IPDM in focus: bitter rot
Bitter rot is often the most serious fruit rot to affect apple orchards in warmer, humid growing conditions.
Quick facts
Identification and impact
- Bitter rot is most prevalent in areas with hot, humid summers.
- The first fruit infection usually occurs in late spring to early summer when temperatures above 21oC and rainfall combine to create conducive conditions. Wet fruit during warm nights is also problematic.
- Symptoms can appear as early as three weeks after petal fall but are more common later in the summer.
- Most symptoms develop as the fruit approaches maturity, sometimes mistaken for target spot. Bitter rot and target spot can be distinguished by their differing ‘ooze’ colouration – bitter rot ooze is salmon pink in colour compared to the cream-coloured ooze of target spot.
- Bitter rot infection typically appears as sunken, round lesions with concentric circles of spore masses. Internally, the disease is characterised by a conical area of brown rotting flesh.
Prevention and management
- Prevention through orchard sanitation is the first and most critical control measure.
- Removal of diseased fruit (fallen and hanging) can help reduce disease pressure. It is also recommended to remove all ‘mummies’ as it is likely these will be infected.
- Scab fungicides from groups M3 and M9 are effective, and timing generally suits incorporation into the orchard scab program.
- Careful selection of fungicides that avoid disruption to predatory mites and other beneficial species providing biological control of secondary pests is important.
Historically, bitter rot has been responsible for major losses in orchards including entire crop destruction. The advent of broad-spectrum fungicides led to a significant decline in its impact and most now regard it as an occasional problem. However, orchardists in regions with warmer, wetter spring–summer periods report a re-emergence of this disease. The industry’s transition to more disease-specific fungicides is speculated to be contributing to this re-emergence.
Small lesions caused by bitter rot. Photo: Alan McWaters, QLD DAF
Damage
Damage in Australia is almost entirely confined to the fruit. Where infection pressure is extremely high, leaves can become infected with the disease causing small, red flecks which enlarge to irregular brown spots. In rare cases this can lead to premature leaf loss but is unlikely to have an economic impact.
Fruit damage
There are two types of fruit damage depending on whether the infection is caused by sexual or asexual spores. In Australia, infection by asexual spores is more common. Circular lesions become larger and sunken as the disease progresses. Significant amounts of ooze containing spores develop in concentric-like circles on the surface of the lesions around the point of infection. Under moist, humid conditions, spore masses appear creamy and the ooze is salmon to pink in colour.
Lesions developed following infection by sexual spores are less sunken than those caused by asexual spores. They are brown and are more irregularly shaped. They also tend to produce fewer spores.
Regardless of which type of spore causes the infection, the single feature of the brown and conical rot extending from the surface lesion can be used in diagnosis. This gives a characteristic ‘V’ shape when the fruit is cut so as to bisect the lesion.
Note: Infected fruit that have not yet developed symptoms are said to have latent infections. Bitter rot is a fungal disease that can develop in storage if fruit have latent infections at harvest, though post-harvest fungicide treatments are usually not effective. If there is a history of bitter rot in stored fruit, then orchard sanitation and green tip copper application, followed by a concerted spray program, is recommended.
Leaf damage
Damage to leaves is not common in Australia but could be confused with Alternaria leaf blotch or damage caused by drift from desiccant-type herbicides. If leaf symptoms are prevalent, testing by a specialist diagnostic service for confirmation is recommended so that appropriate control measures can be deployed.
Similar damage – careful to spot the difference
Fruit which is damaged by hail or mechanical injury can develop sunken brown lesions that appear to be bitter rot. Damage due to sunburn can also cause some misdiagnosis, especially when the sunburn has a red halo around it, similar to early stages of bitter rot. The presence of concentric spores distinguishes asexual bitter rot infections from other types of damage. Infection arising from sexual spores is more difficult to distinguish and samples should be sent to a specialist diagnostic laboratory.
Prevention and good orchard management
Sanitation
Sanitation is critical for effective control. As the disease survives through winter on mummified fruit or dead wood, removing all fruit and prunings from the orchard floor and destroying them is important. However, mulching is an option if left on the orchard floor and/or treating with urea ground sprays to hasten decomposition.
