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Diseases of white clover - 3: leaf spot diseases | AG0733 |
Rod Clarke, Knoxfield
November, 1999 |  |
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Pepper spot
Importance:
Pepper spot, caused by the fungus Leptosphaerulina trifolii, is one of the most common diseases of white clover, but usually does not cause serious herbage losses. When severe, this disease can reduce both yield and quality, especially crude protein content. Oestrogen levels in the leaves can rise which may affect animal reproduction. This disease commonly occurs from late autumn to early spring.
Symptoms:
Round, brown to black spots with darker margins, up to 2 mm in diameter, occur 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.
Biology:
Survival: The fungus carries-over in its perfect stage, in fruiting bodies (perithecia), on infected plant debris and is reported to be seed-borne.
Environmental conditions: Pepper spot is favoured by cool moist weather.
Dispersal: Infected crop debris can be wind-blown over short distances. Under moist conditions ascospores are ejected from the fruiting bodies and dispersed by wind and rain.
Host Range: Leptosphaerulina trifolii has been recorded on pasture legumes such as lucerne, white clover, red clover, subterranean clover and medics.
Control:
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.
Common leaf spot
Importance:
Common leaf spot, caused by the fungus Pseudopeziza trifolii, is a common disease of pasture legumes, occurring in late autumn and winter. This disease does not kill plants, and rarely causes serious economic losses. When severe however, plants can be defoliated, reducing vigour, hay quality and yield. Oestrogen levels in the leaves can rise which may affect animal reproduction.
Symptoms:
Round, brown to black spots with darker margins, 1 to 3 mm in diameter occur across the upper leaf surface. In the centre of the upper surface spots, a minute light coloured jelly-like fruiting body, apothecia, develops. Infected leaves cup, turn yellow, then brown before falling to the ground. Elongated spots occur on stems and petioles.
Biology:
Survival: The fungus carries over on infected plant debris on the soil surface.
Environmental conditions: Common leaf spot develops under cool moist conditions.
Dispersal: Infected crop debris can be wind-blown over short distances. Under moist conditions ascospores are ejected from the fruiting bodies and dispersed by wind and rain.
Host Range: Pseudopeziza trifolii occurs on white clover, red clover, subterranean clover and other Trifolium spp. Kidney vetch and birdsfoot trefoil have also been reported as hosts.
Control:
If stand is to be cut for hay, this should be done as soon as possible to minimise losses from leaf drop and build up of spore numbers.
Black/sooty blotch
Importance:
Black or sooty blotch, caused by the fungus Cymadothea trifolii, is a relatively common disease of pasture legumes, but rarely causes losses. If severe, yields are affected through stunting and partial defoliation of the plants. Horses, cattle, and sheep have been reported to have shown toxic effects and deaths after grazing infected stands. Oestrogen levels in the leaves can rise which may affect animal reproduction.
Symptoms:
Dark-green to black, textured, angular spots up to
1 mm in diameter occur on the undersides of the leaves, and if severe almost cover the entire surface. Infected leaves gradually dry and become brown, leaflets often assume an upright position as they wither.
Biology:
Survival: The fungus carries over on infected plant debris and has been reported to survive in the soil for five years.
Environmental conditions: This disease is most prevalent in low, wet areas especially during late summer and early autumn.
Dispersal: Spores are spread by wind and rain splash.
Host Range: Cymadothea trifolii occurs on a wide range of pasture legumes including white clover, red clover and subterranean clover, lucerne and medics.
Control:
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.
Stemphylium leaf spot
Importance:
Stemphylium leaf spot, caused by the fungi Stemphylium spp., occurs on a range of pasture legumes, but does not cause serious losses in white clover. This disease often occurs in association with other foliar disease.
Symptoms:
The leaf spots are oval or irregular, slightly sunken, and dark brown with lighter centres and surrounded by a pale yellow halo. Older spots may be concentrically ringed, resembling a target. A single lesion can cause a leaflet to yellow and fall.
Biology:
Survival: The fungi are carried-over on infected plant debris, seed and in the soil.
Environmental conditions: Stemphylium leaf spot may develop under warm moist conditions.
Dispersal: Spores of these fungi are spread by wind and rain splash.
Host Range: Stemphylium spp. occur on clovers, lucerne, lupin, broad bean and tomato.
Control:
Hay crops should be harvested as early as possible to reduce losses through leaf fall.
Stagonospora leaf spot
Importance:
This is a minor disease of white clover, caused by the fungus Stagonospora sp. It often occurs in association with other foliar diseases.
Symptoms:
The leaf spots are large (5 to 6 mm diameter), often occurring at the tips, and have diffuse margins with a bleached centre with small light brown dots, fruiting bodies (pycnidia). Infected leaves drop soon after lesions form.
Biology:
Survival: The fungi carry-over in the fruiting bodies (pycnidia) on infected plant debris and lower stems.
Environmental conditions: Stagonospora leaf spot may develop under warm (20 to 25°C) wet conditions.
Dispersal: Under wet conditions, spores ooze from the fruiting bodies and are rain splashed or carried by irrigation or rain water to near-by plants.
Host Range: Stagonospora sp. occurs on lucerne, white clover and sweet clover.
Control:
Due to the minor importance of this disease, no specific control measures are recommended.
There are no resistant cultivars.
Downy mildew
Importance:
Downy mildew, caused by the fungus Peronospora trifoliorum, rarely causes serious damage in white clover. Infected seedlings sown in autumn may die or lose vigour.
Symptoms:
Young foliage at the tip is usually the first to be affected. The leaflets become light green and the margins curl downwards. The light purple to grey colouring and downy growth which characterise this disease are present on the undersides of affected leaflets.
As the disease progresses, the leaves may develop a scorched appearance. Young stems can also be attacked, resulting in wilting and death of shoots.
Biology:
Survival: The fungus carries-over in infected plant material.
Environmental conditions: Downy mildew develops under damp humid conditions.
Dispersal: Spores on the undersides of the leaves are dispersed by air movement.
Host Range: Peronospora trifoliorum occurs on white clover, red clover and lucerne.
Control:
Keeping pastures short, or cutting for hay 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.
Powdery mildew
Importance:
Powdery mildew caused by the fungus Erysiphe trifolii occurs on most pasture legumes but rarely causes significant losses.
Symptoms:
This disease is evident on the leaves as a light grey, superficial, powdery fungal web on the upper surface. When infection is severe, leaves turn yellow and then brown.
Biology:
Survival: The fungus persists within the stand, in small amounts, usually on the lower leaves.
Environmental conditions: Powdery mildew is favoured by warm dry conditions.
Dispersal: Spores on the upper surface of the leaves are detached and carried by air movement to infect other plants.
Host Range: Erysiphe trifolii occurs on white clover, red clover, lupins and trefoil.
Control:
Keeping pastures short, or cutting for hay 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.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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