Long‐term phosphite application maintains species assemblages,richness and structure of plant communities invaded by Phytophthora cinnamomi

The impact of the plant pathogen Phytophthora cinnamomi and the fungicide phosphite on species assemblages, richness, abundance and vegetation structure was quantified at three sites in Kwongkan communities in the Southwest Australian Floristic Region. Healthy and diseased vegetation treated with phosphite over 7–16 years was compared with non‐treated healthy and diseased vegetation. After site differences, disease had the greatest effect on species assemblages, species richness and richness within families. Disease significantly reduced cover in the upper and lower shrub layers and increased sedge and bare ground cover. Seventeen of 21 species assessed from the families Ericaceae, Fabaceae, Myrtaceae and Proteaceae were significantly less abundant in non‐treated diseased vegetation. In diseased habitats, phosphite treatment significantly reduced the loss of shrub cover and reduced bare ground and sedge cover. In multivariate analysis of species assemblages, phosphite‐treated diseased plots grouped more closely with healthy plots. Seven of 17 susceptible species were significantly more abundant in phosphite‐treated diseased plots compared with diseased non‐treated plots. The abundance of seven of 10 Phytophthora‐susceptible species was significantly higher along transects in phosphite‐treated vegetation. Comparison of the floristics of healthy non‐treated with healthy‐treated plots showed no significant differences in species assemblages. Of 21 species assessed, three increased in abundance and only one decreased significantly in phosphite‐treated healthy plots. In three Kwongkan communities of the SWAFR, P. cinnamomi had a profound impact on species assemblages, richness, abundance and vegetation structure. There was no evidence of adverse effects of phosphite treatment on phosphorus‐sensitive species, even after fire. Treatment with phosphite enhanced the survival of key susceptible species and mitigated disease‐mediated changes in vegetation structure. In the absence of alternative methods of control in native communities, phosphite will continue to play an important role in the protection of high priority species and communities at risk of extinction due to P. cinnamomi.     

Containment and spot eradication of a highly destructive,invasive plant pathogen (<Emphasis Type="Italic">Phytophthora cinnamomi</Emphasis>) in natural ecosystems

The invasive plant pathogen Phytophthora cinnamomi (Stramenopila, Oomycota) has been introduced into 15 of the 25 global biodiversity hotspots, threatening susceptible rare flora and degrading plant communities with severe consequences for fauna. We developed protocols to contain or eradicate P. cinnamomi from spot infestations in threatened ecosystems based on two assumptions: in the absence of living hosts, P. cinnamomi is a weakly competitive saprotroph; and in the ecosystems we treated, the transmission of the pathogen occurs mainly by root-to-root contact. At two P. cinnamomi-infested sites differing in climate and vegetation types, we applied increasingly robust treatments including vegetation (host) destruction, fungicides, fumigation and physical root barriers. P. cinnamomi was not recovered at three assessments of treated plots 6–9 months after treatments. Given the high rates of recovery of P. cinnamomi from untreated infested soil and the sampling frequency, the probability of failing to detect P. cinnamomi in treated soil was <0.0003. The methods described have application in containing large infestations, eradicating small infestations and protecting remnant populations of threatened species.     

Defining Plant Resistance to Phytophthora cinnamomi: A Standardized Approach to Assessment

Phytophthora cinnamomi is a soil‐borne plant pathogen that causes devastating disease in agricultural and natural systems worldwide. While a small number of species survive infection by the pathogen without producing disease symptoms, the nature of resistance, especially under controlled conditions, remains poorly understood. At present, there are no standardized criteria by which resistance or susceptibility to P. cinnamomi can be assessed, and we have used five parameters consisting of plant fresh weight, root growth, lesion length, relative chlorophyll content of leaves and pathogen colonization of roots to analyse responses to the pathogen. The parameters were tested using two plant species, Zea mays and Lupinus angustifolius, through a time course study of the interactions and resistance and susceptibility defined 7 days after inoculation. A scoring system was devised to enable differentiation of these responses. In the resistant interaction with Z. mays, there was no significant difference in fresh weight, root length and relative chlorophyll content in inoculated compared with control plants. Both lesion size and pathogen colonization of root tissues were limited to the site of inoculation. Following inoculation L. angustifolius showed a significant reduction in plant fresh weight and relative leaf chlorophyll content, cessation of root growth and increased lesion lengths and pathogen colonization. We propose that this technique provides a standardized method for plant–P. cinnamomi interactions that could be widely used to differentiate resistant from susceptible species.     

