Paul D. Hildebrand

Interaction of ozone and negative air ions to control micro-organisms

Aims:  The aims of this study were to investigate the effect of ozone and/or negative air ions (NAI) on the viability of bacteria.

Infection, carbohydrate utilization, and protein profiles of apple, pear, and raspberry isolates of Erwinia amylovora

The bacterium Erwinia amylovora causes fire blight of about 200 species within the Rosaceae family. Several theories on why this pathogen is limited to this family have been proposed. The utilization by the pathogen of sorbitol, the primary photosynthate of rosaceous plants, has been one such theory. In this study, the carbohydrate contents of apple (Malus) and raspberry (Rubus) were quantified, and isolates of E. amylovora from these respective hosts were examined in relation to carbohydrate utilization, cross infection, and protein profiles to better understand possible reasons for host-range restrictions within the Rosaceae. Raspberry, unlike most rosaceous plants, contained sucrose as the primary photosynthate, while apple contained primarily sorbitol, except for the flower nectar, which contained primarily sucrose but no sorbitol. Apple isolates caused severe fire-blight symptoms on both apple and raspberry flowers and vegetative growing points, despite the differences in photosynthate content. Conversely, raspberry isolates failed to cause symptoms on apple shoots or flowers although they were easily recovered. Accordingly, there was no evidence that photosynthates play a role in delimiting the host range of E. amylovora. In contrast, twodimensional sodium dodecyl sulphate – polyacrylamide gel electrophoresis (SDS–PAGE) profiles of proteins secreted in a hrp (hypersensitive response and pathogenicity)-inducing medium were distinctly different for the apple and raspberry isolates. Differences in flagellin proteins (known pathogenicity factors), in a major outer membrane protein (OmpA), in a periplasmic ABC transporter, and in a heat-shock protein (Hsp70) were observed. Thus, these differences in protein profiles are more likely to be responsible for delimiting the host range of E. amylovora than host carbohydrate content

A quantitative method for inoculating strawberry leaves with Xanthomonas fragariae, factors affecting infection, and cultivar reactions

Abaxial surfaces of strawberry (Fragaria × ananassa) leaves were quantitatively inoculated with the causal agent of angular leaf spot, Xanthomonas fragariae, using a rigidly mounted spray gun device to examine factors affecting infection. Lesions were uniformly distributed within the inoculation sites, and their numbers increased as the distance between the spray nozzle and leaf surface decreased and the duration of the inoculation burst increased. Leaves of plants exposed to darkness before inoculation had low numbers of lesions, but numbers increased markedly after only 0.5 h of light and became maximal after 4 h, demonstrating the importance of inducing stomatal opening prior to inoculation. Exposing plants to a high or low preinoculation humidity did not affect lesion numbers, but incubating plants at a high postinoculation humidity resulted in greater numbers of lesions than those incubated at low humidity. Exposing plants to mist for 3 days following inoculation did not further increase lesion numbers. Plants exposed to frost (–1.5 ®C for up to 1.5 h) before inoculation had fewer lesions than those exposed to no frost, a result of frost-induced closure of stomata. Nineteen isolates obtained from diverse geographic regions were assessed for their aggressiveness on five strawberry cultivars, but no isolate × cultivar interaction was observed. The isolates exhibited a continuum of increasing aggressiveness, but no substantial differences were observed. Twenty-two strawberry cultivars were evaluated for their resistance to X. fragariae. A continuum of increasing resistance was observed except for the cultivar ‘Tristar’, which appeared to be substantially more resistant than the others. In a separate test, two clones, US 4808 and US 4809, which have been released for resistance breeding purposes, were more resistant than the standard cultivar ‘Honeoye’, but they were not completely resistant. The inoculation procedure described here will be useful for quantitatively assessing angular leaf spot of progeny in disease resistance breeding programs.

Breeding Strawberry Plants Resistant to Angular Leafspot Disease

The bacterium Xanthomonas fragariae causes the economically important angular leafspot disease to which a high level of resistance has not been found within the cultivated strawberry Fragaria × ananassa. The 2002 introduction of resistance donors ‘US 4808’ and ‘US 4809’ by the Fruit Laboratory at the Beltsville Agricultural Research Centre provided sources of resistance at the octoploid level derived from F. virginiana. Moving this resistance through three generations of back crossing to different recurrent F. × ananassa parents along with screening progeny for resistance with a spray inoculation method, followed by selection based on horticultural traits, has created improved resistant genotypes. Resistant × susceptible crosses have produced from zero to 21% resistant progeny with a mean of 4.8% for 25 crosses. The presence of undesirable traits, such as small and soft fruit, variegation, pistillate flowers, and susceptibility to powdery mildew, has slowed progress, but this linkage drag can be overcome by increasing screened population size and additional backcross generations. Two resistant × resistant crosses were made in 2009, yielding 32.2 and 32.7% resistant seedlings.

