Assessment of Pierce's disease susceptibility in Vitis vinifera cultivars with different pedigrees

Pierce's disease (PD) of grapevine is caused by the bacterium Xylella fastidiosa. In this study, an integrated approach was applied to assess PD susceptibility among different Vitis vinifera cultivars that incorporated disease severity, bacterial pathogen abundance and loss of stem xylem hydraulic conductivity. It was hypothesized that levels of PD susceptibility in V. vinifera can be attributed in part to the host anatomical features that are shaped by its pedigree background. Two popular wine grape cultivars were initially selected from the occidentalis group, Merlot and Cabernet Sauvignon, and one from the orientalis group, Thompson Seedless. The more recently bred table grape cultivar Scarlet Royal, that has mixed pedigree parentage, was also included. PD susceptibility was compared to the known PD resistant b43-17 V. arizonica/candicans wild grape species from North America. The data indicated that Thompson Seedless was ranked as the most susceptible to PD because it significantly exhibited the most severe disease symptoms at 12 weeks post-inoculation and hosted the highest X. fastidiosa titre of the cultivars, and lost over 90% of its stem hydraulic conductivity. In contrast, the other three cultivars displayed less susceptibility to PD. The way in which the xylem anatomy could impact PD susceptibility in V. vinifera cultivars is discussed, together with how grape pedigrees and their cognate centre of domestication may have influenced xylem anatomical features. This work provides a reference framework to further test the hypothesis that V. vinifera cultivars with wide xylem vessels may be more prone to PD decline.     

Aesthetic growth of a native tree species with desirable characteristics for urban green areas in arid and semiarid environments

Having an aesthetic form is one of the desirable aspects of trees used to create or reforest urban green areas, which is often achieved by procedures that require a great effort. The understanding of factors affecting the architecture of plants in natural conditions may be useful to promote a desirable shape during the growth process. Under the hypothesis that trees in high luminosity conditions have a more aesthetical growth, in this study we explore the relationship between plant shape and photosynthetically active radiation (PAR) in Bursera fagaroides, a characteristic species of the tropical dry forest and xerophytic shrub-lands useful for urban green areas in semiarid environments. To evaluate trees shape we calculate a symmetry index (I_S) by using two diameters of the crown, perpendicular to each other, and the total height. Measurements were made over three periods (before, during and after the growth period); and PAR was also measured for each tree. Linear regressions were used to analyze the relationship between the symmetry of trees and PAR received during the growth period. The post-growth symmetry of trees showed a positive relationship with the PAR received by trees during the growth period, suggesting that PAR effects can be harnessed to develop post-germination strategies for the production of trees with a more aesthetical growth in the studied species. Effect of PAR on the shape of plants may represent a practical option to promote aesthetical growth of trees and shrubs with importance for creation and restoration of urban green areas.     

Stem rot of jewel orchids caused by a new forma specialis,Fusarium oxysporum f. sp. anoectochili in Taiwan

Stem rot of Anoectochilus formosanus (Af) caused by Fusarium oxysporum (Fo) is a major limiting factor to jewel orchid production in Taiwan. Fo causes discoloration in vascular tissues. However, some newly collected Fo isolates from Af stem rot do not cause vascular discoloration, suggesting changes may have occurred in the pathogen. Among recent Fo isolates from Af there are two colony types, the cottony alba (CA) and the sporodochial (S). In order to confirm that both colony types cause Af stem rot, 200 isolates were obtained from diseased stems in Nantou County and characterized by colony type and whether or not the infected plants had vascular discoloration. Isolates of both the CA and S types caused stem rot of Af; some isolates in each colony type caused vascular discoloration whilst others did not. Pathogenicity tests with 22 isolates resulted in stem rot disease severity ratings on Af of 3·1–4·0 and 2·1–4·0 with CA and S type colonies, respectively. The same isolates failed to cause disease on Cattleya, Dendrobium or Phalaenopsis plants. Phylogenetic analysis of partial intergenic spacer sequences showed that these isolates were distinguishable from other formae speciales of Fo and could be separated into two groups correlated with the CA or S type colonies with high bootstrap. Based on pathogenic, morphological and molecular characterizations, the Fo that causes stem rot of Af is proposed to be a new forma specialis, F. oxysporum f. sp. anoectochili, with different pathotypes.     

Impervious surface thresholds for urban tree site selection

Impervious surfaces are a ubiquitous urban feature that increase temperature and tree drought stress and are a demonstrated indicator of Acer rubrum L. tree condition and insect pest abundance. We examined the relationship between A. rubrum condition, impervious surface cover, and Melanaspis tenebricosa (Comstock) abundance, a primary herbivore of urban A. rubrum, in eight cities across the southern distribution of A. rubrum. We predicted that the effects of warming, due to impervious surface, would be greater in warmer southern cities than in cooler northern cities. We found that impervious surface was a robust predictor of tree condition, but this effect was not significantly affected by background temperature. Melanaspis tenebricosa abundance was a function of impervious surface and background temperature, with greatest abundances occurring at mid latitudes. Based on these relationships, we developed impervious surface thresholds to inform site selection for A. rubrum throughout the southeastern USA. Planting criteria based on habitat characteristics should maximize urban tree longevity and services provided.     

