The cause of Pink Spot Disease on Guavas in South Africa.

 Guavas, Psidium guajava, are fruit bearing trees in the Myrtle Family, Myrtaceae, closely related to Eucalyptus. They are native to Mexico, Central America and the Caribbean Islands, but have been introduced to many countries as a commercial fruit crop, the largest producers being Mexico, India and Brazil. Like most commercially grown fruit trees, Guavas are prone to a variety of fungal diseases. Anthracnose causes blackening, mummification and rotting of fruit, which is caused by the fungus Colletotrichum gloeosporioides; this has been reported to have caused the loss of 50% of the annual crop in Puerto Rico, and is widespread in many other Guava-growing countries. Guava Dry-Rot is caused by the fungus Lasiodiplodia theobromae, and affects as much as 40% of the Guava crop in parts of South India.

In South Africa around 40 000 tonnes of Guava are produced annually. The crop here has been reported to suffer from Anthracnose, Blossom End Rot, which is caused by the fungus Phomopsis psidii, and Pink Spot. Pink Spot starts as small pink or red spots on the surface of the Guava fruit and leaves, which grow over time and eventually rot. The fungus Colletotrichum gloeosporioides (the cause of Anthracnose) has been recovered from large, rotten Pink Spot patches, and was thought to be the cause of this disease too, but it has not been isolated from less advanced cases, and is now thought to be a secondary infection taking advantage of weakened plants.

In a paper published in the South African Journal of Science on 22 September 2014, Adriaana Jacobs and Mariette Truter of the Biosystematics Programme of the Plant Protection Research Institute of the South AfricanAgricultural Research Council and Maritha Schoeman of the Crop ProtectionProgramme of the Institute for Tropical and Subtropical Crops of the South African Agricultural Research Council discus the results of a study of Guava Pink Spot from plants growing at Nelspruit in Mpumalanga Province, South Africa, which used genetic analysis to try to determine the Fungi responsible for the disease.

 

Symptoms of pink spot on Guava fruit and leaves: (a) initial slight depression with brick red margin, (b) green fruit showing dark reddish brown lesion, (c) a mature spot and (d) pink spot symptoms on leaves. Jacobs et al. (2014).

Jacobs et al. were able to recover DNA from the fungi Mycosphaerella acaciigena, Mycosphaerella keniensis and Mycosphaerella heimii, as well as at least one other unknown species of Mycosphaerella from Pink Spot infections from Guava fruit at Nelspruit. Fungi of the genus Mycosphaerella have previously been associated with a variety of plant diseases, and are hard to identify by morphological examination alone, usually requiring genetic studies for identification. Interestingly Mycosphaerella heimii has previously been reported to cause disease on Eucalyptus and Mycosphaerella acaciigena infects Acacia trees. Fungi of the genus Mycosphaerella are usually thought to be host-specific (i.e. one species of fungi only infects one species of plant) although the fungi have been known to switch to other hosts under some circumstances. The presence of multiple species of the fungi on Guavas in South Africa suggests that this introduced crop-plant may be particularly vulnerable to Mycosphaerella infection.