Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2016
Publication Date: 6/1/2016
Citation: Rosskopf, E.N., Hong, J.C. 2016. First report of bacterial stem rot of ‘Heirloom’ tomatoes caused by Pectobacterium carotovorum subsp. brasiliensis in Florida. Plant Disease. 100(6)1233. https://doi.org/10.1094/PDIS-09-15-0961-PDN.
Interpretive Summary: Many small farms that sell direct to consumers are beginning to grow older varieties of vegetables commonly referred to as 'heirloom' varieties. Very little is known about the genetics of these varieties, particularly where resistance or susceptibility to plant diseases is concerned. In a recent research plot using heirloom tomato varieties, a severe plant wilt occurred just as plants were beginning to set fruit. This is particularly devastating as the farmer had already invested in the inputs needed to produce a crop of tomatoes when the plants began to die. Symptoms included severe wilting, necrosis of upper leaves, and a water-soaked appearance of the stem. When the stem was split, the center was hollow and the walls were necrotic. A bacterium was consistently isolated from the infected stems and found to be related to the tomato fruit soft rot pathogen Pectobacterium carotovorum, but was a different subspecies than has been previously found in the state. This was demonstrated by sequencing of a portion of the DNA as well as amplification with a subspecies specific primer set. The pathogen, Pectobacterium carotovorum subsp. brasiliensis, has been found to cause black leg and tuber rot of potato in Kenya and New Zealand.
Technical Abstract: In the spring of 2014, a field experiment was established to evaluate the growth of ‘heirloom’ tomato types in a vertical garden hydroponic system. During bloom, approximately 40% of the established plants of the variety ‘Black Prince’ were severely wilted with necrotic upper leaves. Stems of infected plants appeared water-soaked and were hollow and necrotic internally. Diseased tissue was surface sterilized with 10% commercial bleach solution, ground in sterile deionized water, and serial dilutions were plated onto 1/10th-strength tryptic soy agar. Multiple bacterial colony types were present, but a single colony type, transparent and colorless during the first 48 hours, turning translucent blue with age, was dominant. Individual colonies were isolated and grown in tryptic soy broth and bacterial DNA was isolated. The isolate was putatively identified as Pectobacterium carotovorum, based on the symptoms produced, but comparison of the tomato stem isolate with tomato fruit isolates led to the amplification with primers Br1f and L1r (1), from which only the stem isolate produced an amplicon of approximately 320 bp, characteristic of Pectobacterium carotovorum subsp. brasiliensis (Pcb), the causal agent of soft rot and blackleg of potato in New Zealand (2). Amplification, sequencing, and BLAST analysis of the 16S rRNA DNA sequence 99% sequence homology to the deposited sequence of Pcb NZEC1 GenBank Accession JQ771053.1. To confirm pathogenicity, a single isolate was used to inoculate ‘Black Prince’ tomato plants. Five plants each were syringe injected with either sterile water, 10 µl of 1 x105 cfu/ml P. carotovorum fruit rot isolate, or 10 µl of 1 x105 cfu/ml Pcb and injection sites were wrapped with Parafilm to prevent contamination. Three of the five Black Prince tomato plants inoculated with Pcb were wilted within 3 days of inoculation and the bacterium was re-isolated, identify confirmed, and sequence deposited. None of the control plants, nor the plants inoculated with the fruit rot pathogen developed wilt symptoms in either of two repeated inoculations. To the best of our knowledge, this is the first occurrence of bacterial stem rot in heirloom tomatoes in Florida caused by Pectobacterium carotovorum subsp. brasiliensis.