Detection of cacao mild mosaic virus (CaMMV) using nested PCR and evidence of uneven distribution in leaf tissue

Abstract

Distribution of improved germplasm of Theobroma cacao is essential for meeting the increased demand for cocoa beans. In cacao, the introduction of new diseases is prevented by ex-changing material through a national and international quarantine system. In 2020, virus symptoms were observed on plants in a quarantine greenhouse, and Cacao mild mosaic virus (CaMMV) was detected in one plant using published diagnostic primers. However, no virus was detected in other symptomatic plants. To address high pathogen diversity and low virus titer in recently infected plants, a nested PCR test was developed based on 15 CaMMV sequences from Trinidad and Puerto Rico. The test was validated on a subset (n = 30) of plants in the greenhouse, of which 29 tested positive. Most infections are thought to have occurred during the later stage of the quarantine period, possibly due to spread by mealybugs. However, phylogenetic analysis revealed the presence of three strains, suggesting that it was introduced on scionwood from multiple sources. Results of PCR assays on different leaf tissues indicate that the virus is unevenly distributed and that petiole tissue should be used in molecular diagnostics. The movement of infected scionwood is a major dissemination pathway for CaMMV but can be managed through careful screening.