Although they are commonly found in Queensland, the severity of leaf and stem diseases depends on the particular cultivar, and the vigour and density of the stand, as well as the climate. These diseases reduce hay quality and quantity and, in severe cases, stunt and weaken the plants. Often, two or more pathogens affect individual plants simultaneously, making it difficult to determine the effects of each particular disease.

Three types of organisms are responsible for leaf and stem diseases — viruses, bacteria and fungi.

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Viruses

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Alfalfa mosaic virus

Occurrence

• This virus occurs rarely. It is more likely to appear in the cooler months.

Symptoms

• Most affected plants show no symptoms. When symptoms do occur, they vary from a faint leaf mottling to yellowing of the leaf tissue, together with a puckering and reduction in the size of new leaves. Severely affected plants die.

Infection and spread

•Common weeds such as milk thistle, burr medic and nightshade can be infected. The virus is spread among plants by aphids.

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Bacteria and phytoplasmas

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Bacterial leaf and stem spot

Cause

•The bacterium Xanthomonas campestris pv. alfalfae.

Occurrence

• This uncommon disease, occasionally occurring in summer, can be severe in young plants.

Symptoms

• First signs are circular, water-soaked areas that enlarge into definite spots 1-3 mm wide with tan centres and yellow margins. The spots merge, causing the leaflets to become papery and finally to fall. On stems, the spots are greasy and circular, often with a purple margin.

Infection and spread

•Other legumes, eg. burr medic, crimson clover, lablab and mungbean, can be infected. The bacterial cells are spread during wet, humid conditions.

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Witches broom

Cause

• A phytoplasma-like organism.

Occurrence

• This is a common, occasionally severe disease.

Symptoms

• Infected plants are severely stunted and appear bunchy due to the development of many short, spindly shoots from the crown and buds along the stems. Leaves become small, rounded, puckered and pale green. Flowers are rare, and are distorted and pale green, with leaf-like structures replacing floral parts. The disease also reduces plant vigour.

Infection and spread

•The organism is transmitted by a small, pale-green leafhopper, and symptoms may take several months to appear. Infected plants may appear to recover in cool periods, but symptoms recur with high temperatures.

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Control of leaf and stem diseases

• Use the recommended sowing rates and maintain good weed control to allow free movement of air through the stands, and to lower humidity. Ensure the stand is vigorous. When leaf or stem diseases are serious, cut the stand and remove the hay. This reduces the source of infection and minimises spread.

• Rovral® liquid fungicide at 0.5-1 L/1000 L/ha is registered currently for the control of Stemphylium and Leptosphaerulina leaf spots. Use at the high rate on early growth (10-14 days old) and follow up with the low rate 10-14 days later. Withholding period to cutting or grazing is seven days.

• Where stemphylium leaf spot is a problem, use resistant cultivars. In general, the greater the winter activity of a cultivar, the more susceptible it is to infection.

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