A guide to identifying and managing diseases in your clover pasture

1. White clover mosaic virus (WCMV)

WCMV is common in clover pastures that are more than three years old and can significantly reduce dry matter yields. It also stunts the growth of clover and makes it less likely to thrive in a pasture sward.

Affected plants develop yellow markings along the veins. This is typically more noticeable on actively growing plants in spring and autumn. Affected plants may lack vigor but rarely die.

WCMV is typically transmitted by mechanical means such as grazing livestock, tractors and mowers. At present there are no effective control measures or WCMV resistant clover cultivars.

2. Alfalfa mosaic virus (AMV)

AMV can significantly reduce yields in clover stands that are older than two years. It causes an interveinal light green or yellow mottle on the leaves and a stunting of the plant.

Depending on the virus strain, clover cultivar and how cool the temperature is AMV can go unnoticed or even cause plant death.

Infected seed is the most likely primary source of infection. AMV is transmitted by a variety of aphids. At present there are no effective control measures or no AMV resistant cultivars.

3. Clover phyllody

This is caused by mycoplasma, a virus like organism. Affected clover plants are stunted, with yellowing of veins and bronzing of old leaves. In some cases, flowers get transformed into clusters of green leaves.

The clover phyllody mycoplasma is carried over by other hosts such as the leaf hopper. It does not transmit via seed.

It spreads mostly during hot dry summers. While spraying might help, there are no cost-effective measures to control this disease on a large scale.

1. Clover rot

Clover rot is caused by the fungus Sclerotinia trifoliorum. Outbreaks of clover rot are most frequent in irrigated pastures. It can significantly affect clover yields. Young seedlings are particularly susceptible.

This fungus causes tiny brown spots on the leaves. Over time these spots join together and the leaf withers off. The fungus can also infect the stems and shoots, eventually killing the plant.

A severely affected patch of pasture can quickly turn into a mass of rotting plant material. Dead leaves and stems can be covered by small, black, irregular, dormant resting bodies called sclerotia. This can lie dormant for up to 5 years and re-infest a clover field when conditions are right.

2. Grey mould

Grey mould is caused by the fungus Botrytis cinerea. It is a minor disease of white clover which affects scattered plants, but never a whole stand.

Cool moist conditions and lush growth, favour the development of grey mould. They are typically transmitted by wind. This fungus can also survive on infected plant debris, seed and as resting bodies (sclerotia) in the soil.

3. Wart disease

Wart disease is caused by the fungus Physoderma trifolii when excessive moisture is present. The disease usually occurs in wet areas, such as river flats or in poorly drained soils.

Small swellings (or galls) develop on all parts of the plant. The galls eventually join and causes twisted leaves, stems and flowers.

Under excessively wet conditions the galls decay and the resting spores release zoospores which move to infect nearby plants.

4. Common leaf spot

Common leaf spot is caused by the fungus Pseudopeziza trifolii. As the name suggests, it is a common disease of pasture legumes, occurring in late autumn and winter.

The disease does not kill plants, and rarely causes serious economic losses. When severe however, plants can be defoliated, reducing hay quality and yield.

Round, dark spots occur across the upper leaf surface. Infected leaves turn yellow, then brown and finally wither to the ground. Elongated spots can also occur on the affected stems.

The fungus carries over on infected plant debris on the soil surface. Infected crop debris and spores can propagate via wind over short distances.

5. Pepper spot

Pepper spot is caused by the fungus Leptosphaerulina trifolii. It is quite similar to the common leaf spot disease and occurs in clover between late autumn and early spring, as it prefers cool moist weather conditions.

It usually does not cause serious herbage losses. However, when severe, this disease can reduce both yield and quality, especially crude protein content.

Affected plants develop round, dark spots on the leaves, stems and runners. The spots are usually confined to the upper leaf surface, and may be so numerous that the leaves appear covered in soot. Severe infections can cause the leaves to cup, turn brown and die prematurely.

Both infected plant debris and seed can further propagate this disease.

Keep pasture short; this will inhibit disease development by lowering humidity within the stand, and reducing the level of infected material which could carry-over the disease from season to season.

6. Black or sooty blotch

Black or sooty blotch, is caused by the fungus Cymadothea trifolii. It is a relatively common disease of pasture legumes, but rarely causes losses. This disease is most prevalent in low, wet areas especially during late summer and early autumn.

Severe cases may affect yields as it stunts and partially defoliates plants. In some cases, horses, cattle, and sheep have been reported to have shown toxic effects and deaths after grazing infected stands.

Infected plants develop dark-green to black spots on the undersides of the leaves. Infected leaves gradually dry, become brown, and wither.

The fungus carries over on infected plant debris and has been reported to survive in the soil for up to five years. Spores are spread by wind and rain splash.

Keeping pastures short will help inhibit disease development by lowering humidity within the stand, and reducing the level of infected material which could act as a carry-over source.