Identity, Pathogenicity, and Genetic Diversity of Lasiodiplodia Species Associated with Stem-End Rot of Avocado in China.

Abstract

Stem-end rot (SER) causes brown necrotic lesions in the pulp near the base of the fruit pedicel and is one of the most devastating postharvest diseases of avocados in all avocado-growing regions of the world. China's avocado industry is growing very rapidly, and the planting area is expanding, but little is known about the pathogens and genetic diversity of avocado SER. To determine the causal agents of SER, avocado fruits were sampled from the main avocado-producing areas in China during 2020 and 2021. Fungal isolates were obtained from SER symptomatic avocado fruits and identified by morphology combined with phylogenetic analysis of internal transcribed spacer (ITS), translation elongation factor 1-α (EF1-α), and β-tubulin (TUB2) gene sequences. All 101 isolates belonged to Lasiodiplodia spp.; four Lasiodiplodia species were identified, namely, L. pseudotheobromae (59.41%), L. theobromae (24.75%), L. mahajangana (7.92%), and L. euphorbiaceicola (1.98%); and six others are classified as Lasiodiplodia sp. (5.94%). There were only slight morphological differences in colonies and conidia of these four species of Lasiodiplodia. The pathogenicity tests showed symptoms of SER, and 92.08% of the isolates exhibited a high level of virulence on avocado (disease index >70), related to the disease severity on avocado fruits. All tested isolates grew well under the temperature ranging from 23 to 33°C. There was a significant difference in mycelial growth between the four species of Lasiodiplodia after treatment with high or low temperatures. The growth of L. pseudotheobromae was the fastest at 13 to 18°C but was the lowest at 38°C (P < 0.05). The red pigment could be produced by all tested isolates after culturing for 7 days at 38°C. The mycelial growth rate was the fastest on PDA medium, and the slowest on the OMA medium but promoted spore formation (P < 0.05). In addition, the genetic diversity of pathogenic Lasiodiplodia species associated with SER collected from avocado, mango, guava, and soursop fruits was determined. A total of 74 isolates were clustered into four main ISSR groups by the unweighted pair-group method with arithmetic mean analysis, and the classification of this group was related to the host. Extensive diversity was detected in the Lasiodiplodia populations. The diverse geographical origins and host species significantly influenced the population differentiation, and most of the genetic variation occurred within populations (P < 0.001). This is the first study to identify the major pathogens of avocado SER in China, survey their occurrence and pathogenicity, and include a comparative analysis of genetic diversity with Lasiodiplodia spp. causing SER on other fruit hosts. Collectively, the Lasiodiplodia species complex affecting avocado showed high pathogenicity and diversity, while L. pseudotheobromae was the most frequently isolated species in China. The results of this study provide insights into the aspects of the epidemic of SER disease caused by Lasiodiplodia species, which will help in developing strategies for the management and control of SER in avocado.