Black Core Rot Being Studied

Josh McGill Diseases, International, Research

A new Hort Innovation project aims to shed some light on how black core rot happens in Australian groves so the citrus industry can learn how to better manage the disease.

Black core rot caused by the fungus Alternaria spp. can be a problem for some citrus growers in the southern growing areas in Australia. Citrus infected with Alternaria spp. can develop black core rot inside the fruit without showing any external symptoms.

Black Core Rot
Black core rot in mandarin

Alternaria spores are airborne and are believed to enter citrus fruit through wounds or micro-cracks at the stylar end during early fruit development after petal fall. After infection of young fruit, the fungus establishes a dormant infection in the stylar end of the fruit. Especially in citrus varieties with poorly formed navels, the fungus will start growing and enter the fruit from this initial infection point after the button senesces.

The problem may not be detected until after harvest when the fruit reaches the consumer or the juicing plant. Besides the internal black rot, the fruit also develops a bitter flavor. In addition, the disease can give rise to early fruit drop resulting in yield loss.

Application of fungicides is at present not giving consistent control. One way for growers to reduce the impact of the disease is to delay harvesting until most of the infected fruit drops.

“Growers have to make a decision to apply fungicides before knowing that they will have the problem,” says project leader Andre Drenth. “If the decision is made to take a proactive approach based on black core rot problems experienced in previous years in certain blocks, there is no solid base on which decisions concerning choice of product, timing and frequency of application can be made. The return on investment will only become evident after harvest and even then, the effectiveness of applied management strategies is hard to determine.”

The risk of losses due to black core rot would be reduced by knowing which environmental conditions and crop stage have the highest risk of infection for certain varieties. Information on which products to apply and the timing of application is also needed.

This project will start with a survey and interviews with growers and industry stakeholders. The survey and interviews will establish a better understanding of the occurrence and impact of the disease and the effectiveness and limitations of current management practices.

Field trials will be undertaken to better understand the disease. “Based on an improved understanding of the basic biology of the disease, the research team will test various disease management options which are compatible with the existing control programs,” says Drenth. “Based on these experiments, we hope to explore choices and timings of chemical and other control options for citrus black core rot.”

This project is a collaboration between the University of Queensland and the New South Wales Department of Primary Industries

Source: Citrus Australia

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