Interaction Between Diaporthe rhusicola and Neofusicoccum mediterraneum Causing Branch Dieback and Fruit Blight of English Walnut in California, and the Effect of Pruning Wounds on the Infection

Botryosphaeriaceae and Diaporthaceae species are the causal agents of branch dieback of English walnut in California. In this study, the effects of the interaction between Neofusicoccum mediterraneum and Diaporthe rhusicola were evaluated in vitro by using mycelial plugs or spore suspensions and in vivo by inoculating shoots and epicarps (hulls) of walnut. Single inoculations of each species and different coinfection treatments were performed under laboratory or field conditions. The influence of shoot age and susceptibility of bark or pith tissues to N. mediterraneum and D. rhusicola infection after pruning was also evaluated. In in vitro experiments, spore germination of D. rhusicola was significantly (P < 0.0001) reduced in the presence of N. mediterraneum spores. When D. rhusicola was inoculated at 4 days before N. mediterraneum, a delay in lesion development in shoots and hulls was observed compared with the other two interaction treatments. One- to 2-year-old shoots were more susceptible to infection and colonization by N. mediterraneum than 3- to 4-year-old shoots. In young shoots, inoculation in the pith tissue resulted in longer lesions than those observed on shoots inoculated in the bark. No significant differences were observed between the development of internal or external necrosis and the age of the shoots, or the susceptibility of bark and pith to D. rhusicola infection. This information is essential to better understanding the complex situation of this walnut disease toward developing control management strategies.

Ascorbic Acid, Thiamin, Riboflavin, and Vitamin B6 Contents Vary between Sweetpotato Tissue Types

Sweetpotato is considered a good source of ascorbic acid (vitamin C) and certain B vitamins. These water-soluble vitamins (WSV) play essential roles in sustaining human health. Besides the root, sweetpotato vegetative tissues are also edible and considered high in nutritional value. Despite the availability of general reference values for sweetpotato WSV content in the root and leaves, little is known about the distribution of these vitamins in specific sweetpotato root and vegetative tissues. The objective of this study was to determine the ascorbic acid (AA), thiamin (B1), riboflavin (B2), and vitamin B6 content in a range of foliar tissues including buds, vines, young petioles, young leaves, mature petioles, and mature leaves and root tissues including the skin, cortex, and pith tissue at the proximal, distal, and center regions of the root. Among foliar tissues of ‘Beauregard’ sweetpotatoes, the AA content was highest in young leaves (108 to 139 mg/100 g fresh weight) and lowest in mature petioles (7.2 to 13.9 mg). No thiamin was detected in foliar tissue, whereas mature leaves contained the highest riboflavin and vitamin B6 content (0.22 to 0.43 mg and 0.52 to 0.58 mg, respectively). In root tissues of ‘Beauregard’ and ‘LA 07-146’ sweetpotatoes, the AA content was lower in the skin (1.9 to 5.6 mg and 2.54 to 3.82 mg, respectively). The AA content in the cortex and pith tissue at the proximal, distal, and center of the root was generally similar. The thiamin content was variable among root tissues, whereas the skin contained the highest riboflavin content and the lowest vitamin B6 content across root tissues of both cultivars. The results of this study confirmed earlier reports suggesting that sweetpotato leaves can be a good source of multiple WSV in the human diet.

First Report of Tomato Pith Necrosis (Pseudomonas corrugata) on Tomato (Solanum lycopersicum) in Washington

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.

Day and Night Temperature Differential (DIF) or the Absence of Far-red Light Alters Cell Elongation in `Celebrity White' Petunia

`Celebrity White' hybrid petunia plants (Petunia ×hybrida Hort. Vilm-Andr.) were grown either in chambers constructed of CuSO4-filled panels acting as spectral filters removing the far-red light (-FR) or in environmental control chambers under temperature treatments of 24 °C day/18 °C night (+DIF) or 18 °C day/24 °C night (-DIF). Growth responses for plants grown under CuSO4 filter (-FR) or -DIF temperatures were similar in that both treatments resulted in decreased internode length, increased stem diameter, and decreased cell length and cell diameter in epidermal, cortical, and pith tissues. Reduced cortical cell length contributed the largest percentage to internode length reductions compared to epidermal and pith tissue for the -FR treatment while reductions in cell length of all three tissues contributed to internode reduction of -DIF-treated plants. Chlorophyll a increased for plants grown under -FR, but decreased for plants grown in -DIF when compared to the appropriate controls.

First Report of Black-Foot Disease, Caused by Cylindrocarpon obtusisporum, of Grapevine in California

Black-foot disease, caused by Cylindrocarpon obtusisporum (Cook & Harkn.) Wollenweb., impacts young table and wine grape (Vitis vinifera) plantings throughout California. Two- to 5-year-old grapevines showed reduced vigor with small-sized trunks, shortened internodes, uneven wood maturity, sparse foliage, and small leaves with interveinal chlorosis and necrosis. In cross-section, trunks showed dark brown to black streaking in a few to most of the vascular elements. Symptoms included a reduction in root biomass and root hairs with sunken, necrotic root lesions. Pith of affected vines was compacted and discolored. Isolations made from roots, vascular elements, and pith tissue consistently yielded colonies of C. obtusisporum as verified by descriptions in standard texts. Koch's postulates were completed by dipping the roots of cv. Carignane seedlings in a 108 spore per ml suspension for 30 min. Plants were repotted in an artificial soil mix and held in a controlled environment facility at 24°C. Typical black-foot symptoms developed on 92% of the plants within 8 weeks. Control plants dipped in distilled water remained healthy. Cylindrocarpon destructans, a species closely related to C. obtusisporum, was first reported to cause “black-foot disease” on young vines in 1961 (2). In 1975, C. obtusisporum was reported to produce similar “black-foot” symptoms (1). We propose the common name Cylindrocarpon black-foot disease be used with both species. References: (1) S. Grasso et al. Vitis 14:36, 1975. (2) D. R. Maluta and P. Larignon. Viticulture 11:71, 1991.

127 STEM PITH TISSUE FREEZING RESISTANCE IN CABBAGE (BRASSICA OLERACEA VAR.CAPITATA)

Winter survival of cabbage seed crops is limited by the freezing resistance of the lower stem pith tissue. Both tolerance of extracellular freezing and avoidance of lethal temperatures are components of stem pith tissue freezing resistance. The avoidance mechanism involves the formation of ice within the pith tissue at relatively warm temperatures (little undercooling) and the subsequent release of heat of fusion, followed by significant slowing of the freezing rate so that stem temperatures are mitigated against ambient temperatures for several hours.