Virulence tests of Neofusicoccum parvum, Lasiodiplodia theobromae, and Phytophthora palmivora on Theobroma cacao

Authors: Puig, Alina; Keith, Lisa; Matsumoto Brower, Tracie; Gutierrez, Osman; MARELLI, JEAN-PHILLIPE - Mars, Inc

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/2021
Publication Date: 2/10/2021
Citation: Puig, A. S., Keith, L. M., Matsumoto, T. K., Gutierrez, O. A., & Marelli, J. P. (2021). Virulence tests of Neofusicoccum parvum, Lasiodiplodia theobromae, and Phytophthora palmivora on Theobroma cacao. European Journal of Plant Pathology. 159:851–862. https://doi.org/10.1007/s10658-021-02210-1.
DOI: https://doi.org/10.1007/s10658-021-02210-1

Interpretive Summary: A new pathogen, Neofusicoccum parvum, was recently found causing disease on cacao pods on Oahu and Hawaii Islands, but its potential impact on the Hawaii cacao industry is unknown. To determine the infection routes and virulence, inoculations with N. parvum were done on unwounded and wounded stems and pods alongside well-known pathogens such as Lasiodiplodia theobromae and Phytophthora palmivora for comparison purposes. One stem inoculated with an isolate of N. parvum developed symptoms, compared with half of the unwounded stems inoculated with P. palmivora. L. theobromae did not cause lesions on any unwounded stems. N. parvum isolates caused disease symptoms on 40 to 100 percent of unwounded pods of GNV 164 and GNV 360, but almost no lesions on ICS 95, SHRS 21 and SHRS 33. Disease severity on wounded pods was similar among N. parvum, L. theobromae, and P. palmivora. In addition to showing the ability of N. parvum to infect unwounded pods, the variation in infection rates suggests that clones have differing levels of resistance to this pathogen. This is the first report of L. theobromae causing lesions in unwounded cacao pods and P. palmivora causing infections in unwounded stems. This research improves our understanding of the infection cycle of three economically important cacao pathogens and may potentially lead to development of effective disease management strategies.

Technical Abstract: Neofusicoccum parvum is a recently reported pathogen affecting Theobroma cacao L., and has been isolated from symptomatic pods on Oahu and Hawaii Islands. Determining infection routes and virulence are essential for assessing the impact of N. parvum on cacao production and developing effective disease management strategies. Infection routes were determined by inoculating unwounded stems and pods with six isolates of N. parvum alongside Lasiodiplodia theobromae and Phytophthora palmivora. Fifty percent of unwounded stems inoculated with P. palmivora developed lesions, but only a single lesion developed following inoculation with N. parvum (isolate H44). L. theobromae and the remaining N. parvum isolates did not induce lesion development on unwounded stems. In contrast, all N. parvum and L. theobromae isolates produced lesions on 40–100% of unwounded pods of GNV 164 and GNV 360. Low incidences of infection were observed in unwounded pods of ICS 95 (0–66.7%), SHRS 21 (0–75%), and SHRS 33 (0–20%). On wounded pods, all pathogen species produced similar size lesions, ranging from 1.90 to 7.60 cm four days after inoculation. Results from this study show that all three species can produce high rates of pod infection on some clones in the absence of wounds, but stem infection is less likely. In addition, this is the first report of L. theobromae infecting cacao pods and P. palmivora infecting stems in the absence of wounds.