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 improved 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 (living) and abiotic (non-living) threats. This book focuses 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 book.
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 any resistance mechanisms a plant has and compromise it at a cellular level resulting in development of disease 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. This can result in wilting or symptoms of nutrient deficiency. Diseases that affect leaves can cause leaf death. This directly affect a plants ability to produce sugars via photosynthesis. Other diseases (e.g. systemic viral diseases) impair translocation of sugars, produced during photosynthesis, to the grain which can produce shriveled grain, and diseases such as bunts and smut can destroy developing grains. Not all symptoms are visual, diseased plants can have reduced grain protein as the diseases prevent the translocation of sugars in the plant.
Pathogen vs Disease
In plant pathology the terms pathogen and disease are used regularly, and it is important to understand the different meanings of these two terms. The pathogen is the organism that causes the disease. For example, the fungal pathogen ‘Puccinnia striiformis’ causes the disease ‘Stripe rust in wheat’.
The disease is the effect of the pathogen on the plant or what we often see as symptoms. We can have a pathogen present, but not have disease develop (for example in a resistant variety, or if conditions are not suitable for disease development). However, we cannot have disease without the pathogen.
