Pathogenicity of Phoma betae isolates from red beet (Beta vulgaris) at seed farms in Canterbury, New Zealand

Phoma betae is an economically important pathogen of red beet causing pre-emergence seedling damping, leaf spot and root rot. However, the pathogenicity of P. betae is unknown in New Zealand despite the economic importance of this pathogen. Twenty-five isolates were collected from a survey of red beet seed farms in Canterbury, New Zealand during 2016/2017 and three of these PB101 (from seeds), PB103 (from roots) and PB106 (from leaves) were used for pathogenicity testing of two red-beet cultivars. Isolate PB106 was further used to investigate its effects on spinach and fodder beet as well as red beet under greenhouse conditions. All three P. betae isolates were pathogenic on both red-beet cultivars tested, causing leaf-spot symptoms. Isolates PB101 and PB106 produced significantly larger leaf-spot lesions (P<0.001) compared with PB103. Phoma betae isolate PB106 was pathogenic to both red-beet cultivars, spinach and fodder beet but fodder beet was less susceptible than the other species tested. Regardless of cultivar, P. betae is an important pathogen of beets and is capable of causing leaf spots

Californian thistle (Cirsium arvense): endophytes and Puccinia punctiformis

Californian thistle (Cirisum arvense) is a troublesome weed in pastures and cropping systems. The fungal biocontrol agent Puccinia punctiformis, commonly referred to as thistle rust, performs inconsistently on C. arvense. Problems with P. punctiformis establishment and control of C. arvense may be attributable to differing plant endophytic populations in various environments. This article provides an overview of the relationships between endophytes and their host, but also between endophytes and pathogens with a focus on rust pathogens. This review provides insights into reasons why P. punctiformis performs inconsistently and identifies gaps in our knowledge. Filling these gaps may help to improve performance of this classical fungal biocontrol agent

Distribution of Puccinia punctiformis in above-ground tissue of Cirsium arvense (Californian thistle)

Cirsium arvense (Californian thistle) is a problematic weed in agricultural systems throughout New Zealand and the rust fungus Puccinia punctiformis is a potential biological control agent for this weed. Puccinia punctiformis can systemically infect thistles but the movement of the pathogen in planta is not fully understood. This research determined the level of infection in planta caused by P. punctiformis at a single time point. The concentration of P. punctiformis DNA in planta was determined to ascertain the location of the fungus within naturally field-infected C. arvense. Quantitative polymerase chain reaction was undertaken on above-ground symptomatic and asymptomatic C. arvense tissue at various locations within leaves (top, middle and bottom) and the main stem. All C. arvense shoots had detectable amounts of P. punctiformis but the concentration was 100× greater in symptomatic compared with asymptomatic shoots. In general, the concentration of fungus progressed up the leaves with a significant effect between locations (P<0.001). Puccinia punctiformis was found in planta but broadscale disease of C. arvense does not occur and the reason for this is unknown

Short communication: Latent detection of downy mildew (Peronospora pisi) in bioassays against Pisum sativum

Downy mildew of peas is caused by the obligate parasite Peronospora pisi, which occurs sporadically throughout temperate pea-growing regions across the world. To screen pea lines against this biotrophic pathogen, a suitable and reproducible in vitro method using living plant material is required. Field screening can be influenced by environmental factors, thus giving variable results. The aim of this study was to develop a method that could reliably be used to screen pea cultivars against P. pisi in a laboratory setting. A range of bioassays were used to test various methods of inoculation, utilizing sporangia and naturally infested soil. Latent infection was achieved by planting seeds in soil collected from a site with a known history of P. pisi infection and directly inoculating young pea plants with sporangia. Out of the 108 plants which survived the experimental period, only two plants expressed visible signs of disease; however, through a two-step nested PCR process we detected latent infection in 24 plants. This research highlights the importance of considering the presence of latent infection when screening pea lines against downy mildew

Phenotypic characterization of the Hordeum bulbosum derived leaf rust resistance genes Rph22 and Rph26 in barley

