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This physiological disorder of the Pacific Rose variety is wide spread with more severe symptoms than we have seen in the last two or three years.
So far no one has found a satisfactory cure for the problem although many cures have been suggested.
The following are some general observations on the incidence and expression of the symptoms.
There is a strong relationship between crop load and symptom expression. The heavier the crop load the more severe the symptom expression and the earlier in the growing season symptoms appear.
Soils with high water holding capacity tend to hold onto their foliage better and show symptom expression much later in the growing season than soils, which are prone to suffer water stress.
Periods of dull cloudy weather, particularly if prolonged and accompanied by substantial rainfall tend to bring on severe symptom expression. This reaction appears to be more apparent if the wet period has been preceded by a long period of sunny weather.
Cooler more southern districts tend to show less severe leaf symptoms than Hawke's Bay. The Wairarapa, for instance, shows lower levels of symptom expression.
Occasionally broad-spectrum fungicide programmes have reduced incidence compared to unsprayed trees. Anecdotal evidence suggests zinc based fungicides such as Ziram applied over the summer period appear to be effective.
Among micronutrients Zinc and Manganese deficiencies have often been implicated with leaf drop symptoms in apples. Low Zinc levels have been measured on some Pacific Rose orchards, which have a history of severe symptom expression.
There is also anecdotal evidence to suggest that orchards which receive foliar nutrient sprays containing trace elements tend to show less severe symptoms.
At this stage the only thing known for certain about Pacific Rose necrotic leaf spot is that if you grow the variety you had better get used to looking at necrotic leaf spots on it.
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