John W. Marshall

A phylogenetic analysis of the SSU rRNA from members of the Plasmodiophorida and Phagomyxida.

The Plasmodiophorida and Phagomyxida are orders of zoosporic eukaryotes of uncertain phylogenetic affinities. We have obtained ribosomal DNA sequence information from several species in these groups. Phylogenetic analyses of these sequences have confirmed that they form a monophyletic grouping that clusters most closely with a rhizopod assemblage consisting of sarcomonads and chlorarachneans. The Phagomyxida sequences appear distant enough from those of the Plasmodiophorida to justify a separate order. The relationships between the Plasmodiophorida remain unclear, with species from the same genus showing significant degrees of divergence.

Differentiation of Australasian potato cyst nematode (PCN) populations using the polymerase chain reaction (PCR)

Abstract Molecular examination of the ribosomal internal transcribed spacer (ITS) region in potato cyst nematodes (PCN) is described. The ITS was amplified and sequenced from a number of PCN collections. A low level of sequence variation was found between Globodera rostochiensis, G. pallida, and a Peruvian PCN collection, but no variation within Australasian collections of species was noted. Polymerase chain reaction (PCR) primers based upon the G. rostochiensis–G. pallida sequence differences were designed and successfully used to identify mixed PCN species in a single PCR reaction.

Fusarium graminearum, F. cortaderiae and F. pseudograminearum in New Zealand: molecular phylogenetic analysis, mycotoxin chemotypes and co-existence of species

Fusarium graminearum and F. pseudograminearum are important plant pathogens in New Zealand and around the world. Headblight and crown rot diseases of cereals caused by these species are responsible for large economic losses due to reduction in seed quality and contamination of grain with tricothecene mycotoxins. In the current study we have used two different molecular phylogenetic approaches, AFLPs and gene genealogies, to gain insight into the evolutionary relationships between F. graminearum, and F. pseudograminearum in New Zealand. The worldwide genetic diversity of F. graminearum clade is represented by at least eight biogeographically distinct species (previously designated as lineages of F. graminearum). Our analysis demonstrated that this clade is represented by F. graminearum (= F. graminearum Lineage 7) and F. cortaderiae (= F. graminearum Lineage 8) in New Zealand. Through our analysis we also confirm the presence of F. pseudograminearum in New Zealand as a first record for this organism. Information on species is necessary for preventing the inadvertent intercontinental introduction of genetically unique foreign pathogens associated with world trade. The ability to place species information into a worldwide context enabled postulation that the New Zealand representatives of F. graminearum clade originated from at least two regions, and probably on at least two hosts. Correlation of species descriptions with biogeographical and host information revealed evidence for co-localisation of F. graminearum clade species with potential for genetic outcrossing in the field. Mycotoxin analysis showed F. graminearum (= lineage 7) isolates produce either nivalenol (NIV) or deoxnivalenol (DON). In contrast, F. cortaderiae isolates produced only NIV. These findings support earlier observations that mycotoxin production in the F. graminearum clade is not species specific, but suggest maintenance of chemotype diversity through speciation may have been restricted to a subset of species.

Effect of production environments on radiation interception and radiation use efficiency of potato (Solanum tuberosum) grown in Canterbury, New Zealand

Abstract Four experiments were conducted over two growing seasons during 1995/96 and 1996/97 at Lincoln University and New Zealand Institute for Crop & Food Research, Lincoln, Canterbury, to assess whether radiation interception (RI) alone is a good predictor of potato (Solanum tuberosum) total dry matter (TDM) grown under different environments. In the two field experiments production environments were altered by different rates of nitrogen application and in bag experiments, the different production environments were created with the presence of different amounts of potato cyst nematode (Globodera rostochiensis) per ml of soil. The different production environments significantly affected leaf area index (LAI), RI, radiation use efficiency (RUE), and TDM. Increased soil fertility significantly increased RI, RUE, and TDM where as the presence of potato cyst nematode significantly decreased RI, RUE, and TDM. There was a strong (P < 0.001) positive linear relationship between RUE and applied nitrogen, whereas there was a linear, negative relationship between RUE and the numbers of potato cyst nematode per ml of soil. On the basis of this study it was concluded that under different production environments both RI and RUE must be known to accurately predict the TDM of potatoes.

Detecting the presence and distribution of Globodera rostochiensis and G. pallida mixed populations in New Zealand using DNA probes

Abstract The distribution of the potato cyst nematode Globodera rostochiensis (Woll.) pathotype Rol and G. pallida (Stone) pathotype Pa2/3 in New Zealand was determined using dot‐blot hybridisation with species‐specific DNA probes. Both species of nematode were present in New Zealand, with the main foci in intensive cropping areas (market gardens) around the urban centres of Auckland, Christchurch, and Dunedin. A number of domestic gardens were also infested. Of the 64 populations tested, 24% were pure Ro 1, 20% were predominantly Ro 1 with a low frequency or trace of Pa2/3, 33% were pure Pa2/3, and 6% were Pa2/3 with a trace of Ro 1. The remaining 17% contained an equal proportion of both species. The presence of trace Pa2/3 and the impact of mixed species populations on the long‐term success of nematode‐resistant potato cultivars is discussed.

