I am hoping you can help me with a problem we have with young immature (green) fruit bearing on our citrus (late naval oranges). The problem is we have small brownish indented spots developing on the skins which looks similar to scale however have been told it could be winter spot (which we have never heard of).
Allister and Lee
Black spots on citrus can be caused by Alternaria, or by Colletotrichum (Glomerella) in New Zealand. Copper has been observed to cause phytotoxic symptoms when applied to young satsuma mandarins, and the only other fungicide available to organic growers is sulphur - although sometimes the calcium products can have some fungicidal effect mainly to do with changing the pH on the surface of fruit. Neither has been tried in New Zealand for controlling 'black spots' on citrus, and if used should be tried on just a few fruit first in case they cause phytotoxic symptoms.
For more info, the following paper can be accessed on NZ Plant Protection site:
Everett K.R. and Machin T. (2005). The Efficacy Of New And Old Fungicides On Glomerella cingulata On Satsuma Mandarins. New Zealand Plant Protection 58: 84-88.
Other relevant papers are:
Everett, K.R. and Rees-George, J. (2006). Re-Classification of an Isolate of Guignardia citricarpa from New Zealand as Guignardia mangiferae by Sequence Analysis. Plant Pathology 55:194-199.
Everett, K.R. and Rees-George, J. 2006. Species-specific PCR primers for Guignardia citricarpa and Guignardia mangiferae. New Zealand Plant Protection 59: 141-145.
The plant protection journal often contains papers on organic systems.
Other organic measures:
To help prevent plant pests and diseases, leaf litter should be removed or broken down quickly with the addition of nitrogen. Also pruning to optimise airflow and keeping undergrowth down, will promote quick drying after wet weather, to prevent outbreaks.