Aims: Two introgression lines (ILs), 182Q20 and 200A12, which had chromosomal segments introgressed from Hordeum bulbosum in H. vulgare backgrounds, were identified to show seedling resistance against Puccinia hordei, possibly attributed to two resistance genes, Rph22 and Rph26, respectively. This study characterized the phenotypic responses of the two genes against P. hordei over different plant development stages. Methods and Results: Using visual and fungal biomass assessments, responses of ILs 182Q20, 200A12 and four other barley cultivars against P. hordei were determined at seedling, tillering, stem elongation and booting stages. Plants carrying either Rph22 or Rph26 were found to confer gradually increasing resistance over the course of different development stages, with partial resistant phenotypes (i.e. prolonged rust latency periods, reduced uredinia numbers but with susceptible infection types) observed at seedling stage and adult plant resistance (APR) at booting stage. A definitive switch between the two types of resistance occurred at tillering stage. Conclusions: Rph22 and Rph26 derived from H. bulbosum were well characterized and had typical APR phenotypes against P. hordei. Significance and Impact of the Study: This study provides important insights on the effectiveness and expression of Rph22 and Rph26 against P. hordei during plant development and underpins future barley breeding programmes using non-host as a genetic resource for leaf rust management

Fruit drop in two kiwifruit varieties and the use of two Bacillus-based biofungicides

Recently, fruit drop in two green varieties of kiwifruit (Actinidia deliciosa; VarA and VarB) has increased towards the harvest date. The efficacy of two biofungicides, applied post-flowering, to ameliorate the effects of early fruit drop in VarA and VarB during the 2017–18 growing season was tested. Treatments were applied to a single bay, with buffer bays and rows adjacent. Treatments were two different Bacillus-based biofungicide products; Serenade® Max (a.i. B. subtilis QST713 strain) and Triple-X® (a.i. B. amyloliquefaciens BS 1b). There was an untreated control. All fruits in the canopy, within the treated bay, were counted and recorded at 4-weekly intervals, over 5 months until harvest. At the VarA site, there was no statistical difference (P>0.1) in the percentage of fruit drop between the control (7%), Triple-X® (5%) and Serenade® Max (10%) treated vines. At the VarB site, fruit drop differed statistically (P<0.1) between Triple-X® treated vines (5%) and the control (10.5%), with Serenade® Max treated vines (6.5%) having intermediate fruit drop. The use of Triple-X® may be an option to assist with reducing fruit drop in kiwifruit

The infection of the fruit of ‘Cavendish’ banana by Pseudocercospora fijiensis, cause of black leaf streak (black Sigatoka)

Black leaf streak (black Sigatoka) caused by Pseudocercospora fijiensis (teleomorph Mycosphaerella fijiensis) is the most destructive leaf disease of banana world-wide. It is capable of destroying plantations unless controlled and successful commercial production is dependent on frequent applications of fungicides. A study was made to determine whether the pathogen is capable of infecting the fruit of ‘Cavendish’ banana and completing its life cycle on banana skin as the sole substrate. Emerging and recently emerged bunches of ‘Cavendish’ banana were inoculated in the field with carbendazim-resistant strains of P. fijiensis, incubated in situ in a humid environment for 6 days and then allowed to develop and mature naturally. The fruit were harvested approximately 16 weeks after inoculation, ripened to green tip stage, and skins incubated either as whole skins in humid containers or as thin strips of skin removed from the fruit and incubated on water agar modified with penicillin, streptomycin and carbendazim in Petri dishes. Supradermal fascicles of P. fijiensis were found on the incubated skin strips and whole skins within 5 days. From a total of approximately 300 field inoculated fruit, 18 colonies of P. fijiensis were detected. Small pieces of skin, each supporting a single fascicle, were excised, incubated individually on modified water agar and monitored for the production of conidia. It was shown that the fungus colonised, and was able to sporulate profusely on the excised skin pieces. This finding has implications for the transfer of strains of P. fijiensis between countries via commercial banana shipments and represents a potential pathway of entry to countries currently free of the disease

Effects of amendments to agar media on the in vitro growth of the entomopathogenic fungus Ophiocordyceps robertsii (Hook.) G.H. Sung, J.M. Sung, Hywel-Jones & Spatafora (Hypocreales: ophiocordycipitaceae)

