Multigene Phylogeny and Pathogenicity Trials Revealed Alternaria alternata as the Causal Agent of Black Spot Disease and Seedling Wilt of Pecan (Carya illinoinensis) in South Africa

Abstract

The pecan (Carya illinoinensis) industry in South Africa is growing rapidly, and it is becoming increasingly crucial to understand the risks posed to pecans by fungal pathogens. Black spots on leaves, shoots, and nuts in shucks caused by Alternaria species have been observed since 2014 in the Hartswater region of the Northern Cape Province of South Africa. Species of Alternaria include some of the most ubiquitous plant pathogens on earth. The aim of this study was to use molecular techniques to identify the causative agents of Alternaria black spot and seedling wilt isolated from major South African pecan-production areas. Symptomatic and non-symptomatic pecan plant organs (leaves, shoots, and nuts-in-shucks) were collected from pecan orchards, representing the six major production regions in South Africa. Thirty Alternaria isolates were retrieved from the sampled tissues using Potato Dextrose Agar (PDA) culture media and molecular identification was conducted. The phylogeny of multi-locus DNA sequences of Gapdh, Rpb2, Tef1, and Alt a 1 genes revealed that the isolates were all members of Alternaria alternata sensu stricto, forming part of the Alternaria alternata species complex. The virulence of six A. alternata isolates were tested on detached nuts of Wichita and Ukulinga cultivars, respectively, as well as detached leaves of Wichita. The A. alternata isolates were also evaluated for their ability to cause seedling wilt in Wichita. The results differed significantly between wounded and unwounded nuts of both cultivars, but not between the cultivars. Similarly, the disease lesions on the wounded detached leaves were significantly different in size from the unwounded leaves. The seedling tests confirmed that A. alternata is pathogenic and that A. alternata causes black spot disease and seedling wilt of pecans. This study is one of the first documentations of Alternaria black spot disease of pecan trees and its widespread occurrence in South Africa.

Random amplified microsatellites (RAMS) analysis ascertains genetic variation of Alternaria alternata causing black spot disease on Carya illinoinensis in South Africa

“…A total of 364 A. alternata isolates sampled from eight geographical locations were used in this study ( Supplementary Table S1 ). Following the isolation procedures leading to single hyphal-tip cultures as described ( Achilonu et al, 2023c ), stock cultures were transferred into 2 mL cryogenic tubes (Merck Millipore, Pretoria, South Africa) containing 0.85% saline (NaCl)—15% glycerol solution and stored at −80°C until required.…”

“…The ecological imbalance caused by biotic or abiotic factors across major production regions in South Africa have favoured the emergence of pathogenic microorganisms such as Alternaria alternata on pecans, thereby influencing the productivity and quality of pecan nuts ( Achilonu et al, 2023c ). Alternaria alternata has 9 to 11 chromosomes and a genome of up to 33.6 Mb ( Akamatsu et al, 1999 ; Nguyen et al, 2016 ).…”

“…Alternaria alternata causes black spot disease or Alternaria black spot (ABS) in plants ( Kaur M. et al, 2020 ). Symptoms of ABS on plant tissues occur as dark brown circular lesions that enlarge, and eventually cause leaf death or rotten nuts ( Achilonu et al, 2023c ). The pathogen has been reported to cause ABS on crops such as citrus ( Tran, 2018 ), shell ginger ( Yan et al, 2022 ), cabbage ( Rahimloo and Ghosta, 2015 ), persimmons ( Kurt et al, 2010 ), pomegranates ( Berbegal et al, 2014 ), pears ( Terakami et al, 2016 ), sunflowers ( Kgatle et al, 2018 ), and rubber trees ( Cai et al, 2008 ).…”

“…In South Africa, ABS is a common disease in pecan orchards, and it is caused by A. alternata ( Achilonu et al, 2023c ). In recent years, pecan farmers have stressed that the disease has become a growing problem, especially in major pecan production areas of the country.…”

In Vitro Evaluation of Azoxystrobin, Boscalid, Fentin-Hydroxide, Propiconazole, Pyraclostrobin Fungicides against Alternaria alternata Pathogen Isolated from Carya illinoinensis in South Africa

“…The production of pecans ( Carya illinoinensis ) in South Africa (SA) currently shows yield limitations mainly due to the development of diseases [ 1 , 2 ]. Alternaria black spot (ABS), caused by Alternaria alternata , is one of the more prevalent diseases in SA pecans [ 3 , 4 ]. At present, no ABS disease on pecans caused by species of Alternaria has been reported elsewhere in the world.…”