Effect of relative humidity, temperature, light and plant age on infection of lowbush blueberry by Sphaerulina vaccinii

Leaf spot and stem canker caused by Sphaerulina vaccinii is associated with premature defoliation in lowbush blueberry resulting in reduced yields. In this study, we investigated the impact of free water, relative humidity (RH), temperature, light, and plant age on leaf infection under controlled conditions. On potato dextrose agar, germination of conidia was usually polar. Growth was minimal at 5 and 10°C, increased at 15 and 20°C, was maximal at 25°C and decreased at 30°C. Percentage of germinated conidia on inoculated blueberry leaves incubated in dark controlled-humidity chambers for 3 days (25°C) was 86.0, 90.5, 81.3 and 28.3% in free water, 100, 97.5 or 95% relative humidity (RH), respectively. Germination did not occur at 90 or 85% RH. Infection of inoculated plants, however, was not favored by free water, but rather by high RH (>95%) and a 14-h photoperiod (180 µmol/m2/s). Infection failed in continuous darkness, continuous light or continuous darkness followed by 4, 8 or 12 h of light. Light and scanning electron microscopy showed that hyphal penetration into stomata on abaxial leaf surfaces was strongly tropic. When germ tubes grew in close proximity to a stomate, a penetration hypha formed at about 90° angles to the germ tube and took the closest path to the stomate. Stomatal penetration was usually direct, but occasionally appressorium-like hyphal swellings formed over stomatal openings. When inoculated plants were exposed to high RH (>95%) at various temperatures, infection occurred after 4 days at 10°C, after 3 days at 15°C and after 1 day at 20 and 25°C. Infection failed to occur at 30°C. Disease severity also increased with duration of the humid period. When leaves were examined microscopically, those that had been incubated for 6 days showed a substantially greater network of epiphytic growth with more stomatal penetrations compared to those incubated for 3 days. Infection was substantially reduced when the humid period was interrupted by alternating days of low RH (60%). Two-week-old leaves were 2.7 times more susceptible than 8-week-old leaves.

Clethodim suppresses hair fescue in lowbush blueberry

Hair fescue (Festuca filiformis) is a tuft-forming perennial grass of concern in lowbush blueberry (Vaccinium angustifolium) as tufts form dense sods that reduce lowbush blueberry yield and inhibit harvest. Although generally tolerant to ACCase-inhibiting herbicides, injury to some Festuca spp. following clethodim applications has been reported. ACCase-inhibiting herbicides are important for non-bearing and bearing year perennial grass management in lowbush blueberry but have not been extensively evaluated for hair fescue management. The objectives of this research were to 1) determine the relative efficacy of foramsulfuron, fluazifop-p-butyl, sethoxydim, and clethodim on hair fescue, and 2) determine if foramsulfuron tank mixtures with fluazifop-p-butyl, sethoxydim, and clethodim improve hair fescue suppression. None of the herbicides evaluated caused unacceptable injury to lowbush blueberry. Foramsulfuron (35 g a.i. ha-1) reduced hair fescue total tuft density, flowering tuft density, and flowering tuft inflorescence number. Fluazifop-p-butyl (250 g a.i. ha-1) and sethoxydim (495 g a.i. ha-1) caused variable levels of visual injury to hair fescue and did not reduce total tuft density, flowering tuft density, or flowering tuft inflorescence number. Clethodim (91 g a.i. ha-1), however, caused >50% visual injury to hair fescue and reduced hair fescue total tuft density, flowering tuft density, and flowering tuft inflorescence number. Fluazifop-p-butyl and sethoxydim tank mixtures with foramsulfuron did not increase hair fescue suppression relative to foramsulfuron alone. A foramsulfuron + clethodim tank mixture provided equivalent hair fescue suppression as either herbicide applied alone. Foramsulfuron and clethodim should therefore be used in rotation rather than tank mixture to manage hair fescue in lowbush blueberry

Identification and Characterization of Sphaerulina vaccinii as the Cause of Leaf Spot and Stem Canker in Lowbush Blueberry and its Epidemiology

Septoria leaf spot and stem canker is an important disease of lowbush blueberry, but the causal pathogen has not been accurately identified. Based on sequence analysis of the ITS, TEF-1α, RPB2, LSU and β-tubulin genes, the pathogen aligns closely with the genus Sphaerulina. The phylogenetic analyses based on these loci demonstrate that while the pathogen is closely related to the species Sphaerulina amelanchier, it is sufficiently distinct to warrant a new species designation. No ascomata of the teleomorph were found, however, ascospores recovered from leaves fit, morphologically, with the genus Mycosphaerella. The morphological data also support a new species designation. Based on the host that this pathogen infects, we propose the name as Sphaerulina vaccinii and the disease as Sphaerulina leaf spot and stem canker. Under field conditions, it appears that initial inoculum originates from pycnidia on overwintered leaves and stem lesions (cankers) on fruiting stems. More than 90% of the initial inoculum was released during the flowering period from late May through June. Leaf spots began to appear in early June and disease severity increased in a linear manner over time. Secondary inoculum production from diseased foliage was minimal and not considered important epidemiologically. Defoliation resulting from disease began in early July and increased in a non-linear manner thereafter. Manual defoliation of blueberry stems at various times prior to harvest showed the substantial extent to which premature defoliation can affect yield. Stem lesions were also shown to have an impact on yield, even though stems were not killed.

