Citrus biosecurity threats

The disease

• This leaf and fruit spot disease is caused by the fungus Pseudocercospora angolensis.
• Severe infection of trees can result in premature abscission of young fruit and leaves.
• The juice content of diseased fruit is markedly reduced, making them unsuitable for fresh consumption or processing.
• Yield losses of 50–100% can occur and production may cease where the disease is endemic.

Stem symptoms are seldom found. When infection of stems occurs, the lesions are dark-brown and usually occur as extensions of petiole lesions. Several lesions at the stem tip result in dieback. Lesions on other parts of the stem coalesce, become corky, and crack. Usually, profuse growth of secondary shoots occurs at the base of the dead stem.

Host range

• All Citrus spp.
• African cherry orange (Citropsis tanakae), round kumquat (Fortunella japonica).

Current distribution

• Yemen
• Various countries in Africa, as far south as Angola, Mozambique, Zambia and Zimbabwe.

Method of spread

• Infected plant propagation material: citrus propagation material (trees, cuttings, grafts, budwood, rootstock seedlings), and propagation material of other hosts.
• Infected fruit.

Preventative actions

• Quarantine procedures for importation of citrus propagation material, fruit and other hosts.
• Plant certified disease-free citrus trees.
• Awareness and surveillance to ensure early detection and rapid implementation of control measures.
• Do not bring illegal plant material into SA and onto your farm.

Post-bloom fruit drop

Symptoms The first symptoms of post-bloom fruit drop (PFD) may appear two to seven days after infection. Initially, peach to orange-coloured lesions are formed on the petals of open flowers (floral rot or blossom blight), later turning reddish-brown and necrotic. Although unopened and even pinhead flower buds may be affected, petals on open flowers are more susceptible to infection. Whole flower clusters may be infected, resulting in entire branches with orange to brown petals clinging to inflorescences. Flowers affected by PFD start developing into yellowish fruitlets, but fruit development ceases prematurely. The fruitlets abscise, and the remaining calyces and floral disks, commonly called buttons, persist on the twigs for the life of the twig. These persistent buttons are uniquely characteristic to the disease and are not known to be produced by any other disorder. Leaves surrounding infected inflorescences are usually twisted and distorted, with twisted laminae and enlarged veins.

The disease

• Colletotrichum abscissum, belonging to the Colletotrichum acutatum complex, is the causal agent of PFD.
• PFD is a destructive disease of citrus and causes damage to blossoms (floral rot or blossom blight), which perpetuates the disease.
• The disease results in up to 80% early fruit drop in favourable climatic conditions.
• PFD is very difficult to control.

Host range All Citrus spp.

Current distribution

• Brazil
• USA

Method of spread

• Infected plant propagation material: citrus propagation material (trees, cuttings, grafts, budwood, rootstocks).
• Airborne spores are wind dispersed from infected tissue.

Preventative actions

• Quarantine procedures for importation of citrus propagation material.
• Plant certified disease-free citrus trees.
• Awareness and surveillance to ensure early detection and rapid implementation of control measures.
• Do not bring illegal plant material into SA and onto your farm.

BOX:

Fact sheets produced by CRI Biosecurity Division – January 2024 For more information on this disease, or if you find anything unusual, contact Wayne Kirkman from the CRI Biosecurity Division: biosecurity@cri.co.za / 084 458 0349.

Disclaimer: The material in this publication is for general information only, and no person should act, or fail to act based on this material without obtaining professional advice. CRI and all persons acting for CRI expressly disclaim liability with respect to anything done in reliance on this publication.