The abundance of ground-dwelling invertebrates in a Victorian forest affected by ‘dieback’(Phytophthora cinnamomi) disease

Abstract Phytophthora cinnamomi (cinnamon fungus) is a pathogenic soil fungus that infects plant communities along the southeastern coast of Australia and the southwestern corner of Western Australia. Infection of native plant communities with P. cinnamomi regularly leads to dramatic changes in both the structural and the floristic characteristics of these communities. This study aimed to assess the effect of P. cinnamomi induced changes in plant community attributes upon the abundance and diversity of invertebrates in an area of open forest in the Brisbane Ranges, Victoria. Pitfall trapping was conducted continuously for 1 week within each season over a 3 year period. Invertebrates were sorted to order level, and abundances were compared between times (season and year) and infection status for taxa with normalized distributions (Coleoptera, Collembola, Dermaptera and ants). Non-parametric comparisons were made for other groups (Aranae, Blattodea, Diptera, Hemiptera, Orthoptera, ant ‘morpho-species’ and unidentified larvae) to assess differences on the basis of infection status. Significant differences on the basis of infection were uncommon and, where identified, elevated abundances were more commonly observed at sites infected by P. cinnamomi. Consistent temporal effects (season and year) were observed in normalized data sets. Abundances both within individual taxa and from pooled counts were generally weakly associated with ground-level habitat features. Overall, the impact of P. cinnamomi on vegetative structure and floristics was not reflected in different abundances of ground-dwelling invertebrates.     

Bioclimatic analysis of the distribution of damage to native plants in Tasmania by Phytophthora cinnamomi

Climatic profiles were generated by the computer program BIOCLIM for three sets of sites in native vegetation in Tasmania: (i) 308 sites at which Phytophthora cinnamomi was isolated from diseased plants Pc+ ive; (ii) 322 sites in healthy plant communities from which P. cinnamomi could not be recovered Pc?ive; and (iii) 801 sites representing the climatic range across Tasmania. A discriminatory analysis comparing the first and third sets indicated that seven of the 16 climatic indices available for analysis were good discriminators of the distribution of damage by P. cinnamomi. The analysis suggests that damage to native vegetation due to P. cinnamomi is unlikely on sites where annual mean temperature does not exceed 7.5°C or annual mean rainfall is < 600 mm. Two maps were produced to indicate those areas of Tasmania that have climates suited to damaging interaction between P. cinnamomi and native vegetation. The first was based on those sites that had annual mean temperature more than 7.5°C and annual mean rainfall less than 600 mm. The second included those sites that matched the values of the Pc+ ive set for all seven good discriminators. The two approaches produced similar results. Areas in which even the most favourable microsites are unlikely to support pathogenic activity by P. cinnamomi constitute less than 20% of the land area. Twelve substantial areas of native vegetation that occur in climates suited to infection by P. cinnamomi. but for which no record of the fungus exists, have been identified.     

Spatial model for predicting the presence of cinnamon fungus (Phytophthora cinnamomi) in sclerophyll vegetation communities in south‐eastern Australia

Abstract The pathogen Phytophthora cinnamomi causes extensive ‘dieback’ of Australian native vegetation. This study investigated the distribution of infection in an area of significant sclerophyll vegetation in Australia. It aimed to determine the relationship of infection to site variables and to develop a predictive model of infection. Site variables recorded at 50 study sites included aspect, slope, altitude, proximity to road and road characteristics, soil profile characteristics and vegetation attributes. Soil and plant tissues were assayed for the presence of the pathogen. A geographical information systyem (GIS) was employed to provide accurate estimations of spatial variables and develop a predictive model for the distribution of P. cinnamomi. The pathogen was isolated from 76% of the study sites. Of the 17 site variables initially investigated during the study a logistic regression model identified only two, elevation and sun‐index, as significant in determining the probability of infection. The presence of P. cinnamomi infection was negatively associated with elevation and positively associated with sun‐index. The model predicted that up to 74% of the study area (11 875 ha) had a high probability of being affected by P. cinnamomi. However, the present areas of infection were small, providing an opportunity for management to minimize spread into highly susceptible uninvaded areas.     

