What is it ?
Glomerella, or bitter rot, is a fungal disease which occurs on pipfruit throughout the world. It is often referred to as a ‘summer rot’. The fungus is found on a number of other host plants including citrus, avocado, grapes and figs.

What does it look like ?
Glomerella mainly shows up on the fruit, particularly towards harvest. Early in the season, the symptoms are small grey-brown flecks on the skin. Closer to fruit maturity, Glomerella appears as circular, firm, light brown spots which may be slightly sunken. These lesions increase in size up to about 3 cm diameter and can turn dark brown-black within a few days.

Spores are produced within the lesions appearing at first as blisters and then as distinctive circles in shades of pink or orange . In older lesions, these spore halos can turn dark brown or black.

As the disease develops, it spreads towards the core. A cross section of the fruit shows the rotting tissue as a V-shape from the skin to the core. Eventually the whole fruit will rot, sometimes secondary infections contribute to the decay.

Glomerella is not often seen on leaves. When it does, it appears as small reddish flecks which develop into brown spots about 0.5-1.5mm wide.

What conditions are needed ?
Warm, wet, humid conditions are ideal for Glomerella.

The spores can survive during the winter in mummified fruit or cankers (rare in New Zealand). These spores are carried to developing fruit by rain, insects and wind. If conditions are favourable, the spores will germinate and penetrate the fruit through the skin.

In New Zealand, the main infection period for apples is from late November through until January, although infection sometimes occurs around blossom time. The lesions continue to grow and spread especially at warm temperatures. Lesions on infected fruit will also spread in storage.

Where is it found ?
Glomerella is a major problem in Auckland, Waikato and sometimes Poverty Bay, but it occurs in other fruit growing regions in New Zealand. Granny Smith and Sturmer are particularly susceptible to Glomerella.

Why is it a problem ?
The disease can cause major fruit losses in a warm, wet season. All fruit showing even the smallest lesion must be graded out of a line, otherwise rotting may develop further in coolstore. Secondary infections can also develop.

What are the current chemical controls ?
Protectant fungicide cover is necessary from mid to late November until harvest if Glomerella is likely to be a problem. Mancozeb, ziram and captan are the main fungicides currently used for Glomerella control.

Chlorine is added to the water dump or rinse at the packhouse to prevent the spread of spores.

Are there any other forms of control ?
Orchard hygiene is extremely important. Removing and destroying all mummified fruit at pruning will prevent the carryover of the spores. Cankers and dead wood should also be pruned out in the winter.

Fresh infections should be removed and either burnt or buried to stop the spread of the disease.

References
Atkinson, J.D. 1971. Diseases of Fruit Trees in New Zealand. A.R. Shearer, Government Printer, Wellington, New Zealand.

Brook, P.J. 1960. Lenticel spot, bitter rot and ripe spot of apples. The Orchardist of New Zealand. February, 1960 : 20-29.

Jones, A.L. and Aldwinkle, H.S. (eds). 1991. Compendium of Apple and Pear Diseases. The American Phytopathological Society, United States of America.