Understanding Ceratocystis species A: growth, morphology, and host resistance

Abstract

Ceratocystis Wilt of ‘Ōhi‘a (Metrosideros polymorpha) is a newly discovered vascular wilt disease that has the potential to devastate native forests across the state of Hawai‘i, USA. To better understand the causal agent of the disease, C. sp. A morphology, temperature/medium dependent mycelial growth and spore production, and virulence of three isolates were assessed. In addition, individuals of four varieties of M. polymorpha from Hawai‘i Island were inoculated with C. sp. A to compare disease severity, the number of days to death of the individual, and the number of survivors. Isolates P14-1-1, P15-80, and P16-7 were all similar in morphological measurements and growth. Mycelial growth was highest at 25-30°C and on 10% V8 agar and malt yeast extract agar (MYEA) for all isolates tested. Spore production of isolate P14-1-1 was greatest at 25°C and on MYEA. Mean disease severity and mean number of days to death were not significantly different among the three isolates. Disease severity was lower in vars. incana (38.84 ± 5.08) and newellii (36.11 ± 11.01) compared to vars. glaberrima (59.17 ± 4.05) polymorpha (70.08 ± 3.64). Log mean number of days to death was higher for vars. glaberrima (3.77 ± 0.08) compared to polymorpha (3.43 ± 0.05). There were no differences in log mean number of days to death among the rest of the varieties. There was a significant relationship between variety and plant health. Vars. incana and polymorpha contributed the most to the difference, with a higher number of var. incana (11) and a lower number of var. polymorpha (0) survivors than expected.The similarities between the isolates support the hypothesis of a single recent introduction of C. sp. A and suggest that variation in pathogenicity and virulence is low within the population that exists on Hawai‘i Island. Also, the results suggest that resistance is present in M. polymorpha varieties in nature, and that var. incana may be more resistant than the other three varieties.