boarders and lowest within the field.

Virus incidence increases when cassava is growing vigorously. Thus, plant density impacts the spread of the virus, with low density fields encouraging faster disease propagation than high density fields. In dry areas, rainfall can be a limiting factor for cassava growth so higher rainfall will be associated with higher incidence of disease. Populations of whitefly will increase with rainfall, but heavy rains may impede whitefly spread and thus decrease incidence of virus. Timing of planting can also play an important role in the severity of disease, with cassava planted in march showing a 74% incidence rate of CMV, compared with 4% in August. Seasonal distribution of the virus will vary with the climate. In tropical rain forest type climates, where it is wet and humid most of the year, rapid virus distribution of the virus occurred from November to June, and slow progress occurred from July to September. This timing correlated with higher and lower temperatures. In a study of the disease in the Ivory Coast of Africa, maximum rate of disease spread was reached by two months after planting. Little to no infection occurs after three months, and variation in spread was due to varying in temperature, radiation and population levels of whitefly.