One of the most important diseases of apples and pears is Black Spot caused by fungi Venturia inaequalis (apple) or Venturia pirina (pear).
In order to make decisions on spraying and giving priority to different blocks of fruit, it is very important to check spray history for each block and consider the kick back period for fungicides.
A variety of fungicide sprays with differing modes of action are available for this disease. When and how they are used depends upon their mode of action.
Protectant fungicides
Protectant fungicides prevent the spores from germinating or penetrating leaf tissue and must be applied to the surface of susceptible tissue before infection occurs.
These fungicides are applied routinely at 7 to 10 day intervals or according to anticipated infection periods.
Occurrence of infection can be predicted with an accurate weather forecast and the Mills table (Table 1).
Curative fungicides
Curative fungicides control the Black Spot fungus inside leaves and fruit.
These chemicals can penetrate plant tissues to eliminate or inhibit lesion development.
The ability of these fungicides to stop infections is limited to a few hours, or up to a few days (depending upon the specific fungicide), and often varies with temperatures during the first 24 to 48 hours after infection.
Some fungicides can inhibit the fungus even later into the incubation period (the time between infection and the appearance of symptoms).
Eradication of Black Spot lesions after they appear does not usually occur, but can be achieved with the proper rate and timing of certain fungicides.
Selection of fungicides
The selection of fungicides for management of Black Spot is based on several factors, including the entire spectrum of other diseases that must be managed at that time, the potential for resistance in the Black Spot fungus to the selected chemical, the history of the disease in a particular orchard, the final market for the fruit and other social and economic factors.
Good horticultural practices, such as proper site selection, tree spacing and annual pruning facilitates chemical control by improving spray coverage and reducing the length of wet periods.
Control powdery mildew
Some of the fungicides used for control of apple and pear Black Spot also affect another important disease, powdery mildew.
This disease is caused by a fungus called Podosphaera leucotricha. Both fungi belong to the same group, Ascomycota and have very similar physiology.
By using the fungicides that can control both diseases, costs of production can be significantly reduced.
Besides a spectrum of activity, other properties of fungicides must be taken into consideration such as resistance and cross-resistance properties, growth promotion or retardation, effect on beneficial organisms etc.
Maintain natural resistance (continued next month)
See this article in Tree Fruit Nov 2016
