In New Zealand, potato crop yields of 90 t ha−1 are achievable but at 55 t ha−1, the average is becoming economically unsustainable. In 2012/13, a grower-initiated survey found that Rhizoctonia solani and Spongospora subterranea (soil-borne pathogens) and soil compaction were widespread in 11 Canterbury potato crops. Targeted areas in these crops had measured yield losses of 0 (healthy plants) – 42 (diseased, resource constrained plants) t ha−1, limiting the farmer-measured field yield to a 56 t ha−1 average. In 2014/15, growth, water use and health of three contrasting crops were measured. Near-potential yield was achieved in one crop when disease incidence and severity were low and resources adequate. The other two crops yielded less than potential; one had an inadequate water supply and the other widespread soil-borne disease. Any suspected links between soil-borne disease, soil compaction and yield were further investigated by using them as factors in replicated experiments reported elsewhere.

Powdery scab, caused by Spongospora subterranea, is an important potato disease. Greenhouse experiments in 2017/18 and 2018/19 on (very susceptible) ‘Agria’ seed tubers assessed if field-collected soils had different powdery scab-suppressive capabilities and identified factors involved in disease suppression. 2017/18: 12 geographically diverse soils with either S. subterranea added at planting or not added; 2018/19: six single-type soils used, to determine if powdery scab suppression was ‘general’, or ‘specific’ (transferable; possibly involving microorganisms), and if suppression was associated with soil physical, chemical, or biological factors (bacteria/fungi). For both seasons, S. subterranea soil ammendment increased scab severity on harvested tubers in all soils but one. Powdery scab severity (percent tubers with > 5% surface area covered by scabs) ranged from 0 to 39%. Soil texture, pH, soil organic matter and nutrient contents were associated with powdery scab incidence for some soils but not others. Effects of previous crop rotations on powdery scab were variable: one soil with three recent previous potato crops in rotation was disease-suppressive. All 2018/19 soils displayed some microbe-mediated disease suppression, three being more suppressive than others. Two had possible ‘specific’ Spongospora suppression (less disease when added to the conducive soil). Thus Spongospora -suppressive soils are present in New Zealand, and abiotic and biotic soil factors influenced incidence/severity of powdery scab of potato.

High-yielding potato crops in Canterbury have achieved 90 t ha−1or more. However, soil-borne diseases and poor soil structure are primary factors responsible for reducing this potential to an average of 55 t ha−1. The interaction between these factors was measured by growing the cultivars ‘Russet Burbank’ and ‘Innovator’ in 15 fields, grouped into four categories related to cropping history that influences soil health: soil-restorative (>7 years of grass) and non-restorative (>6 annual crops) with presence or absence of potato crops in the previous 10 years. Although predominantly grass histories with previous potato crops showed greater severity of soil-borne diseases than annual crop history without previous potatoes, this did not lead to yield losses. Improved soil quality increased yields by an average 10 t ha−1 regardless of disease pressure. Modelling suggested that early planting, maintaining crop canopies and managing water supply were also key factors for achieving high potato yields.

Fungi, nematodes and oomycetes belong to the most prominent eukaryotic plant pathogenic organisms. Unicellular organisms from other eukaryotic lineages, commonly addressed as protists, also infect plants. This review provides an introduction to plant pathogenic protists, including algae infecting oomycetes, and their current state of research.

Biological control activity of Trichoderma spp. is heavily influenced by abiotic factors. Effects of pH and water activity (a(w)) in culture media of Trichoderma atroviride LU132 were assessed on conidium production, germination percentage and bioactivity against a soil-borne pathogen Rhizoctonia solani in dual agar culture assays. The effects of pH and buffer capacity in media were tested on T. atroviride colonies using different proportions (50, 100, 200 and 400 mM) of phosphate buffers at pHs of 3.5, 4.5, 5.5, 6.5, 7.5 or 8.5. Water activity experiments tested different a(w) values (0.998, 0.995, 0.985, 0.977, 0.961 and 0.948). Conidium production was reduced with increasing phosphate buffer concentrations from 50 to 400 mM. Maximum germination, and inhibition activity against R. solani was measured for conidia obtained from medium at pH 7.5. Greatest conidium production occurred at a(w) 0.995 and the least at a(w) 0.948. Mean conidium germination was greatest (78%) from media at a(w) 0.961, and least (21%) from a(w) 0.985. Inhibition and overgrowth activities of Trichoderma colonies were greatest at a(w) 0.961 and least at a(w) 0.998. This study has demonstrated that manipulation of culture conditions as such may improve conidium fitness (quantity and quality). However, these factors may not have caused independently effects on the fungus. Effects of physical growth conditions (besides nutritional requirements) are likely to be important for optimum production of biocontrol agents based on T. atroviride LU132, and other similar biocontrol agents. (C) 2015 Elsevier Inc. All rights reserved.

