How do foliar disease outbreaks occur?

A range of winter pulse crops are grown in Australia, most of which can be significantly affected by foliar diseases. This post explains the significance of foliar diseases in winter pulse crops and how outbreaks occur.

The significance of foliar diseases in Winter Pulses

A range of winter pulse crops are grown in Australia, most of which can be significantly affected by foliar diseases.

The impact of foliar diseases is influenced by several key factors including location, rainfall, pre-existing disease inoculum levels, varietal tolerance, disease status of neighbouring crops and type of fungicide program employed.

The impact diseases will have on pulse crops can range from complete yield loss, if not well managed, to negligible affect where best practice tactics are employed.

Multi-pronged approaches to managing diseases are always preferred.

How do foliar disease outbreaks occur?

There are three factors that must be present for a foliar disease outbreak:

• A viable inoculum source
• A susceptible variety
• Favourable weather conditions.

Viable inoculum source

The inoculum (fungal spores) that initiates a disease epidemic is called the inoculum source.

The primary inoculum source will depend upon the disease and its ability to survive on either a living host (e.g. Botrytis Grey Mould) or on dead plant material such as stubble (e.g. Ascochyta Blight).

Volunteers and infected stubble host diseases and provide an inoculum source to initiate new infections in subsequent crops. 

A susceptible variety

Variety selection can help growers manage the risk of losses from foliar diseases. It can also help tailor fungicide applications according to the yield potential and level of disease risk. (See 'Variety Resistance Classification' table below).

Growers and agronomists should be aware that sensitivity shifts to diseases can occur quickly within varieties. A good example of this is the recent breakdown of disease resistance in chickpeas. A mutation in the Ascochyta Blight pathogen has resulted in no current chickpea variety having a rating above MS for this key disease.

Favourable weather conditions

Pathogens have temperature and leaf wetness requirements for spore germination, infection and porulation. At a suitable temperature and leaf wetness, spores germinate and infect the plant.

Once diseases have started to develop, the rate at which they progress depends upon the thermal requirements for each disease. Thermal time maps the accumulation of average daily temperatures across time and can be a useful way to assess likely disease progression and latent period.

Foliar Fungicide Guide for Winter Pulses

Consult the ADAMA Foliar Fungicide Guide for Winter Pulses for more information on crop disease management. If you haven't already downloaded the guide, click 'Download' below for your copy.