Algal Leaf Spot
· Zitrus · Mango
The parasitic alga C. virescens predominantly affects leaves of mango and other hosts, but may also target branches and stems. Infected leaves shows round, slightly raised, green to orange spots of 2-4mm diameter. They are characterized by furry growth (spores of the algae) and indistinct margins. They can merge into patchy areas. In young stems, which are more susceptible to the pathogen, C. virescens may cause cracks in the bark, leading to dieback. On many trees, the leaves of low hanging branches show the worst symptoms. Algal leaf spot occurs commonly in areas of high temperatures and rainfall, and in plants with impaired growth.
Algal leaf spot most commonly appears in locations with high temperatures and rainfall, and where the host plants are not growing well. Poor nutrition, poor soil drainage and too much or too little shade create conditions that favor the disease. The spores need water to germinate. They spread to other trees via splashing rain or wind. C. virescens take up its hosts' water and mineral salts, characterizing it as a ' water parasite'. The algal growth covers the leaves until they shed. Young superficial colonies may be washed off by frequent rains. Only those spores entering the leaves through wounds produce lesions. There is no evidence for penetrations of uninjured cuticles.
When the disease is mild, remove and destroy leaves with spots as well as diseased branches. Additionally, rake up and destroy affected leaves on the ground. When algal leaf spot is severe, spray a Bordeaux mixture or other copper-based products. Sprays should be applied every 2 weeks from the beginning of summer to the end of autumn.
Always consider an integrated approach with preventive measures together with biological treatments if available. Use fungicidal sprays containing copper if chemical control is necessary.