A New Disease Threat To Papaya And Citrus

Phytophthora palmivora, a disease that causes crown and root rot, poses a new threat to subtropical fruit and citrus production in South Africa. Dr Mieke Daneel of the Agricultural Research Council’s (ARC) Tropical and Subtropical Crops division in Mbombela and Dr Wilhelm Botha of the ARC’s Plant Health and Protection division in Roodeplaat explain the disease and how to combat it.

In 2019, the Agricultural Research Council’s Tropical and Subtropical Crops campus received a number of papaya plants (Carica papaya L.) with symptoms of extensive crown and root rot, which causes plants in orchards to collapse. The diseased trees arrived from Malelane in Mpumalanga and Tzaneen in Limpopo. Isolates of Phytophthora palmivora, a root and crown rot pathogen, were obtained from diseased tissue from each of the infected plants, and the identity of the species was confirmed by molecular identification.

P. palmivora infects plant tissue below and above the soil line. At least 138 species from many plant families worldwide, but mainly in tropical climates, are potential hosts. They include species of citrus, various ornamental plants, and papaya. Symptoms of the disease include flower, fruit, stem and root rot. The pathogen is spread primarily with the aid of motile zoospores in water.

P. palmivora can infect a variety of hosts because of its ability to form sporangia and abundant zoospores on diseased tissue when free water is present. The pathogen was first detected in South Africa in 2005 in ornamental nurseries from White River and Malelane. The samples tested in 2019 represented the first report of P. palmivora from papaya in South Africa. Papaya is the most common host infected in high-intensity production areas worldwide, and suffers significant damage and yield loss.

P. palmivora is a regulated pathogen in South Africa and must be reported according to the Reporting Procedure for Quarantine or New Pests of Plants in South Africa.