Is phytoplasma titre correlated with symptom development in Banana Wilt Associated Phytoplasma disease in Papua New Guinea?

Abstract

Banana wilt associated phytoplasma (BWAP) is associated with lethal yellowing in banana in Papua New Guinea (PNG) and the Solomon Islands. The banana wilt associated phytoplasma is also associated with Bogia coconut syndrome (BCS) in coconuts and lethal yellowing in other palms. Although the disease was reported in the early 1990s, the pathogenicity of the phytoplasma in the host has not been investigated. Most research regarding pathogenicity has traditionally focused on phytoplasma’s that cause growth and flower abnormalities. Phytoplasmas are hypothesised to initially exist as endophytes and only become pathogenic once the titre in the host reaches a certain threshold. This has been investigated in crops such as grapevine, plum and apple, however this has not been investigated for BWAP. We sought to determine the relationship between symptom development and phytoplasma titre in banana hosts. Hence, we tested the hypothesis that BWAP titre is correlated with symptom severity in banana. Plants with BWAP symptoms were collected and the presence of phytoplasma confirmed using PCR based diagnostics. All banana plants were individually scored for symptom severity using a scale we developed to score disease severity from 0-3 based on presence and prevalence of certain visual symptoms. Real time PCR was conducted to obtain cycle threshold (Ct) values for banana tissues and to estimate phytoplasma titres. Correlation of symptom development to pathogen titre was determined by comparing Ct values of individual plant parts to the same plant part of different plants expressing different stages of disease severity. Asymptomatic plants were also included in the analyses. Our results suggest that BWAP titre 21 in banana hosts is correlated with disease progression. Future research will focus on identifying pathogenicity genes of the BWAP and the level of the expression thereof at different stages of disease development