Isolation and Identification of Ice Nucleation Active Fusarium Strains from Rapid Apple Declined Trees in Korea

Abstract

In biological particles such as Fusarium species, ice nucleation activity (INA) has been observed. Fusarium strains isolated from apple declined trees in Korea were identified with a multilocus sequence analysis using the tef1 and rpb1 genes. Droplet-freezing and tube-freezing assays were used to determine the INA of the strains, using Pseudomonas syringae pv. syringae KACC 21200 as a positive control and resulting in seven INA+ fungal strains that were identified as F. tricinctum (KNUF- 21-F17, KNUF-21-F18, KNUF-21-F29, KNUF-21-F32, KNUF-21-F38, KNUF-21-F43, and KNUF-21-F44). The effect of Fusarium INA+ KNUF-21-F29 was compared to that of INA– strains on Chrysanthemum morifolium cv. Shinma explants. A higher callus formation and noshoot formation were observed, suggesting that fungal INA could play a role in cold injuries and be a factor to consider in rapid apple decline. To the best of our knowledge, this is the first report of INA fungal strains isolated in Korea.

The Rapid Apple Decline Phenomenon: Current Status and Expected Associated Factors in Korea

“…Recently, Fusarium tricinctum has been reported as the causal agent of apple decline in China ( Zhang et al, 2022 ). In addition, studies have shown that the ice nucleation-active F. tricinctum and Fusarium diversisporum isolated from declined apple trees in Korea may be linked to RAD ( Avalos-Ruiz et al, 2022a , 2022b ). Thus, further research is needed for a comprehensive understanding of RAD as well as the identification of causal agents, which will contribute to combating this disease worldwide.…”

“…In Korea, PSS WSP007 was isolated from apple stem canker, and draft genomic analysis shed light on its life cycle, revealing that it exhibited INA ( Lee et al, 2023 ; Lim et al, 2019b ). At the same time, Fusarium strains obtained from declining apple trees in Korea have also been reported to show INA ( Avalos-Ruiz et al, 2022a , 2022b ). However, our understanding of the specific effects of INA caused by PSS or Fusarium species on different apple rootstocks depending on climatic conditions and geographic regions is still limited.…”

“…Furthermore, INA can be a factor of RAD, highlighting the potential role of ice nucleation-active microorganisms in the decline of apple trees. Research suggests that elevated INA levels may increase the susceptibility to cold injury ( Avalos-Ruiz et al, 2022a , 2022b ; Lee et al, 2023 ; Lim et al, 2019b ), leading to rapid decline due to infection by opportunistic fungi. However, further research is needed to fully understand the complex interactions between INA and other contributing factors like opportunistic pathogens.…”

“…Current Status of Rapid Apple Decline and Associated Factors 539 Such a widespread occurrence has led to misdiagnosis and confusion among farmers in regard to its causes. Several studies have been conducted to determine the factors that can be associated with RAD or apple decline more generally (Avalos-Ruiz et al, 2022b;Lee et al, , 2020bLee et al, , 2021aLee et al, , 2023. However, due to the complex nature of RAD, it is difficult to establish the causative agents and what triggers this phenomenon.…”