Guideline Index

Chapter 2: Limits to Plant Growth

2.7 Plant disease and insects

Pests and diseases can severely impact pastures and contribute to pasture rundown. Crops and pastures should be inspected regularly for pests as well as disease and deficiencies. A combination of biological, chemical and cultural control measures to control pests are considered the most effective and sustainable approach to addressing pest problems.

Insect pests and diseases affect pastures at all stages from germination of the seed (e.g. seedling blight or damping off disease caused by a fungus) through to the final leaf and stem (e.g. Rust on ryegrass, also caused by a fungus – see Figure 2.3).

Figure 2.3  Pustules of crown rust on ryegrass leaves.   (Source: )
Figure 2.3 Pustules of crown rust on ryegrass leaves.
(Source: )

Diseases can cause a significant loss of pasture production as well as effect forage quality. A diseased plant will also be more susceptible to other environmental stresses. Depending on what has caused the disease (fungus, virus, bacteria, nematodes or even another insect) will determine where it occurs on the plant. Generally plant diseases can be classified by the area affected such as those that affect the seedlings, leaf and stem, and root (see Figure 2.4) and crown.

Figure 2.4  Phytophthora root rot in Lucerne (Source:
Figure 2.4 Phytophthora root rot in Lucerne

The effects of insects on pastures will vary. Some can be of minor importance, especially in the case of some insect pests in lucerne. As a perennial crop, lucerne harbours all kinds of insects including those that are pests, predators or beneficials. For example, an insecticidal approach for the control of pests such as heliothis or lucerne leaf roller, could be unnecessary due to the natural control by other insects or virus. Insect control can be an option when insect numbers reach certain populations or thresholds. Thresholds are based on the potential economic damage, the costs to control and the likely yields to be gained from insect control. Monitoring insect populations during susceptible times of the year will prevent needless sprays, for example in the warmer months for aphids present in lucerne.