Citrus

Algal Leaf Spot

Cephaleuros virescens

Other

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In a Nutshell

  • Furry, green to orange spots on leaves.
  • Cracks in the barks of young stems.
  • Defoliation.
  • Distortion of fruit.
  • Increased infection on low hanging branches.

Can also be found in

4 Crops
Citrus
Coffee
Guava
Mango

Citrus

Symptoms

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.

Recommendations

Organic Control

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.

Chemical Control

Always consider an integrated approach with preventive measures together with biological treatments if available. Use fungicidal sprays containing copper if chemical control is necessary.

What caused it?

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.

Preventive Measures

  • Ensure good ventilation by orientating the rows appropriately.
  • Improve growing conditions to minimize plant stress.
  • Avoid waterlogged soils through improved drainage.
  • Irrigate early in the day to ensure rapid drying of the foliage.
  • Avoid overhead irrigation whenever possible.
  • Add mineral fertilizer when growth is poor.
  • Apply sprays containing potassium phosphate.
  • Ensure that plant density allows air circulation for rapid drying of leaves and fruits.
  • Remove weeds from around the trees to reduce competition for nutrients and humidity.
  • Avoid lesions caused by mechanic tools.

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