First report of Curtobacterium flaccumfaciens pv. flaccumfaciens causing bacterial tan spot on soybean in Africa

Authors: PAWLOWSKI, MICHELLE - University Of Illinois; Hartman, Glen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2019
Publication Date: 8/19/2019
Citation: Pawlowski, M., Hartman, G.L. 2019. First report of Curtobacterium flaccumfaciens pv. flaccumfaciens causing bacterial tan spot on soybean in Africa. Plant Disease. https://doi.org/10.1094/PDIS-05-19-1036-PDN.
DOI: https://doi.org/10.1094/PDIS-05-19-1036-PDN

Interpretive Summary: Bacterial tan spot of soybean was first reported in Iowa in 1975. The bacterium occurs on a number of hosts and is considered an important disease of beans where it causes bacterial wilt. The distribution of the disease on soybean is restrictive and has not been reported on soybeans grown in Africa. In Zambia, soybean leaflets from 20 different plants with necrotic lesions covering up to one third of the leaflet from the leaflet margin were selected. Leaf tissue was surface disinfected before being macerated in sterile water. The suspension was streaked onto nutrient dextrose agar and incubated at 30°C in the dark. Yellow bacterial colonies appeared after 7 days. DNA was extracted from bacterial colonies based on specific primers the colonies were positively identified as the cause of bacterial tan spot. To confirm pathogenicity, four 3-week-old plants of soybean cultivar Williams 82 were dusted with carborundum before a cheesecloth soaked in a bacterial suspension was rubbed onto the upper side of the second trifoliolates. Bacterial tan spot symptoms appeared 7 days post inoculation as necrotic lesions. The pathogen was re-isolated and cultured and after 7 days colonies appeared morphologically to be the same bacterium. This first report of this disease on soybeans in Africa is of interest to bacteriologists, crop specialists, plant pathologists, and others interested in geographic distribution of diseases and their movement in the world.

Technical Abstract: Curtobacterium flaccumfaciens pv. flaccumfaciens (Hedges) Collins & Jones (Cff) was first reported as causing bacterial wilt on beans in the state of South Dakota (USA) and first reported causing bacterial tan spot on soybean in Iowa in 1975. The bacterium occurs on a number of hosts and ecological niches including East and South Europe, Australia, Asia, North and South America, and Africa. In Zambia, a sample of leaflets from 20 plants with necrotic lesions covering up to one third of the leaflet from the leaflet margin in what appeared to be bacterial tan spot were selected. Leaf tissue was surface disinfected before being macerated in sterile water. The suspension was streaked onto nutrient dextrose agar (NDA) and incubated at 30°C in the dark. Yellow bacterial colonies appeared after 7 days. The bacterium was rod-shaped, about 3 µm in length, and gram stain positive. For further confirmation, DNA was extracted from bacterial colonies by disruption in Lysing Matrix A and CLS-TC solution. Five microliter subsamples were subjected to PCR using Cff specific primers (CffFOR2 5'-GTTATGACTGAACTTCACTCC-3'; CffREV4 5'-GATGTTCCCGGTGTTCGA-3'). The tested isolates were positively identified as Cff by producing 306 bp amplicons. To confirm pathogenicity, four 3-week-old plants of soybean cultivar Williams 82 were dusted with carborundum before a cheesecloth soaked in a bacterial suspension was rubbed onto the adaxial side of the second trifoliolates (Dunleavy, 1983). Plants were then held at a constant 26°C with a 16-hour photoperiod. Symptoms appeared 7 days post inoculation as necrotic lesions. The pathogen was re-isolated and cultured and after 7 days colonies appeared morphologically to be the same bacterium and positively identified using the Cff specific primer set.