In areas where bitter rot has been a problem during the season, hanging fruit needs to be entirely removed. Diseased or damaged fruit can be put into the inter row for mulching, removing the risk of overwintering spores from the orchard system.
Fruit infected by bitter rot left hanging during the season becomes a disease dispenser. Although labour-intensive, removal and destruction of diseased fruit during summer will reduce damage to the crop and may reduce dependency on sprays.
Preventing the proliferation of bitter rot begins in the orchard. As millions of fungal spores form on a single fruit, wind spread throughout the orchard is a significant risk. Rotting fruit are best removed early in the morning when the air is relatively still.
Monitoring
In areas where bitter rot has been historically problematic, careful monitoring is highly recommended during fruit development. Fruit is susceptible to infection from three weeks after petal fall until harvest. Infection early in the season becomes dormant soon after the spore penetrates the skin, but symptoms of infection do not show until resumption of growth is triggered by warmer weather and humid conditions outlined previously. Fruit susceptibility increases as it matures. If anthracnose leaf spots appear, it is important to be particularly vigilant, especially during summer.
Monitoring of higher regions of the tree, where spray applications for other diseases such as black spot may not have reached, is recommended. First symptoms of bitter rot appear as small brown circular spots, sunken by the time they reach a diameter of about 3mm. The fruiting bodies of the fungus will appear as small black dots arranged in concentric circles in the sunken lesion when its diameter reaches about 12mm.
Mature bitter rot lesion with concentric rings of spores. Photo: Alan McWaters, QLD DAF
Management
Responsible use of pesticides
As with most diseases, application of copper is likely to reduce the number of over-wintering spores available to initiate disease early in the season.
Mancozeb, Dithianon, Ziram and Metiram are registered for the control of bitter rot on pome fruit, including apples and pears. When applied for other diseases such as powdery mildew or black spot, these fungicides will also control bitter rot, if timing coincides with spore release. However, where monitoring indicates that bitter rot is present, a thorough early application is needed as a control measure.
IPDM alert: In orchards using predatory mites to control plant-feeding mites, the application of Mancozeb, Ziram or Metiram may lead to pest mite outbreaks because the fungicides are toxic to predatory mites.
Biological control, biorational pesticides and organics
A number of microscopic biological control agents, including yeasts and bacteria, have been trialled experimentally overseas and have shown promise. None have been developed commercially at this time.
More information
It is always important to remember that the biology of pests and the tactics used to control them vary subtly between countries and change over time. The species, strains and lifecycles may differ to Australia, even if common names are the same.
Shane WW and Sutton TB (1981) “Germination, appressorium formation, and infection of immature and mature apple fruit by Glomerella cingulata”, Phytopathology, 71:454–457.
Velha AC, Stadnik MJ and Wallhead M (2019) “Unravelling Colletotrichum species associated with Glomerella leaf spot of apple”, Tropical Plant Pathology, 44:197–204.
The IPDM Manual provides guidance on developing an IPDM (integrated pest and disease management) plan for your orchard to identify, monitor and manage key pests, diseases and weeds through an integrated approach that aims to disrupt life cycles and prevent susceptibility while encouraging and nurturing beneficial species that can act as natural control agents in your orchard.
For detailed information on bitter rot in Australia’s apple and pear growing regions, including detail on the life cycle of the disease, see page 187 of the IPDM Manual.
Acknowledgement
The PIPS3 Program’s Strengthening cultural and biological management of pests and diseases (AP19002) project has been funded by Hort Innovation, using the apple and pear research and development levy, contributions from the Australian Government and co-investment from Agriculture Victoria. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
This article is part of a series of articles in AFG that focus on implementation of IPDM. It was first published in the Summer 2022 edition of AFG and was prepared by Emily Crawford, Agriculture Victoria, using information from the revised Integrated Pest, Disease and Weed Management manual for Australian Apples and Pears (IPDM Manual), authored by David Williams, Oscar Villata and others, published by Hort Innovation, October 2021.