Two experiments were undertaken to determine the effect of agar amendments on the growth of the asexual (anamorph) stage of the entomopathogenic fungus Ophiocordyceps robertsii (Hook.) G.H. Sung, J.M. Sung, Hywel-Jones & Spatafora (Hypocreales:Ophiocordycipitaceae) in culture. These were part of ongoing research whose aim was to develop methods for augmenting O. robertsii infested ghost moth (Hepialidae) larvae (āwheto) populations in forests, with a view to assisting their potential commercial harvest. The first experiment used full-strength potato-dextrose agar (PDA) with added yeast while the second used a reduced concentration of PDA. Amendments included mycosed host (ghost moth larva) tissue, leaf litter, soil, hinau (Elaeocarpus dentatus (J.R.Forst. et G.Forst.) Vahl var. dentatus) leaves, berries, seeds and roots and combinations of these amendments. The hinau leaf amendment treatment stood out, unexpectedly causing a 53% reduction in fungal growth in the first experiment and a 100% reduction in the second. Several of the other amendments were associated with a small (up to 15%) reduction in fungal growth in both experiments while in the second experiment, with a reduced-agar content, some combinations of amendments that avoided the use of hinau leaves were associated with an increase in fungal growth of 24%–43%. While these experiments did not achieve the original objectives of developing amended media recipes for increasing O. robertsii growth rates, they do indicate that hinau leaves may contain chemicals with anti-fungal properties. The results are discussed in relation to recent discoveries of chemicals with strong anti-fungal properties in the leaves of an Elaeocarpus species in India. More research into the nature of New Zealand endemic Eleaocarpus spp. phytochemistry and its anti-fungal properties is warranted

Cauterising or pruning to minimise spread of cankers caused by Pseudomonas syringae pv. actinidiae in Kiwifruit

Pseudomonas syringae pv. Actinidiae (Psa) causes leaf spotting, cane and leader cankers, or death of kiwifruit vines. Growers require management strategies that minimise the spread of cankers or systemic infection, whilst also minimising canopy loss. This project investigated whether cauterising cankers or pruning beyond the visible canker had any long-term effect in containing progression of the pathogen within the vine. The study included 72 ‘Zesy002’ (commonly known as Gold3) (Actinidia chinensis) vines on one orchard and 87 ‘Hayward’ (A. deliciosa) vines on two orchards, in Te Puke, New Zealand. From one to 30 cane or leader cankers per vine were monitored. Cankers were either left untreated, cauterised using a gas blowtorch, or pruned 40 cm below the lowest symptom. Cankers were delineated initially and canker advance was measured after 1, 3, 7 and 12 months. In ‘Hayward’, pruning proved to be the most effective, and most pruning wounds callused and healed. Cankers spread only where pruning wounds had not callused and healed. Only 11% and 3% of pruned canes on the two orchards, respectively, showed canker advance after 7 months. In comparison, in cauterised and untreated vines the majority (over 80%) of cankers continued to expand, with no obvious difference between these two treatments, and many instances where cankers expanded beyond the 40-cm mark. In Gold3, canker expansion was greater than in ‘Hayward’. Pruning 40 cm below the visible canker margin did not remove the infection from the cane, and cankers continued to expand, in 18.4% of cases. This compares with 27.6 and 27.8% of cankers that expanded beyond the 40-cm mark in the untreated and cauterised vines, respectively, with more than 80% of cankers expanded beyond the original canker margin. The experiments showed that pruning beyond the visible canker reduced the systemic spread of Psa more effectively than cauterising cankers or leaving them untreated

Effect of sclerotial form and isolate on the pathogenicity and virulence of Sclerotinia sclerotiorum on Ranunculus acris

Ranunculus acris (giant buttercup) is a significant weed in dairy pastures in New Zealand, outcompeting grasses and clovers and leading to an increased impacts over time. In recent years, there has been a reduction in efficacy of herbicides registered for the control of R. acris due to evolved resistance. Thus, there is a need for alternative control measures. The fungus, Sclerotinia sclerotiorum, has undergone substantial research to develop it as a mycoherbicide to R. acris using a grain-based formulation. Our study investigated the vegetative survival structure, the sclerote, as a potential source of inoculum for the mycoherbicide. Three bioassays were undertaken to determine: 1) the most effective sclerotial form (whole, ground or powdered) of S. sclerotiorum for the infection of R. acris; 2) the virulence of two S. sclerotiorum isolates (S36 and S37) on R. acris; and 3) if there is a difference in the pathogenicity of S. sclerotiorum on the different chloroplast cytotypes of R. acris. The assays showed that the S. sclerotiorum isolate (S36, S37), inoculum type (whole, ground or powdered), amount of sclerotia applied, and R. acris cytotype all affected necrotic lesion development. By contrast, there was no significant (P>0.05) and consistent difference between the lesions caused by the two isolates of S. sclerotiorum. Sclerotia cut or ground, germinated and infected R. acris tissue quicker than intact sclerotia. There was no difference in the response of the different R. acris cytotypes. Despite the inconsistencies observed between the bioassays, the methods are useful for future screening of potential S. sclerotiorum isolates in a mycoherbicide for R. acris