The goal was to determine the effect of temperature, light and incubation period on production, germination and bioactivity of Trichoderma atroviride LU132 against Rhizoctonia solani.

AIMS:Effects of culture conditions on productivity, germinability and bioactivity of Trichoderma atroviride LU132 conidia were assessed to identify the factors affecting conidium 'fitness' (quantity and quality) and to withstand variable environmental conditions, increase conidial productivity, and perform optimum bioactivity. METHODS AND RESULTS:The interaction effects of temperatures (20 or 30°C) vs hydrocarbon types (dextrose or sucrose in constant C : N 5 : 1) were assessed for bioactivity and colonization potential in pot experiments with ryegrass in the presence of pathogen, Rhizoctonia solani. Trichoderma atroviride produced in different culture conditions increased some growth parameters of ryegrass plant and also reduced the pathogenicity effects of R. solani. For example, Trichoderma colony produced at 20°C with sucrose increased all plant growth parameters and conidia produced at 20°C with dextrose gave the greatest bioactivity. CONCLUSION:The bimodal population cycle in T. atroviride recurred in pot experiments in a manner similar to that previously observed in agar plates but indicating that simulated natural conditions shortened the Trichoderma life cycle. Trichoderma colonized ryegrass root system and symbiotically interacted with ryegrass and greater ryegrass colonization resulted from medium production treatment with dextrose rather than sucrose. SIGNIFICANCE AND IMPACT OF THE STUDY:This study is the first report on the effects of inoculum production conditions on conidium quality of Trichoderma to colonize and to maintain populations in host rhizospheres, and also the ability to promote plant growth and suppress a soil-borne disease. The results of these experiments provide new knowledge on how manipulation of culture conditions of T. atroviride LU132 can influence conidium fitness, as a basis for optimizing commercial production of the fungus as a biological control agent.

Effects of abiotic factors during production (temperature, nutrients, water activity, pH) on conidium fitness (quantity and quality) of Trichoderma atroviride LU132 (a key biocontrol agent) were studied. Conidia from the culturing regimes which resulted in greatest and least bioactivity against Rhizoctonia solani in dual culture assays were selected to assess effects of storage conditions on conidial fitness over time. Further studies assessed interaction effects of temperatures (20 degrees C or 30 degrees C) and sugars (dextrose or sucrose) on conidium germination and bioactivity as fresh conidia, or after 6 months of storage. Biochemical analyses of sugars and fatty acids were carried out to determine relationships between quality variations and cellular characteristics for conidia produced in different culturing conditions. Low trehalose content in conidia (e. g. at 20 degrees C) was associated with the least conidium fitness, although high trehalose content did not necessarily support conidium fitness. High proportions of total fatty acids in conidia were mostly associated with the least conidium fitness. When Trichoderma was grown at high carbon to nitrogen ratio (e. g. at C:N 160: 1), the total conidium fatty acids content increased. This study also indicated that the monosaccharide dextrose is metabolically optimal for T. atroviride LU132 at 20 degrees C while the disaccharide sucrose is optimal at 30 degrees C. These studies indicate that physical growth conditions and nutritional requirements attribute in conidium fitness of T. atroviride LU132, and provide important knowledge supporting optimum production of biocontrol agents based on T. atroviride, and possibly other similar biocontrol agents.

A rotation trial spanning nine consecutive growing seasons was established in 2004 to study cumulative effects of specific onion- and potato-focused crop rotations on soil nutrient levels, soil biological communities, plant productivity and soilborne diseases. Soil microbial activity, as determined by fluorescein diacetate hydrolysis, was greatest in the sustainable' potato rotation in five of the 6 years that the test was carried out. Rhizoctonia solani anastomosis group 3 DNA was first detected in potato monoculture soils in the fifth year, with numbers increasing from then on, but was not detected in the onion monoculture throughout the trial period. Potato yields were greater when a crop other than potato was grown in the previous year compared with when potatoes were the preceding crop. After 2005, mean annual onion yields from the onion monoculture were less than yields from the other rotations. Black scurf on potato tubers was the primary soilborne disease observed during the study, and the incidence of this disease was greater in the potato monoculture than the other rotations after the second year, and least when potatoes had not been grown in the same ground the three previous seasons. This long-term crop rotation study has demonstrated the benefits of a sustainable' rotation where potatoes or onions were grown every fourth growing season, with different crops (oat, broccoli, cabbage, squash) grown in the intervening years, compared with the conventional consecutive biennial crops of potato and onion.

