Importance of thermal adaptation of Amphobotrys ricini in its infection of Asian copperleaf (Acalypha australis)

Abstract

Amphobotrys ricini and Botrytis cinerea are causal agents of gray mold in euphorbiaceous plants, including Ricinus communis and Acalypha australis. In the present monitoring of epidemics of A. ricini and B. cinerea in A. australis, A. ricini was found in A. australis from August to October, when average temperatures ranged from 17.5 to 32.5 °C. In contrast, B. cinerea was not found in A. australis. This result suggests that A. ricini may adapt better than B. cinerea to warm weather. To test this idea, we examined the effects of temperature on mycelial growth and conidial germination of A. ricini and B. cinerea and their aggressiveness in A. australis and R. communis. Results showed that the optimal temperature for mycelial growth was 20–30 °C for A. ricini and 20 °C for B. cinerea. The optimal temperatures for conidial germination ranged from 26 to 30 °C for A. ricini and from 20 to 26 °C for B. cinerea. At 30–32 °C, A. ricini grew more rapidly, had higher conidial germination percentages and longer germ tubes than B. cinerea. When A. australis and R. communis were inoculated with these fungi, A. ricini was more aggressive than B. cinerea at 28 °C, but less aggressive than B. cinerea at 20 °C, during infection of leaves. These results suggest that thermal tolerance plays an important role in epidemics of A. ricini in A. australis. The study elucidated an ecological mechanism for better understanding about host preference of A. ricini in A. australis.