Forest community structure and composition following containment treatments for the fungal pathogen oak wilt

In temperate oak forests in Ohio, USA, we examined variability in forest communities within containment treatment sites for oak wilt (Bretziella fagacearum), a fungal pathogen lethal to susceptible oak species. Containment treatments included quarantine lines in soil for limiting belowground fungal spread and sanitation cutting of 1–3 mature black oak (Quercus velutina) trees within oak wilt infection patches. At 28 sites, we compared tree structure and understory plant communities across a gradient of 1- to 6-year-old treatments and reference forest (untreated and without evidence of oak wilt). While oak seedlings were abundant, oak saplings (1–10 cm in diameter) were absent. In contrast, many native understory plant community measures were highest in oak wilt treatments. Plant species richness 100 m^?2 doubled in treatments, regardless of age, compared with reference forest. Plant cover increased with treatment age, with 6-year-old treatments exhibiting 5?×?more cover than reference forest. Non-native plants averaged only a small proportion (<?0.12) of cover across treatments and reference forest. Variability in understory communities was mostly predictable using treatment age, tree canopy cover, and geographic location, as 20 of 25 understory measures had at least 72% of their variance modeled. While oak wilt treatments did not facilitate oak regeneration nor many conservation-priority species of open savanna-woodland habitats, the treatments did diversify and increase cover of native understory communities with minimal invasion of non-native plants.

     

Community‐level changes in Banksia woodland following plant pathogen invasion in the Southwest Australian Floristic Region

Question: Does the introduced pathogen Phytophthora cinnamomi change Banksia woodland α‐ or β‐diversity and what are the implications for species re‐colonization? Location: High rainfall zone of the Southwest Australian Floristic Region (SWAFR). Methods: We measured pathogen‐induced floristic change along a disease chronosequence, and re‐sampled historic quadrats in Banksia attenuata woodlands of the SWAFR. The chronosequence represents three disease stages: (1) healthy vegetation with no disease expression; (2) the active disease front; and (3) diseased vegetation infected for at least 15 years. Comparative data were obtained by resampling diseased plots that were historically disease‐free when established in 1990. Results: β‐diversity differed substantially for both chronosequence and historic data, while α‐diversity was maintained, as measured by plot species richness and Simpson's reciprocal index. Species of known pathogen susceptibility were significantly reduced in cover–abundance, including the structurally dominant species; Banksia attenuata, B. ilicifolia and Allocasuarina fraseriana. Although these species remained present on diseased sites, there were overall reductions in canopy closure, leaf litter and basal area. These declines were coupled with an increase of species with unknown susceptibility, suggesting potential resistance and capacity to take advantage of altered site conditions. Conclusions: This study highlights the ability of an introduced plant pathogen to alter community floristics and associated stand variables. Species cover–abundances are unlikely to recover due to a reduced seed source, altered site conditions and pathogen persistence at the landscape level. However, maintenance of α‐diversity suggests continued biological significance of Phytophthora‐affected sites and the formation of novel ecosystems, themselves worthy of conservation.     

The Microscopic Examination of Phytophthora cinnamomi in Plant Tissues Using Fluorescent In Situ Hybridization

The microscopic examination of Phytophthora cinnamomi in plant tissues is often difficult as structures such as hyphae, chlamydospores and oospores are frequently indistinguishable from those of other fungi and oomycetes, with histological stains not enabling species differentiation. This lack of staining specificity makes the localization of P. cinnamomi hyphae and reproductive structures within plant tissues difficult, especially in woody tissues. This study demonstrates that with the use of a species‐specific fluorescently labelled DNA probe, P. cinnamomi can be specifically detected and visualized directly using fluorescent in situ hybridization (FISH) without damage to plant or pathogen cell integrity or the need for subculturing. This approach provides a new application for FISH with potential use in the detailed study of plant–pathogen interactions in plants.     

