Desscription: Citrus leaf spot is caused by a fungus called Pseudocercospora angolensis (Phaeromularia angolensis). The disease affects plants in the citrus family including grapefruit, pummelo, limes, mandarins, and rough lemons. The plant parts affected include leaves, fruits and rarely stems. Leaf lesions produce more conidia than those on fruits thus this may be the main source of infection across seasons.
The fungus is transmitted by wind, water splash (during rainfall or irrigation) and also by humans through infected seedlings.
Grapefruit, orange, pummelo, and mandarin are very susceptible, whereaslemon and lime are less susceptible. The brownish/black spots lower the quality of the fruit making them unmarketable.
Disease/Pest Category : Continuous
- Greenish yellow spots on leaves which later turn brown with a yellow halo.
- On fruits brown circular spots with a yellow halo form on young fruit. The spots are usually sunken with raised circular edges giving the fruits a blister like appearance.
- Blister/tumor like growths on fruits.
- Spots on both leaves and fruits produce spores and become black during wet weather.
- Chlorosis/yellowing of leaves due to merging of spots.
- Defoliation (Leaf fall) occurs on severely affected plants.
- Dark brown lesions /spots on stems. These spots may merge causing stem dieback.
Conditions prevailing that contribute to success:
- Prolonged wet weather favors disease development and spread due to rapid production of fungal spores and transmission through splash.
- The disease is spread by air and may also be spread through splash or during overhead irrigation.
- Infected plant residues in the field serve as a source of future infections.
Conditions prevailing that contribute to failure
- Un‐favourable weather conditions that do not favor multiplication of fungal spores would contribute to failure. There is reduced multiplication of spores during dry weather as compared to during wet weather hence reduced spread.
The following management options are recommended:
1. Practise field sanitation by removing and destroying (e.g. by burning) infected plant parts and plant debris to reduce disease inoculum on the farm.
2. Orchards should be established using clean seedlings free from the disease.
3. Ensure crop vigor and good health through fertilizer and manure application.
4. Spray affected plants with a protective fungicide such as mancozeb or a copper based fungicide (such as copper oxychloride, copper sulphate or copper hydroxide) according to manufacturer’s recommended rates.
Ensure proper coverage of the plant during spraying.
Note: Agro‐chemicals should be used in consultation with professional practitioners and considering existing cautionary/safety measures, particularly the manufacturer’s instructions.
Geographic Coverage The disease is said to have occurred first in Angola and Mozambique before
spreading northwards to countries including Kenya. Distribution in Kenya has not been clearly documented but the humid parts of the country are most likely to be affected.
Sources of More Information (http://www.plantwise.org/KnowledgeBank/CountryHome.aspx)
Article courtesy of KALRO factsheet