Fungi

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Common leaf spot

Cause

• The fungus Pseudopeziza medicaginis.

Occurrence

• Although this disease is common between autumn and spring, it has only a minor impact.

Symptoms

• Small, pinhead-sized, brown-black spots often completely cover leaflets and cause yellowing and leaf fall. Raised fruiting structures develop in the centres of spots. Spots may also be present on the stems and leaf stalks.

Infection and spread

• The fungus survives as spores in the fruiting structures on undecomposed residues. In spring and autumn the spores are released and spread by rain and irrigation. Vigorous stands are unlikely to be severely affected.

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Stemphylium leaf spot

Cause

• The fungus Stemphylium vesicarium.

Occurrrence

• This common, occasionally serious disease occurs in autumn and winter. Most highly winter-active lines are severely affected.

Symptoms

• There are two types of symptoms in Queensland. The cool-temperature biotype produces elongated spots with bleached centres and sharply defined dark borders. The warm-temperature biotype produces oval spots with dark centres and brown borders; these lesions grow to form a concentric pattern.

Infection and spread

• The fungus survives in dead stem and leaf material. Spores produced in fruiting structures are spread by wind, rain and irrigation.

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Leptosphaerulina leaf spot, or pepper spot

Cause

* The fungus Leptosphaerulina trifolii.

Occurrence

• This is a common, often severe disease present throughout the year but more severe over winter. No lucerne cultivars are resistant.

Symptoms

• The disease starts as small, black, ‘pepper’ spots which usually enlarge to oval spots 1-3 mm wide with lightbrown centres and darker brown borders. Under ideal conditions large, bleached areas develop, giving leaflets a burned appearance.

Infection and spread

• Fruiting bodies of the fungus develop on fallen infected leaves. During cool, moist weather the spores are discharged and spread by wind onto lucerne foliage.

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Rust

Cause

• The fungus Uromyces striatus.

Occurrence

• This is a common, but usually minor, disease occurring in summer. Most varieties are affected.

Symptoms

• Reddish-brown spore masses rupture through the new leaves, leaf stalks and stems. The pustules may be single or arranged in small circles. When severely affected, leaves shrivel and fall prematurely.

Infection and spread

• The fungus survives as spores, or vegetatively in plants, spreading by wind-blown spores during warm seasons. The disease can build up rapidly under the right conditions, but symptoms pass quickly once the weather changes.

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Downy mildew

Cause

• The fungus Peronospora trifoliorum.

Occurrence

• This is a common, occasionally severe disease, occurring from autumn to spring.

Symptoms

• Upper leaves and shoots are attacked first, with leaves becoming yellow, twisted and rolled. On the underside of leaves, a pale-violet, downy growth is often visible. Infected shoots often become dwarfed and yellow.

Infection and spread

• The fungus survives as resting spores in diseased tissue or vegetatively in shoots and crown buds. The fungus is spread as spores (produced on the downy growth) by wind and splashing rain. Spores germinate only in free water.

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Cercospora leaf spot

Cause

• The fungus Cercospora medicaginis.

Occurrence

• This minor disease is common in late summer and autumn.

Symptoms

• Leaf spots are 2-6 mm wide, roughly circular and light reddish-brown with a yellow halo. Severely affected leaves yellow and fall prematurely.

Infection and spread

• Survival is in the vegetative form in fallen leaves. The fungus spreads by spores during wet, windy weather.

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