Identification of the production and storage factors that affect conidium germination and bioactivity (fitness) will assist the success of biological control agents. Effects of culturing conditions on conidium fitness of Trichoderma atroviride LU132 were examined in different storage conditions over time. Abiotic factors (temperature, nutrients, water activity and pH) during production were studied. Conidia from the culturing regimes which resulted in greatest and least bioactivity against Rhizoctonia solani in dual culture were selected to assess effects of storage conditions on conidium fitness. Fitness of the test conidia was examined after storage at 30 degrees C and at 0% or 50% relative humidity (RH) over 6 months. Fitness declined over time, and the decline was greater for 50% RH than 0% RH, probably through reduced metabolic activity of conidia during long-term storage. Stored conidia were probably affected by dehydration, temperature and other factors such as oxidation, before and during storage, and also by rehydration after storage. The greatest number of conidia and germination percentage resulted from production at 25 degrees C, but greatest bioactivity resulted from those produced at 30 degrees C. No significant effects on bioactivity were detected between the conidium production treatments C : N 5 : 1 and C : N 160 : 1, indicating that C : N ratio in culture medium is not important for conidium survival of T. atroviride.

A major concern in potato and onion production worldwide is the sustainability of crop production systems using short-term rotations. Although rotation crops are known to influence soil microbial communities, few long-term field studies have investigated effects of potato and onion crop rotations on soil microflora and soilborne diseases. A long-term trial (10 years) was established in New Zealand in the 2004–05 season to determine changes in soil characteristics, soil biological communities, plant productivity and soilborne diseases resulting from different crop rotations. Six different crop rotation ‘treatments’ were used: three were onion-focused and three were potato-focused. For both the onion- and potato-focused rotations, one treatment was continual monoculture, one was biennial alternating onion and potato (‘conventional’ rotation, common local commercial practice) and one was a 4-year cycle where the onion or potato crops were grown every fourth season with different crops in the intervening seasons (‘sustainable’ rotation). The results of the rotations were evaluated at the end of 2012–13, nine growing seasons after the trial began. Soils from the three potato-focused rotations had similar microbial biomass carbon (C) and nitrogen (N), total C and N, and enzyme activity, but had different microbial activity parameters. Soils from the three onion rotations differed in organic matter content, total C and N, enzyme activity and carbohydrate utilization. The main factor correlated with potato diseases (stem canker, black scurf) and crop yields was the amount of Rhizoctonia inoculum in the soil. Onion yields in 2012–13 were correlated with the severity of pink root during the growing season. Total microbial activity in soil gave better relationships to soilborne disease incidence (Rhizoctonia stem canker of potato or pink root of onion) and to crop yields, than soil chemical parameters (pH, nutrient status, C:N ratio) or other microbiological parameters (microbial biomass, total culturable bacteria and fungi, and microbial diversity). This research has broadened understanding of effects of crop rotations on soil microbial community characteristics, soilborne diseases and crop productivity of potatoes and onions grown in a soil and an area where intensive vegetable production is common. This knowledge will assist growers to increase crop yields and reduce soilborne pathogens in a sustainable manner. Based on the results from this study, intensive potato and onion production should require these crops to be grown within 4-year crop rotations in order to foster sustainable yields and minimize soilborne diseases.

Double-stranded RNA (dsRNA) elements are ubiquitous in Rhizoctonia solani. Total dsRNA was randomly amplified from a R. solani isolate (RS002) belonging to anastomosis group3PT (AG-3PT), associated with black scurf in potato. Assembly of resulting cDNA sequences identified a nearly complete genome of a novel virus related to the genus Mitovirus (family Narnaviridae), herein named Rhizoctonia mitovirus 1 RS002 (RMV-1-RS002). The 2797 nucleotide partial genome of RMV-1-RS002 is A-U rich (59.06 %), and can be folded into stable stem-loop structures at 5' and 3' ends. Universal and mold mitochondrial codon usages revealed a large open reading frame in the genome, putatively encoding an 826 amino acid polypeptide, which has conserved motifs for mitoviral RNA-dependent RNA polymerase. The full length putative polypeptide shared 25.6 % sequence identity with the corresponding region of Tuber excavatum mitovirus (TeMV). The partial genome of a second mitovirus (proposed name Rhizoctonia mitovirus 2 RS002 (RMV-2-RS002)) was also amplified from RS002. A nearly identical copy of RMV-1-RS002 was detected in two additional AG-3PT isolates. These data indicate that multiple mitoviruses can exist in a single isolate of R. solani AG-3PT, and that mitoviruses such as RMV-1-RS002 are probably widespread in this pathogen. The roles of mitoviruses in the biology of R. solani AG-3PT remain unknown. (C) 2015 The British Mycological Society. Published by Elsevier Ltd. All rights reserved.

