The chain of events that leads to the development of a disease is called the disease cycle – which may be different to the pathogen’s life cycle. The incidence and severity of the majority of plant diseases vary on a distinct cyclic basis. Each cycle includes two alternating phases; the parasitic phase and the survival or oversummering phase. The seasonal nature of the production of annual crops and the seasonal nature of climate are the main factors contributing to the cyclical nature of plant diseases.
Survival over summer is dependent on environmental conditions. Rhizoctonia and take-all survive well during dry summers when there is little break down of plant residues or competition from other organisms in the soil. Rusts survive during a wet summer because conditions suit the growth of host plants.
The first requirement for the establishment of a disease is for the pathogen to come into contact with the host. Disease inoculum is generated by previous infections and liberated into the environment; it may come from the same location or have travelled over great distances. Inoculum may be primary (resulting from infections in the previous season), or secondary (arising from infections in the same season).
Wind is the most important way in which fungal spores, (eg. rust spores), are disseminated over long distances. Water is important for some fungal pathogens (eg. Septoria) to spread, especially over short distances. For other diseases (eg. Rhizoctonia, take-all) the inoculum comes from infected plant debris remaining in the soil.
The size, distribution and genetic diversity of host populations are of great importance in determining the degree and rate of epidemic development. For example, the risk of major losses in our wheat crops would be high if all the cultivars relied on the same stripe or stem rust resistance genes.
A key component of disease control is accurate diagnosis and knowledge of the disease cycle for the particular pathogen. Disease management practices include:
- use of resistant varieties (see a current Cereal Disease Guide relevant to your region)
- crop rotation
- farm hygiene
- use of chemicals.
A particular control strategy needs to be considered in the context of other farm practices. There is no point recommending a control program that would cost more than the crop is likely to return to the grower.
Diagnosing the Causes of Plant Diseases