Departments of Plant Pathology and Horticulture, Washington State University Mount Vernon NWREC, 16650 State Route 536, Mount Vernon 98273
Department of Plant Pathology, Washington State University, PO Box 647521, Pullman 99164
Department of Plant Pathology, Washington State University Mount Vernon NWREC, 16650 State Route 536, Mount Vernon 98273
Tomato pith necrosis was observed on 2.7% of tomatoes grown in rows covered with black polyethylene, various biodegradable plastics, and an experimental spunbond poly(lactic) acid agricultural mulch in high tunnel and open field experimental plots, in western Washington in 2011. Symptoms developed on 3-month-old plants and progressed acropetally until night temperatures dropped to 10°C. Affected plants had chlorotic leaves, produced adventitious roots, and pith tissue was brown and either corrugated or rotted. Similar symptoms were observed again in 2012 on 2.0% of plants, but only in experimental plots with black polyethylene mulch. Diseased stem tissue was homogenized with a mortar and pestle in sterile water and the extract was streaked onto King's medium B (KMB) agar. Colonies were white and smooth initially, and after 5 days had an irregular surface and margin and produced a tan diffuse pigment. One isolate, Pc.Sl.2011, was gram-negative, grew at 37°C on nutrient broth yeast (NBY) agar, did not fluoresce on KMB (3), and was arginine dihydrolase positive. A partial 16S fragment, 1,387 bp, was obtained via PCR with universal 27f and 1492f primers. The resulting sequence exhibited 99% identity to Pseudomonas corrugata Roberts & Scarlett, and has been assigned GenBank Accession KC812729. Pathogenicity of Pc.Sl.2011 was tested in two greenhouse trials with five replications of one tomato plant per treatment. Seeds of ‘Celebrity’ were surface sterilized by soaking in 70% EtOH for 30 s and then 10% NaOCl for 30 s, then rinsed with sterile water and sown into 14 cm diameter pots filled with non-sterile Sunshine Mix #1 (SunGro Horticulture Distribution Inc., Bellevue, WA). Seedlings were inoculated at the four leaf stage using 5 ml NBY broth cultures of Pc.Sl.2011 grown at 28°C for 12 h with agitation. A sterile needle was used to inject 10 μl of either sterile water or a bacterial suspension of 1.0 × 1010 CFU/ml into the axil of the second true leaf. Inoculum concentration was confirmed by NBY dilution plate counts. The plants were incubated in clear polyethylene bags for 4 days and placed in a greenhouse at 21.1 ± 1.2°C with a 14-h photoperiod. The first and second trials were sampled at 8 and 9 weeks after inoculation, respectively. Plants inoculated with sterile water had green pith tissue. However, 60 and 40% of inoculated plants had brown pith tissue around the inoculation site in the first and second trial, respectively, but wilting and adventitious roots were not observed. Stem tissue from the inoculation site of symptomatic plants was homogenized as above, and the extract streaked onto NBY agar plates. Three isolates recovered from inoculated plants from both trials had the same characteristics as the original isolate, including similar colony morphology, ability to grow on NBY at 37°C, and lack of fluorescence on KMB. To our knowledge, this is the first documented report of tomato pith necrosis in Washington. Pith necrosis has been reported previously in high tunnel tomato production (4), where excess nitrogen fertilization occurs with cool evening temperatures (3), and when plastic mulch is utilized (2). In the cool climate of western Washington, successful tomato production requires the use of agricultural mulches and covers that trap heat. Since P. corrugata has been isolated from soil and the tomato seeds of inoculated plants (1), local growers attempting to manage pith necrosis need to select tomato seed lots carefully and avoid applying excess nitrogen, especially when using plastic mulch.
References: (1) V. Catara. Mol. Plant Pathol. 8:233, 2007. (2) E. J. Sikora and W. S. Gazaway. Online. ACES.edu ANR-0797, 2009. (3) C. M. Scarlett and J. T. Fletcher. Ann. Appl. Biol. 88:105, 1978. (4) X. Xu et al. Plant Dis. 97:988, 2013.