Induction of resistance in potato to Spongospora subterranea (which causes powdery scab on tubers) was studied in two controlled glasshouse experiments. β-aminobutyric acid (BABA) was applied at different concentrations (1, 2 or 4 mM) to potato plants which were then inoculated with S. subterranea. Amounts of the pathogen in roots and intensity of Spongospora root galling were assessed. Infection was measured at different times during plant growth, at 3 or 6 weeks after inoculation (first experiment), and 2, 4 or 6 weeks after inoculation (second experiment). In the first experiment, growth of plants was reduced in proportion to the BABA application rate, and the plants had high levels of S. subterranea infection. In the second experiment, at 4 weeks after inoculation, plants treated with BABA had less S. subterranea in their roots and fewer root galls compared with non-treated plants. Six weeks after inoculation, plants treated with 1 mM BABA developed S. subterranea infection, almost to the level of non-treated plants. In contrast, 2 and 4 mM BABA treatments greatly reduced the amount of S. subterranea in roots and numbers of root galls compared with non-treated plants. Repeated applications (at 2 weeks intervals) increased the effectiveness of BABA. A third test indicated that BABA was not directly toxic to S. subterranea resting spores. These results demonstrate that chemically induced resistance reduces host root infection by S. subterranea, and may have disease management potential. This is the first report of chemically induced host resistance to Spongospora.

Trichoderma spp are widely used as biocontrol agents and plant growth promoters Endophytic colonisation of perennial ryegrass (Lolium perenne) by T atroviride strains was studied in laboratory greenhouse and field experiments Four strains of T atroviride were inoculated into in vitro agar cultures or potting mix with ryegrass seeds for colonisation studies The strains were also produced as prill or granule formulations for application in the field experiment Microscopy was used to observe fungal structures in plant tissue segments from pot or fieldgrown plants Segments were also surface sterilised and placed onto Trichoderma selective media Fungal colonies recovered were verified as T atroviride by sequencing the tef1 gene No endophytic colonisation occurred in any noninoculated plants and no Trichoderma endophytic colonisation was observed in fieldgrown plants However microscopy revealed fungal hyphae and reproductive structures characteristic of Trichoderma in root and stem sheath tissues of inoculated plants from in vitro cultures and pots These results were verified by sequencing the tef1 gene This study has demonstrated endophytic colonisation of ryegrass by T atroviride strains which may be related to beneficial effects on plant growth and disease control

Spongospora subterranea f. sp. subterranea causes the potato diseases powdery scab on tubers and galls on roots, and occurs in most potato production areas worldwide. The pathogen was probably introduced to Europe from South America in the 16th century. Three different genotype clusters have been found worldwide: the genetically variable groups from South America (native), and, in contrast, the nearly clonal group outside South America (invasive). An inoculation experiment was carried out with the long-day potato host ‘Agria’ comparing three different native Spongospora resting spore inocula with an invasive one, to determine the infection risk potential on a widely grown potato subspecies. All inocula led to root infection. Invasive tuber lesion sporosori from ‘Agria’ produced the greatest amount of infection, whereas the tuber lesion inoculum from the Venezuelan S. tuberosum ssp. tuberosum host and the root gall inoculum from the Colombian S. phureja host caused the least infections. The inoculum genotypes corresponded to all of the three previously described groups. Most root galls showed the invasive group type, independent of the inoculum. These results suggest that the most successful invasive genotype is still present in native pathogen populations and emphasize the need for continued quarantine vigilance to prevent new virulent recombinants of the pathogen.

Information is reviewed on root infection of potato by the plasmodiophorid Spongospora subterranea f. sp. subterranea. This pathogen has long been recognized as the cause of root galls (hyperplasia) and the economically important disease powdery scab on tubers (modified stolons). The significance for plant productivity of the zoosporangium stages of the pathogen in potato roots has only recently begun to be documented. Two experiments are described that assessed effects of S. subterranea root infection on potato plant root function and productivity. A greenhouse experiment measured root function and plant parameters for eight potato cultivars with markedly different susceptibilities to tuber powdery scab. Water uptake and plant growth were reduced by S. subterranea inoculation in all eight cultivars. The magnitudes of these negative effects, and intensities of root hyperplasia, differed among the cultivars, but were not related to respective susceptibilities to tuber powdery scab. A field trial assessed root function and plant productivity for a cultivar (Iwa) that is very susceptible to Spongospora tuber and root diseases. Soil water content beneath uninoculated plants was consistently less than for inoculated plants, indicating that inoculation reduced water uptake (root function). Inoculation reduced shoot and root dry weights, and reduced weight of tubers per plant by 42%. Spongospora subterranea causes three diseases of potato: root membrane dysfunction, root hyperplasia and tuber powdery scab. The root diseases caused by the pathogen are likely to be important both for powdery scab management and for deleterious effects on potato crop yields.