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Pear scab, or black spot is caused by the fungus Venturia pirina. It infects leaves, shoots, blossoms and fruit, and can cause serious crop loss especially in wet seasons when control measures are inadequate. The disease is found world-wide, wherever pears are grown. The fungus is closely related to apple scab, but although many similarities exist, cross-infection from one host to the other cannot occur.
Symptoms
The symptoms of pear scab are very similar to those of apple scab. Fruit infections appear as olive-green to black spots. Early infections cause large spots that distort the fruit while later infections cause smaller, more superficial spotting. As with apple scab, infections immediately before harvest may produce storage scab i.e. very small black spots that develop on fruit during storage. Leaf infections, which are less common than in apple scab, frequently occur on the underside of leaves. Twig infections, by contrast, are more common than on apple, and appear as small oval blisters on the affected shoot .
Figure 1. Winter Nelis fruit infected with pear scab, showing spotting and fruit distortion
Economic importance
Pear scab is the most serious and widespread fungal disease of pears. Losses from the disease are similar to those caused by apple scab on apples, and control depends mainly on costly spraying programs.
Figure 2. Pear scab lesions on underside of leaf
Disease cycle
The disease cycle of pear scab is similar to that described for apple scab. The main difference is that under Victorian conditions shoot infections on pears are more common than on apples. These shoot infections can provide the fungus with another means of overwintering, although overwintering as ascospores in dead leaves under trees is still the most important source of primary inoculum. Shoot infections are common on Winter Nelis and Beurre Bosc, but are less common on Williams' Bon Chretien and Packham's Triumph.
Infection periods (the time that leaves or fruit must remain wet for infection to occur) of pear scab are similar to those described for apple scab, although few detailed studies have been carried out to confirm this.
Control measures
Principles of control are very similar to those for apple scab. Control is based on a protectant spray program, supplemented by post-infection and autumn eradicant sprays. The period from delayed green tip to petal fall is most important in preventing infection. This corresponds with the period of greatest discharge of ascospores. Apply the first spray when one-third of buds reach the stage of delayed green tip. Apply the second spray about five to seven days later, and the third spray about 10 to 14 days after the first. A fourth spray should be applied at petal fall. Growing plant tissues could become exposed during infection periods in showery weather, and a fifth spray may be needed between delayed green tip and petal fall. Alternatively a post-infection spray may be needed.
Cover sprays applied at 10-14 day intervals after petal fall may be necessary if primary infection has occurred, and if wet weather favourable to infection persists. During dry summer weather no further sprays are necessary. However, if an infection period does occur, the use of an appropriate protectant spray before the wet period, or a post-infection spray shortly after it, should prevent development of summer scab.
Similarly, infection periods immediately before harvest may make sprays necessary in order to prevent the development of scab in storage.
Note that post-infection sprays must be applied within several days of an infection period. For maximum effect they should be applied as soon as the weather clears after such a period.
When scab has been difficult to control, use sanitation practices after harvest to reduce the carryover of the fungus into the next season. Some practical sanitation practices are:
- treating leaves on the tree immediately before leaf fall with a nitrogenous fertiliser to hasten leaf breakdown
- mulching the leaf litter after leaf fall by sweeping and then using a mechanical shredder, slasher or flail mower to chop leaves into small pieces which then break down more rapidly
- combining leaf mulching with a ground application of a nitrogenous fertiliser
When planning your scab spray program, it is wise to include at least two fungicides with different modes of action. This minimises the risk of development of resistance to fungicide. Consult with DPI and chemical resellers for the fungicides and spray timing that is most appropriate in your situation.
Monitoring potential overwintering scab
Check levels of leaf infection in all blocks after harvest to estimate the potential level of scab which may overwinter and initiate primary infection in the following spring.
For effective pest and disease control, correct diagnosis is essential. A commercial diagnostic service is available at the DPI PIRVic Knoxfield Centre. For further information, contact the Diagnostic Service. ph: 03 9210-9222 or fax 03 9800 3521.
The previous version of this note was published in December 1999.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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