Crop diseases are as old as agriculture itself and have often caused hardship for people across the world. Early literature refers to the Romans sacrificing a red dog every year to appease the rust god Rubigus, hoping to ensure a rust free harvest. During 1845-52 potato blight in Ireland caused the Great Famine and was estimated to have resulted in one million deaths and caused mass emigration. More recently the stem rust strain Ug99 has caused international concern as it has spread through Africa and beyond.
On a more local scale, Ascochyta blight severely impacted Wimmera chickpea crops in the late 1990s and brought hardship to many farmers. With, the development of good disease management strategies and cultivars with improved resistance the chickpea industry has since recovered.
Due to their ever changing nature diseases remain a constraint to field crop production, but fortunately modern agriculture is able to minimise the impact of diseases on production through the use of better knowledge and the implementation of integrated control strategies.
Within this manual the general principles of plant pathology and disease identification and control are addressed for the important field crops grown in Victoria.
What is a Disease?
Technically a disease can be defined as any deviation from what may be considered ’healthy’. These broad definitions include diseases caused by environmental or physiological factors as well as living organisms, hence diseases are generally split into two categories; biotic and abiotic. This manual will focus on biotic diseases which are caused by living organisms. Abiotic diseases are caused by non-living factors (e.g nutrition and moisture stress) and are commonly referred to as disorders and will not be a focus of this manual.
In terms of biotic crop diseases, a plant becomes diseased when it is successfully attacked by a living organism (i.e. a pathogen) and symptoms appear. For this to occur the pathogen must overcome the resistance mechanisms the plant has and compromise it at a cellular level resulting in symptoms. These symptoms allow for identification and management of the disease.
Diseases can affect many different parts of the plant and this influences the symptoms that appear. Diseases that affect roots and stem bases cause damage by interfering with water and mineral absorption from the soil. Diseases that cause leaf death will directly affect a plants ability to produce sugars via photosynthesis. Other diseases (e.g. systemic virus diseases) impair translocation of sugars produced during photosynthesis to the grain and diseases such as bunts and smut can destroy developing grains.