Effect of inoculum dose on infection of apple (Malus domestica) flowers by Erwinia amylovora

Abstract The effects of inoculum dose, flower age, and climate (temperature, relative humidity, and rainfall) on the incidence of fire blight following inoculation of apple (Malus domestica) flowers with Erwinia amylovora were studied in an orchard. Disease symptoms were only observed when populations of E. amylovora exceeded 106 colony‐forming units (cfu) on flowers 4 days after budburst. The rate at which these populations were reached depended on the climatic conditions over flowering. Ten cfu of E. amylovora inoculated on flower stigmas were capable of inciting fire blight in an orchard in which temperatures above 15°C and high rainfall provided ideal conditions for bacterial multiplication, infection, and subsequent expression of symptoms in flowers. The results from this study may lead to insights into improved management strategies and provide key information for phytosanitary risk assessments for exports of commercial apple fruit.

Effect of early blight (Alternaria solani) and different nitrogen inputs on radiation interception, radiation use efficiency, and total dry matter production in potatoes (Solanum tuberosum) grown in Canterbury, New Zealand

Abstract Field experiments were conducted in 1995/96 and 1996/97 to investigate the effect of early blight epidemics on photosynthetically active radiation (PAR) interception (RI), radiation use efficiency (RUE), and total dry matter (TDM) production of potatoes (Solanum tuberosum) grown in Canterbury, New Zealand. RI was calculated from green leaf area index (GLAI) corrected for disease. RUE was calculated as the ratio between accumulated TDM and accumulated RI. Early blight reduced RI by 9% in both seasons and RUE by 17% and 28% in 1995/96 and 1996/97 respectively. There was a total loss of 26% and 36% in TDM in 1995/96 and 1996/97 respectively. Nitrogen inputs increased RUE by 3% and 16% as compared to low nitrogen status in 1995/ 96 and 1996/97 respectively. Results showed that RI and RUE, calculated from leaf area index (LAI), not corrected for disease, underestimated the loss in RI and RUE.

Effect of early blight (Alternaria solani) on healthy area duration and healthy area absorption of potatoes (Solanum tuberosum) grown in Canterbury New Zealand with different nitrogen application and stress from potato cyst nematode (Globodera rostochiensis)

Abstract Two experiments were conducted over the 1995/96 and 1996/97 growing seasons, at Lincoln University and New Zealand Institute for Crop & Food Research, Lincoln, Canterbury to assess the effects of nitrogen and potato cyst nematode (Globodera rostocheinsis) on the development of early blight epidemics. The objective was to measure effects on photosynthetically active radiation (PAR) intercepted by crops under multiple pest constraints, and also to compare the effect of early blight, nitrogen, and potato cyst nematode on integrated values for healthy leaf area duration (HAD) and healthy leaf area absorption (HAA). Epidemic development of early blight was rapid in high nitrogen fertility plots and in plots with 30 nematode eggs/ml of soil in 1995/96, but in 1996/97 neither nitrogen nor potato cyst nematode significantly affected the epidemic development. In these trials the intercepted PAR (HAA) was derived from the integral over time of green leaf area index (GLAI) which was used as a measure of HAD. HAD and HAA were derived from leaf area index (LAI) corrected by two methods, the percent reflectance (PR) ratio or percent yellowing (PY). This paper explains the detailed correction methods used and shows that LAI corrected with PR ratio is the best way for calculating GLAI from a diseased crop, which can then be used for calculating HAA. Early blight and potato cyst nematode significantly reduced HAD and HAA whereas nitrogen significantly increased HAD and HAA in both growing seasons. The effect of disease on HAD and HAA was not altered by nitrogen or potato cyst nematode in both growing seasons.

Soil on imported shipping containers provides a source of new Pseudomonad biodiversity into New Zealand

Abstract Exotic soil attached to shipping containers imported to New Zealand was assessed for new species of Pseudomonas (sensu stricto) bacteria to determine whether this soil facilitated the introduction of new biodiversity. Pseudomonads were isolated from exotic soil originating from 19 different countries (termed container isolates (CIs)), and from soils in eight geographically distinct locations throughout New Zealand (termed New Zealand isolates (NZIs)). Partial 16S rRNA gene nucleotide sequences were determined from 40 selected CIs and 38 NZIs. Relationships between CIs, NZIs, and a further 25 typed Pseudomonad species were inferred by 16S rRNA phylogenetic analyses. CIs showed relatedness to a broad range of Pseudomonad species and many CIs grouped with NZIs suggesting they are closely related to Pseudomonads currently in New Zealand. However, unique phylogenetic branchings of CIs also indicate that soils attached to shipping containers may provide a source of new Pseudomonad biodiversity to New Zealand.

Empirical models of the relationships between early blight (Alternaria solani) and yield of potato (Solanum tuberosum) crop grown under different production environments

Abstract Four experiments were conducted over two growing seasons during 1995/96 and 1996/97 at Lincoln University and the New Zealand Institute for Crop & Food Research, Lincoln, Canterbury. A large database was used to assess the effects of production environments on disease yield relationships and to compare different models derived from pathogen‐based disease measurement and host‐based disease measurements. All the models were developed using regression analyses. All the single point (SP), multiple point (MP), and integral models derived from host‐based disease measurements explained more variation than models derived from pathogen‐based disease measurements. SP, MP, and integral models were also compared and integral models were selected as the best for comparing disease effects as they combine the effect of disease over the whole growing season in one variable. Further, integral models were tested for the effect of different production environments on disease yield relationships. It was found that the nitrogen‐altered production environments had no significant effect on disease yield relationships, whereas potato cyst nematode‐altered production environments significantly affected the disease yield relationship. A single model was used for the disease yield relationship for nitrogen inputs, whereas separate disease yield relationships were used for each nematode level.