Impacts of the invasive exotic <Emphasis Type="Italic">Prosopis juliflora</Emphasis> (Sw.) D.C. on the native flora and soils of the UAE

The effects of the invasive exotic Prosopis juliflora shrubs on the natural plant communities and soil chemical characters were assessed in two regions of the United Arab Emirates (UAE). Five sites were selected subjectively: three in Sharja with 73 stands and two in Ras Al-Khima with 37 stands. Stands were located randomly within each site to cover density variation in Sharja and size variation in Ras Al-Khima. Density, frequency, richness and evenness of the associated annual and perennial species were studied in nine quadrats distributed under, at the margin and outside the canopy of a P. juliflora shrub located in the center of each stand. The results indicated that the effect of P. juliflora on the associated flora depends significantly on the density and size of the canopy. Larger individuals and greater densities have significantly greater negative impacts on the associated plants. All the studied community attributes were significantly lower under P. juliflora canopies than outside. Annuals were inhibited more than perennials. The number of annuals with significant reductions in density and/or frequency under P. juliflora canopies was significantly greater than the number of perennials. Density of more than 50% of the associated annuals was significantly inhibited under P. juliflora canopies. Density of P. juliflora seedlings was greater underneath the canopy of the same species than away from them, indicating little or no self allelopathic effect (auto-inhibition) under field conditions. P. juliflora ameliorated some soil characters, through significant pH reduction and increase in K, N and P and organic matters. Hence, plant diversity might be enhanced following eradication of P. juliflora.     

Characterization of vegetation in an Australian open forest community affected by cinnamon fungus (shape Phytophthora cinnamomi): implications for faunal habitat quality

Phytophthora cinnamomi (cinnamon fungus) is a pathogenic soil fungus which infects plant communities such as open forests, woodlands and heathlands in the south-eastern and south-western corners of Australia, leading to devastating effects upon both structural and floristic features of these plant communities. This study undertaken in open forest in the Brisbane Ranges, south-eastern Australia, sought to characterize two study areas using visual classification and several ordination techniques. Additionally, sites were examined for differences in vegetation composition relating to P. cinnamomi infection, and how these may relate to site habitat quality for resident fauna. This characterization had a high degree of conformity (90% to 96%) between the original visual classification and several ordination methods used. Differences in the cover of understorey vegetation including both low and tall shrubs, and the grass-tree Xanthorrhoea australis were recorded between uninfected and post-infected sites, however the cover of litter or bare ground did not infer infection status. There were no consistent significant differences in projective crown cover of Eucalyptus spp. between uninfected and infected sites, nor in the number of hollows suitable for den or nest sites for arboreal mammals and birds. Hence, the impact of P. cinnamomi upon faunal habitat quality at the sites was considered more likely to relate to changes in ground level vegetation structure than to effects of the pathogen on the dominant Eucalyptus species. Based on the small proportion of sites with detectable isolates of the pathogen (25%), and the slow rate of movement of the disease front during this study (1 to 5 m/3 yr), the current state of disease progression appeared relatively quiescent compared to infestations previously documented in the same area a decade or two earlier.     

Effect of Brassica Biofumigant Amendments on Different Stages of the Life Cycle of Phytophthora cinnamomi

The oomycete plant pathogen Phytophthora cinnamomi causes a highly destructive root rot that affects numerous hosts. Integrated management strategies are needed to control P. cinnamomi in seminatural oak rangelands. We tested how biofumigation affects crucial stages of the pathogen's life cycle in vitro, in infested soils under laboratory conditions and in planta. Different genotypes of three potential biofumigant plant species (Brassica carinata, Brassica juncea, Brassica napus) were collected at different phenological stages, analysed for their glucosinolate contents, and subsequently tested. The most effective genotypes against mycelial growth and sporangial production were further tested on the viability of chlamydospores in artificially infested natural soils and in planta on Lupinus luteus, a host highly susceptible to P.cinnamomi. Brassica carinata and B. juncea genotypes inhibited mycelial growth, decreased sporangial production, and effectively inhibited the viability of chlamydospores in soil, but only B. carinata decreased disease symptoms in plants. Effective genotypes of Brassica had high levels of the glucosinolate sinigrin. Biofumigation with Brassica plants rich in sinigrin has potential to be a suitable tool for control of oak root disease caused by P. cinnamomi in Spanish oak rangeland ecosystems.     

