Fungus causing root and crown rot newly detected in SA’s papayas

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Several papaya plants (Carica papaya L.) with symptoms of extensive crown and root rot, which caused plants in orchards to collapse, were received at the Agricultural Research Council’s (ARC) Tropical and Subtropical Crops campus during 2019. Diseased trees were from Malalane in Mpumalanga and Tzaneen in Limpopo.

Isolates of the Phytophthora palmivora fungus, a root and crown rot pathogen, were obtained from diseased tissue of each of the infected papaya plants. The identity of the species was confirmed by molecular identification. P. palmivora infects plant tissue below and above the soil line. It is capable of infecting a variety of hosts and has been recorded in at least 138 species of plant families mainly in tropical climates. Symptoms include flower, fruit, stem, and root rot.

First detection in South African papayas

P. palmivora was first detected in South Africa in 2005 in ornamental nurseries in White River and Malalane, Mpumalanga. The result of the current study at the ARC Plant Protection Research is the first record of P. palmivora found in papayas in the country.

The fruit is the most common host to be infected by this fungus in high-intensity production areas worldwide. Significant damage and yield loss occur. P. palmivora is a regulated pathogen in South Africa. It must be reported, according to the Reporting Procedure for Quarantine or New Pests of Plants in South Africa of the Department of Agriculture, Land Reform and Rural Development.

A papaya tree with root rot.

Of interest is the simultaneous occurrence of the disease on papayas from two production regions which are more than 400km apart, in the north-eastern parts of South Africa. This suggests that the disease may be linked to extreme climate conditions, such as high temperatures and severe drought, prevalent in both regions during late 2019.

Increasing drip irrigation daily proved to be ineffective in cooling and reducing stress on the plants, as the rate of evaporation and transpiration exceeded that of water absorption by the roots. Extreme temperatures and drought conditions prevailed during November and December 2019, ideal conditions for P. palmivora infections. – Plant Protection Newsletter, ARC