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By Nick Sakovich
In general, we in California are free from fruit and leaf spotting diseases. This is largely due to our weather patterns, unlike areas such as Florida where high rainfall, temperatures and humidity provide good environments for many diseases. Thus, for many years growers here will see little of the fungal disease Septoria Spot. Since we have had our fair share of rain however, this year growers will be seeing the fruit symptoms of this disease. It most prevalent during cool, moist conditions. The more rain we get, the worse this disease seems to be.
Septoria can cause fruit spotting of oranges, grapefruit and lemons, which will downgrade fruit and even necessitate extensive culling. The disease is of lesser importance in California's coastal plantings, but as rainfall increases, so does it's degree of severity.
This fungus first gains entry into the tree through weakened tissue, such as frost or pest damage. Infective spores are spread throughout the tree in dew or rain water. Thus, the more rain we get, the greater the spread of the disease. On the fruit, the spots are small, tan to reddish-brown depressions 1-2 mm in diameter, usually penetrating no deeper than the oil bearing tissue. On lemons, these small spots can spread to form large brown blotches during storage period. Often, the typical 'tearing-staining' can be seen of the fruit. Septoria may be confused with copper injury, however upon close examination, the depressions or pits caused by copper injury will be lacking fungal fruiting bodies.
It is too late to do anything about Septorea leaf spot this year. Preventive copper sprays in the early winter, just after the first rain, are appropriate. Unlike brown rot control, thorough coverage of the entire tree is essential. In years of heavy rainfall, retreatment may be necessary.
One other disease common to cool moist weather, especially fog, is Botrytis. The fungus infects through injuries, particularly injury to the bark by cold temperatures. Infected twigs may dieback several inches. Botrytis is mainly a problem on coastal lemons andoccasionalllh on Valencia oranges. During prolonged we periods, the fungus infects blossoms and twigs. Fruit set is lowered as blossoms rot. Infected petals often spread the disease to small adjacent fruit as contact is make. At the point of contact, the fungus stimulates cell growth of the fruit and ridges develop. These ridges are particularly susceptible to scarring. Ridged fruit, with or without scarring is downgraded. Control in coastal areas where the trouble occurs is difficult because blossoms develop almost continuously and cool fogs are frequent, providing the best conditions for invasion. Under these conditions it would be necessary to spray every few days to give adequate protection. General preventive measures such a s avoiding mechanical or chemical injury, protecting against frost and brown rot, and pruning regularly, help reduce the incidence of Botrytis.