Effect of sugar alcohols, antioxidants and activators of systemically acquired resistance on severity of bacterial angular leaf spot (Xanthomonas fragariae) of strawberry in controlled environment conditions

Abstract Bacterial angular leaf spot of strawberry caused by Xanthomonas fragariae is an increasingly important disease for which few management strategies are available. The resistance status of most cultivars is unknown and of those cultivars tested, few have significant levels of resistance. Copper formulations are available and have limited effectiveness but phytotoxicity is a problem with repeated applications. This controlled environment study examined the efficacy of foliar applications of various antioxidants, such as α-tocopherol and mannitol, along with the plant activators acetylsalicylic acid and acibenzolar-S-methyl, and the fungicide fosetyl-Al, for disease control. Mannitol and acibenzolar-S-methyl consistently provided excellent control of disease, while fosetyl-Al and α-tocopherol were significantly less effective. Other antioxidants and sugar alcohols similar to mannitol did not provide the same level of angular leaf spot control. Interestingly, interactions were observed between various sugar alcohols and strawberry cultivars in suppressing disease. The mode of action of sugar alcohols against angular leaf spot is likely not related to their antioxidant activity. Sugar alcohols, especially mannitol, may represent a new and non-toxic approach to bacterial angular leaf spot control.

Epidemiology of fire blight of floricane fruiting red raspberry caused by Erwinia amylovora

Current information on the epidemiology of fire blight of raspberry (Rubus idaeus), caused by Erwinia amylovora, is sparse and based on the assumption of a similarity to fire blight of apple (Malus ×domestica) and pear (Pyrus communis). To test this assumption, epidemiological studies of raspberry fire blight were conducted in controlled environment and in a planting of raspberry selection K81-6, which is highly susceptible to this disease. The bacteria moved down to and overwintered in canes, within 6–10 cm of the soil line, but not at the soil line or in roots. The initial inoculum, responsible for the disease cycle, originated from systemically infected fruiting laterals that develop on floricanes in the spring. These laterals produced bacterial ooze that attracted ants (Tetramorium caespitum) in search of nectar, which then transmitted the inoculum to healthy flowers. Also, tarnished plant bugs (Lygus lineolaris), meadow spittle bugs (Philaenus spumarius), and earwigs (Forficularia auricularia) were shown to vector the pathogen to healthy raspberry leaves and canes through feeding wounds. Infected canes produced copious amounts of bacterial ooze days before other symptoms of fire blight infection became evident. Flowers were infected primarily through the nectaries, and green fruit could be infected only through wound inoculations. Nearly ripe and ripe fruit were not susceptible to infection. The implications of subtle epidemiological differences between apple and raspberry fire blight on disease management strategies are discussed.

Application of the MaryBlytTM version 4.3 to the prediction of fire blight in raspberry

Fire blight of apple (Malus ×domestica) and pear (Pyrus communis), caused by Erwinia amylovora, is a well-known disease and has been extensively researched. Fire blight of raspberry (Rubus idaeus), caused by a distinct strain of E. amylovora is less well known but appears to be similar in its behavior. The purpose of this study was to determine if the MaryBlytTM version 4.3 forecasting system for predicting fire blight in apples and pears could be applied to the prediction of fire blight in raspberry. Seven years of weather data and field observations from a naturally infected plot of Kentville raspberry selection ‘K81-6’ were entered into the MaryBlyt version 4.3 forecasting system. This system predicted flower infection events and the subsequent appearance of symptoms within 0-3 days of the observed symptoms for 2000-2006. In 4 of the 7 years, the appearance of systemically infected raspberry cane laterals was within ≤3 days of the MaryBlyt version 4.3 prediction for canker blight symptoms in apple when a modified degree-day threshold specifically for raspberry was used. Also, changing the degree-day base temperature from 12.7 °C to 4.0 °C reduced the error of prediction, but the error for three of the years was still unacceptably high. This study also showed that insect-vectored infections of primocanes cannot be successfully modeled by adaptations of the MaryBlyt version 4.3 program. However, a simple degree-day model independent of MaryBlyt was developed to predict flowering.