Key Plant, Key Pests: Sycamore (Platanus spp.)

This series of Key Plant, Key Pests publications are designed for Florida gardeners, horticulturalists, and landscape professionals to help identify common pests associated with regional flora. This new 6-page publication of the UF/IFAS Environmental Horticulture Department helps identify the most common pests found on trees in the sycamore group, Platanus spp., and it provides information and general management recommendations for sycamore lace bug, bacterial leaf scorch, powdery mildew, anthracnose, and canker stain. Written by Matthew Borden, Kelly Laplante, Juanita Popenoe, Adam Dale, Caroline R. Warwick, and Brian Pearson.https://edis.ifas.ufl.edu/ep601

Impacts of Vaccinium arboreum Rootstocks on Vegetative Growth and Yield in Two Southern Highbush Blueberry Cultivars

Vaccinium arboreum Marsh is a small tree adapted to low-organic matter soils and is one of the few ericaceous species that tolerates soil pH greater than 6.0. It has a deep root system and is more drought tolerant than cultivated blueberry. The use of V. arboreum as a rootstock for commercial blueberry production has been studied previously in young blueberry plantings. The objective of the current study was to expand on earlier work and evaluate growth, productivity, and tolerance to bacterial leaf scorch (Xylella fastidiosa) in established plantings of own-rooted vs. grafted southern highbush blueberry (SHB). Two field plantings of grafted and own-rooted ‘Meadowlark’ and ‘Farthing’ SHB were established in May 2011: one at the University of Florida–Institute of Food and Agricultural Sciences (UF-IFAS) Plant Science Research and Education Unit in Citra, FL, and the other at a commercial blueberry farm in Archer, FL. At both sites, four rootstock–scion combinations were grown in either pine bark-amended or nonamended soil. Canopy volume was greater in grafted compared with own-rooted ‘Meadowlark’ at both locations throughout the 4 years of the study (2015–18), whereas canopy volume in ‘Farthing’ was not consistently different. For both cultivars and both locations, canopy volume was greater on amended compared with nonamended soil. Although canopy growth was not consistently increased in the grafted compared with own-rooted plants, yield was greater in grafted plants of both cultivars at both locations. Cumulative yield over the 4 years was similar between grafted plants grown on both amended and nonamended soil, and was significantly greater than yield of own-rooted plants on nonamended soil, suggesting the use of this rootstock may decrease the requirement for pine bark amendment. In general, grafted plants produced larger berries, with no negative impacts on fruit soluble solids, titratable acidity, or firmness. ‘Meadowlark’—an SHB cultivar that exhibits high sensitivity to bacterial leaf scorch—displayed decreased development of bacterial leaf scorch symptoms when grafted onto V. arboreum compared with own-rooted plants. These results indicate the potential benefits of grafting SHB onto V. arboreum rootstock, particularly under marginal soil conditions. However, a complete economic analysis that also takes into account any differences in longevity between the two systems must be done to determine whether the benefits of using grafting are feasible financially for the grower.

Pecan Bacterial Leaf Scorch, Caused by Xylella fastidiosa, Is Endemic in Georgia Pecan Orchards

Pecan bacterial leaf scorch (PBLS), caused by Xylella fastidiosa, can cause severe disease in some pecan cultivars, resulting in yield loss. Only recently has some information been obtained regarding the distribution and extent of the disease in pecan in any state in the United States. With emphasis on a susceptible cultivar, Cape Fear, we sampled a total of 91 trees in eight orchards from the southwestern and central production areas in Georgia (GA) and found 60.4% of trees sampled infected, most showing symptoms of PBLS. Further multilocus sequence typing from 16 of these trees confirmed presence X. fastidiosa. The results confirm that X. fastidiosa is widespread geographically in GA, and different cultivars may be infected. This is the first definitive report confirming X. fastidiosa causing PBLS in different pecan producing areas and cultivars in GA.

Bacterial Leaf Scorch in the District of Columbia: Distribution, Host Range, and Presence of Xylella fastidiosa Among Urban Trees

A survey of urban trees affected by bacterial leaf scorch (BLS) caused by Xylella fastidiosa was conducted in the District of Columbia during 2011 and 2012. Over 20 species of urban trees were evaluated at 95 sites. Symptomatic and asymptomatic foliage from trees with BLS symptoms and foliage from neighboring asymptomatic trees were sampled. An X. fastidiosa-specific enzyme-linked immunosorbent assay (ELISA) and a polymerase chain reaction assay were used to detect and identify the strains from environmental samples. Symptomatic trees testing ELISA-positive for X. fastidiosa occurred most frequently with Quercus palustris, Q. rubra, Ulmus americana, and Platanus occidentalis. The bacterium was also less frequently identified on eight other symptomatic and five asymptomatic tree species. On infected trees, the bacterium was also detected on the asymptomatic portion of seven tree species. All strains were identified as the X. fastidiosa subsp. multiplex genotype ALSII except on Morus alba, where the genotype ALSI and the subsp. sandyi were detected. The occurrence of crown dieback was found significantly associated with X. fastidiosa-infection on Q. palustris, Q. rubra, U. americana, and P. occidentalis. Because this pathogen continues to perpetuate uncontrolled in urban environments, there is a pressing need to identify long-term management strategies that abate disease.

Distribution of Xylella fastidiosa in Blueberry Stem and Root Sections in Relation to Disease Severity in the Field

Xylella fastidiosa causes bacterial leaf scorch, a new disease of southern highbush blueberry in the southeastern United States. Infections occlude the xylem of affected plants, causing drought-like symptoms and, eventually, plant death. To assess the likelihood of mitigation of bacterial leaf scorch through cultural practices such as pruning or hedging of affected plants, we determined the localization and population density of X. fastidiosa in naturally infected blueberry plants with varying levels of bacterial leaf scorch severity. Stem segments were sampled from the current season's growth down to the base of the plant, as were root segments on plants that were either asymptomatic or had light, moderate, or severe symptoms in three plantings affected by the disease. Stem sap was extracted from each segment and population densities of X. fastidiosa were determined using real-time polymerase chain reaction with species-specific primers. Detection frequencies were lowest (but non-zero) in sap from asymptomatic plants and highest in plants with severe symptoms. In asymptomatic plants, detection was generally least frequent (0 to 20.0%) in top and root sections and highest (4.6 to 55.6%) in middle and base stem sections. As disease severity increased, detection frequencies in roots increased to >80% in two plantings and to 60% in the third planting. Overall, detection frequencies were highest (>80%) in middle and base stem sections of plants from the moderate and severe disease classes. The lowest bacterial titers (averaging 0 to 2.1 × 101 CFU per 50 μl of sap) were observed in top and root sections of asymptomatic plants, whereas the highest titers (generally between 104 and 105 CFU per 50 μl of sap) were obtained from middle, base, and root sections of plants from the moderate and severe classes. The presence of the bacterium in middle and base stem sections at low disease severity indicates rapid distribution of X. fastidiosa in affected plants. Because the pathogen accumulates in the roots at moderate and high disease severity levels, management strategies such as pruning and mowing are unlikely to be effective in curing affected plants from bacterial leaf scorch.