Fall-bearing Year Herbicides and Spring-nonbearing Year Foramsulfuron Applications for Hair Fescue Management in Lowbush Blueberry

Hair fescue (Festuca filiformis) is a tuft-forming perennial grass that reduces yields in lowbush blueberry (Vaccinium angustifolium) fields. Nonbearing year foramsulfuron applications suppress hair fescue, but there is interest in increasing suppression through foramsulfuron use in conjunction with fall-applied herbicides. The objective of this research was to determine the main and interactive effects of fall-bearing year herbicide applications and spring-nonbearing year foramsulfuron applications on hair fescue. The experiment was a 5 × 2 factorial arrangement of fall-bearing year herbicide (none, terbacil, pronamide, glufosinate, dichlobenil) and spring-nonbearing year foramsulfuron application (0, 35 g·ha−1) arranged in a randomized complete block design at lowbush blueberry fields in Portapique and Stewiacke, Nova Scotia, Canada. Spring-nonbearing year foramsulfuron applications did not reduce total tuft density or consistently reduce flowering tuft density, flowering tuft inflorescence number, or flowering tuft seed production. Fall-bearing year pronamide applications reduced hair fescue density for the 2-year production cycle, although additional bearing year density reductions occurred when pronamide was followed by spring-nonbearing year foramsulfuron applications at Stewiacke. Fall-bearing year dichlobenil applications reduced total and flowering tuft density at each site, although reductions in flowering tuft inflorescence number and seed production were most consistent when followed by spring-nonbearing year foramsulfuron applications at Stewiacke. Suppression extended into the bearing year at each site, and dichlobenil should be examined further for hair fescue control. Fall-bearing year glufosinate applications reduced hair fescue total tuft density at each site and flowering tuft density and flowering tuft seed production at Stewiacke. Fall-bearing year glufosinate applications followed by spring-nonbearing year foramsulfuron applications also reduced nonbearing year flowering tuft inflorescence number and bearing year hair fescue seedling density at Stewiacke, indicating that this treatment may reduce hair fescue seedling recruitment at some sites. Fall-bearing year terbacil applications did not suppress hair fescue and are not recommended for hair fescue management in lowbush blueberry.

Evaluation of terbacil-based herbicide treatments for hair fescue management in lowbush blueberry

Hair fescue is common tuft-forming perennial grass weed that reduces yields and hinders mechanical harvest in lowbush blueberry fields. PRE terbacil applications traditionally controlled hair fescue, but currently only provide suppression in most fields. Terbacil use has not, however, been evaluated in conjunction with other currently registered herbicides in lowbush blueberry. The objective of this research was to evaluate a range of terbacil-based herbicide treatments for hair fescue management in lowbush blueberry. The experiment was conducted at three lowbush blueberry fields in Nova Scotia, Canada. Spring non-bearing year terbacil applications (2000 g ai ha-1) exhibited variable efficacy on hair fescue with reduced total tuft density at one site and reduced flowering tuft density and flowering tuft inflorescence number at two sites. Suppression was limited to the year of application only. Terbacil followed by (fb) foramsulfuron (35 g ai ha-1) did not improve suppression. A terbacil tank mixture with glufosinate (750 ai ha-1), however, reduced flowering tuft density and flowering tuft inflorescence number at each site and reduced total tuft density at one site, suggesting improved suppression with terbacil+glufosinate relative to terbacil alone. Terbacil+glufosinate fb foramsulfuron gave additional reductions in total tuft density at two sites and reduced bearing year flowering tuft density at two sites, indicating that hair fescue suppression with this herbicide combination extends into the bearing year. Although less effective than the industry standard pronamide applications, terbacil+glufosinate or terbacil+glufosinate fb foramsulfuron could be used as part of a weed management program for hair fescue in lowbush blueberry.

Effects of fall bearing year glufosinate applications, spring non-bearing year glufosinate applications, and spring non-bearing year foramsulfuron applications on hair fescue in lowbush blueberry

Hair fescue is a common perennial grass that reduces yields in lowbush blueberry fields. This grass is suppressed with non-bearing year foramsulfuron applications, though suppression may be improved through use of sequential glufosinate and foramsulfuron applications. The objective of this research was to determine the main and interactive effects of fall bearing year glufosinate applications, spring non-bearing year glufosinate applications, and spring non-bearing year foramsulfuron applications on hair fescue. The experiment was a 2 by 2 by 2 factorial arrangement of fall bearing year glufosinate application (0, 750 g ai ha-1), spring non-bearing year glufosinate application (0, 750 g ai ha-1), and spring non-bearing year foramsulfuron application (0, 35 g ai ha-1) arranged in a randomized complete block design at lowbush blueberry fields located in Parrsboro and Portapique, NS, Canada. Fall bearing year glufosinate applications, spring non-bearing year glufosinate applications, and spring non-bearing year foramsulfuron applications alone provided inconsistent hair fescue suppression. Fall bearing year glufosinate applications followed by spring non-bearing year foramsulfuron applications, however, reduced non-bearing year total tuft density, flowering tuft density, and flowering tuft inflorescence number at each site and reduced seed production at Portapique. Sequential fall bearing year and spring non-bearing year glufosinate applications or sequential spring non-bearing year glufosinate and foramsulfuron applications reduced flowering tuft density and flowering tuft inflorescence number at each site but did not consistently reduce total tuft density. Sequential herbicide treatments reduced bearing year seedling density and may therefore contribute to hair fescue seed bank management in lowbush blueberry.