Diseases of Lucerne - 2: Fungal Leaf Diseases
Note Number: AG0727
Published: November 1999
Updated:
This Agriculture Note provides information on the importance, symptoms, and biology of pepper spot, common leaf spot, Stemphylium leaf spot, Stagonospora leaf spot, downy mildew and rust in lucerne.
Pepper spot
Importance:
Pepper spot, caused by the fungus Leptosphaerulina trifolii, is one of the most common diseases on lucerne, but usually does not cause serious herbage losses. When severe, this disease can reduce both yield and quality, especially crude protein content. The greatest damage in lucerne occurs on young regrowth after hay cutting.
Symptoms:
Small, pin-point size brown spots surrounded by a pale halo, develop on the leaflets. These enlarge and acquire a tan centre with an irregular brown border. When severe large numbers of spots may kill the leaves.
Biology:
Survival: The fungus carries-over in its perfect stage, in fruiting bodies (perithecia), on infected plant debris and is said 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 lucerne, white clover, red clover, subterranean clover and medics.
Control:
Foliar sprays for lucerne are registered in some Australian states, but may not be economic on other pasture legumes.
Common leaf spot
Importance:
Common leaf spot caused by the fungus Pseudopeziza medicaginis, is a common disease of lucerne that does not kill plants, and rarely causes serious economic losses. When severe though, plants can be defoliated, reducing vigour, hay quality and yield.
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 medicaginis occurs on lucerne and medics.
Control:
If a 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. Sprays have been effective in controlling the disease in the USA, but there are no chemicals registered for such use in Australia.
Stemphylium leaf spot
Importance:
Stemphylium leaf spot, caused by the fungus Stemphylium botryosum, is a common disease of lucerne which can cause defoliation when severe. This leaf spot often occurs in association with other foliar diseases.
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. Stems can be blackened.
Biology:
Survival: The fungi are carried-over on infected plant debris (leaves and stems), 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 botryosum occurs on lucerne, clovers, lupin, broad bean and tomato.
Control:
Foliar sprays have been registered in some Australian states, but usually no control measures are necessary. Hay crops should be harvested as early as possible to reduce losses through leaf fall.
Stagonospora leaf spot
Importance:
Stagonospora leaf spot, caused by Stagonospora meliloti, is of minor importance and often occurs in association with other foliar diseases.
Symptoms:
The leaf spots are large (5-6 mm diam.), often occurring at the tips, have diffuse margins with a bleached centre with small light brown dots, fruiting bodies (pycnidia). Infected leaves drop soon after lesions form. Stems and tap roots may also be affected by this disease (See Diseases of lucerne-3).
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 carried by irrigation or rain water to near-by plants.
Host Range: Stagonospora meliloti occurs only on lucerne 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 can cause infected seedlings sown in autumn to die or lose vigour. Yield losses are usually restricted to the first cut.
Symptoms:
Young foliage at the tip are 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 develops, 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 crown buds and crown shoots. Seed transmission although reported is thought to be unimportant.
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 lucerne, white clover and red clover.
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.
Rust
Importance:
Rust, caused by the fungus Uromyces striatus var medicaginis, is a very common disease of lucerne, with severe outbreaks capable of causing losses through defoliation, and nutritive value. The nitrogen fixing capacity of affected plants is also reduced. The disease is most damaging on seedlings and in stands shut up for seed or hay.
Symptoms:
Reddish-brown pustules, surrounded by a yellow halo occur on the underside of leaves, on stems and petioles. When severe, leaves wilt and fall prematurely.
Biology:
Survival: Uromyces striatus spores and/or fungus mould (mycelium) carry-over in live plant tissue to initiate new infections when conditions suit.
Environmental conditions: Warm moist weather favours rust development on lucerne.
Dispersal: Rust spores are spread by wind and rain splash.
Host Range: Uromyces striatus occurs on lucerne, medics and sweet clover.
Control:
Graze or cut for hay to reduce losses and humidity within the stand. This practice also removes rust inoculum from the paddock. Some cultivars are more resistant than others to rust infection.
Acknowledgement
This Information Note was developed by Rod Clarke, November 1999.