No detection of seed transmission of citrus tatter leaf virus in ‘Meyer’ lemon

Journal of Plant Diseases and Protection - Citrus tatter leaf virus (CTLV), a strain of apple stem grooving virus, is a virus of citrus that is of commercial importance for trifoliate orange...     

The effect of grafting on nitrogen use in determinate field-grown tomatoes

Grafting tomato (Solanum lycopersicum L.) onto disease resistant rootstocks has grown in use in North America over the past two decades. Rootstocks have traditionally been bred and used for their resistance packages to numerous soil-borne diseases but some rootstocks appear to improve scion yield and vigour in comparison to non-grafted plants in conditions lacking disease challenge. In this study, the tomato rootstocks ‘Maxifort’ or ‘RST-106’ were used to determine if vigour improved ‘Tribute’ scion traits, especially yield, and if marketable yield could be maintained in grafted plants at reduced nitrogen fertiliser inputs. Plants were grown in an open-field plasticulture production system at five rates of nitrogen from 0 kg ha^?1 to 224 kg ha^?1 applied via drip irrigation. Marketable yield was significantly affected by nitrogen rate and rootstock in both years. ‘Tribute’ grafted onto ‘Maxifort’ rootstock had the greatest, most consistent impact and increased marketable yield 15% and 30% in 2013 and 2014, respectively, compared to the non-grafted ‘Tribute’. Our findings suggest that some rootstocks can increase scion growth and yield but do not differentially respond to decreased nitrogen rates.     

Effect of temperature on virulence of Rhizoctonia solani on soybean leaves and seedlings

Isolates representing 11 anastomosis groups (AGs) of Rhizoctonia solani from various geographic locations and host plants were tested for virulence on soybean leaves at 15, 20, 25, 30, and 35°C, and on soybean seedlings at 20, 25, and 30°C. Numbers of infection cushions formed on soybean leaves were determined using light microscopy. Isolates of AG-1 IA, AG-1 IB and AG-5 were more virulent on soybean leaves at 20, 25, and 30°C than isolates of AG-1 IC and AG-4. Maximum numbers of infection cushions were formed on soybean leaves by AG-1 (IA, IB, and IC), AG-4, and AG-5 at 25 and 30°C. The other AGs tested did not form infection cushions on soybean leaves although some caused minimal disease severity. Isolates of AG-1 IA formed significantly more infection cushions and caused greater disease severity than AG-1 IB and other isolates at 35°C. Maximum seedling infection, based on per cent area of hypocotyl region covered by lesions occurred at 25 C for AG-1 (IA, IB, and IC) and AG-4. Isolates of AG-5 caused greater seedling infection at 20°C than at 25 and 30°C. The other AGs tested caused only minimal damage to the seedlings. Isolates of AG-4 and AG-5 are not known to cause Rhizoctonia foliar blights of soybean in Louisiana, but their potential to be destructive foliar pathogens is confirmed.     

C-cell Distribution in Ovine Thyroid Gland

The number and percentage of C-cells per unit area were investigated in 2-3-year-old sheep by an immunoperoxidase technique, using a digital-image analysis. C-cells were distributed throughout the thyroid lobes but were not present in either the isthmus or the superior and inferior poles of the thyroid. C-cells were more concentrated in the deep central region of the lobes and decreased gradually toward the periphery. There was a high correlation between the number and the percentage of C-cells per unit area of thyroid gland in sheep. Significant differences were not present between male and female sheep of 2-3 years of age.     

Scanning electron microscopy of early infection in the uredial stage of Puccinia sorghi in Zea mays

An SEM study was made of the infection process of Puccinia sorghi in Zea mays. A uredospore germ tube grows across epidermal cells and along their anticlinal walls, often branching and altering direction of growth. The fungus, on attaining a stoma, delimits an appressorium over it. Infection peg initials enlarge linearly and centripetally along the appressorium base, forcing open the stomatal slit. Having penetrated the stomatal aperture, the infection peg develops a substomatal vesicle. From the vesicle, two short primary infection hyphae develop synchronously, a septum later forming between the vesicle body and each hyphal base. A further septum divides the primary hypha into two cells. Secondary infection hyphae emerge later from the fully expanded vesicle on the proximal side of each vesicle/primary hypha septum. Secondary hyphae are narrower than primary hyphae, form their proximal septum some distance along the hypha, develop asynchronously, and proliferate to form the intercellular mycelium. Infection processes and epidermal stripping are discussed.