Establishing the cause of marbling and corky flesh in plums

Increased reports of marbling and corky flesh in plums during the last few seasons prompted an HTS based investigation of plums displaying these symptoms. A new pathogen, a novel viroid provisional named plum viroid I (PlVd-I) was discovered. The clear association of PlVd-I with the observed fruit symptoms is strong evidence that this viroid is most likely the causative agent. The project aimed to gather a better understanding of the disease to assist in the development of a management strategy

High throughput sequencing was used to identify the causative agent in affected plum trees by comparing the virome profiles of symptomatic (marbling/corky flesh) and asymptomatic plum tree samples. An RT-PCR detection assay for PlVd-I was developed. Additional samples were collected from different cultivars and the association between PlVd-I and the symptoms validated. An experimental orchard were established to evaluate the symptom expression of PlVd-I in different plum cultivars and also to test graft and mechanical transmission. The temporal distribution of PlVd-I was also assessed.

The association between the presence of PlVd-I and symptomatic fruit was shown to be 97%. High-throughput sequencing of atypical samples lead to the first detection of plum bark necrosis stem pitting-associated virus (PBNSPaV) in South Africa. Hop stunt viroid (HSVd) was also identified, however no association with symptoms was established. PlVd-I was shown to be graft transmissible, but was not mechanical or seed transmitted in the experimental setup of this project. It was shown that the RT-PCR can detect PlVd-I throughout the season. The presence of PlVd-I in symptomatic apricots was also detected.

The correlation between the presence of PlVd-I and marbling and corky flesh symptoms remains 97%. Although it is believed that the primary mode of spread is through infected plant material, graft transmission was also demonstrated. No significant seasonal variation in PlVd-I concentration was detected but testing for PlVd-I at the end of the season can result in invalid tests due to lower RNA quality. The presence of PlVd-I in apricots was detected. Latent infections of PBNSaV and an ilarvirus was identified in plums and apricots respectively using HTS.

Plum marbling is a disease associated with the presence of PlVd-I. Symptom expression varies between cultivars and ranges from mild to severe marbling patterns on the skin and mild to severe corky flesh. The viroid is transmitted through grafting of infected plant material. Seed transmission was not observed in this study and mechanical transmission remains a low probability since no positive plants could be identified in the deliberated mechanical transmission experiment. A tree with marbling disease cannot be cured, but the presence of the viroid can be tested and the tree identified. To prevent the accidental propagation of infected material, infected trees should be removed.