Survival of Phytophthora cinnamomi and Fusarium verticillioides in commercial potting substrates for ornamental plants

Live plants, particularly when accompanied by soil or potting substrates, are considered the main pathway for international spread of plant pathogens. Modern, rapid shipping technologies for international plant trade increase the probability of plant pathogen survival during transport and the subsequent chances of disease outbreaks in new locations. The survival of two model pathogens, an Oomycete, Phytophthora cinnamomi, and a filamentous fungus, Fusarium verticillioides, was studied in two different commercial potting substrates (peat and peat‐free) under glasshouse conditions in the absence of a plant host. Survival rates were analysed at 2, 7, 12 and 17 months after substrate inoculation. Fusarium verticillioides had the longest survival rate, and was still present at 17 months. In contrast, P. cinnamomi survived up to 7 months but was not recovered after 12 or 17 months. There was no significant difference in the number of colony‐forming units (CFUs) of either pathogen in the two substrates, except at 2 months, when higher numbers were recovered from peat substrates.     

Regulation of defence responses in avocado roots infected with Phytophthora cinnamomi (Rands)

Phytophthora cinnamomi occurs worldwide and has a host range in excess of 1,000 plant species. Avocados (Persea americana Mill) have been described as highly susceptible to this soil-borne pathogen. Here, the regulation of defence responses in avocado root seedlings inoculated with P. cinnamomi mycelia is described. A burst of reactive oxygen species (ROS) was observed 4 days after inoculation. The higher physiological concentration of H₂O₂ induced by P. cinnamomi on avocado roots had no effect on in vitro growth of the oomycete. Total phenols and epicathecin content showed a significant decrease, but lignin and pyocianidins exhibited no changes after inoculation. Also, increased nitric oxide (NO) production was observed 72 h after treatment. We studied the effects of one NO donor [sodium nitroprusside (SNP)], and one NO scavenger [2- to 4-carboxyphenyl-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (CPTIO)] to determine the role of NO during root colonisation by P. cinnamomi mycelia. Pretreatment of the roots with CPTIO, but not with SNP, inhibited root colonisation suggesting an important role for NO production during the avocado–P. cinnamomi interaction. Our data suggest that although defence responses are activated in avocado roots in response to P. cinnamomi infection, these are not sufficient to avoid pathogen invasion.     

Phosphite stimulated histological responses of Eucalyptus marginata to infection by Phytophthora cinnamomi

Phosphite is used to protect plants from the soil borne pathogen, Phytophthora cinnamomi. Although phosphite stimulates resistance to P. cinnamomi, this is the first histological study of its effect on Eucalyptus marginata, an economically important forest tree in Western Australia. Clonal lines of E. marginata, considered resistant and susceptible to P. cinnamomi, were underbark inoculated with P. cinnamomi. 4 days later, they were treated with 0, 2.5, 5 or 10 g L⁻¹ phosphite. Transverse hand sections were stained for suberin and lignin, and histological responses to infection were examined. Defence responses were stimulated at all phosphite concentrations in both clonal lines, and the genotypic difference in lesion length was eliminated within 8 days of treatment. In the resistant line, suberin production was stimulated while in the susceptible line both lignin and suberin were stimulated. By 2 days after treatment, phosphite stimulated a faster rate of suberin production in the resistant line than the susceptible line, but by 4 days after treatment, there was no difference in the increase between the lines. Damage caused by P. cinnamomi was found to extend furthest in the cortex and outer phloem in transverse sections in both genotypes. In the presence of P. cinnamomi, phosphite stimulated mitosis as part of the defence response, with meristematic activity involved in the compartmentalisation of damaged tissue (formation of periderm) and closure of healthy tissue (callus). Phytotoxicity had a detrimental effect in healthy tissues and this was more apparent in the resistant line, where it did not provide the best protection from lesion extension and plant mortality, suggesting phytotoxicity could disrupt defence responses. Phosphite increases the capacity of susceptible and resistant E. marginata clonal lines to wall-off and contain P. cinnamomi colonisation through lignin and suberin deposition, and increased meristematic activity.     

Changes in Respiration of Seedling Roots Inoculated with Phytophthora cinnamomi

An increase in rate of respiration was recorded for intact roots of seven native Australian species 16 h after inoculation with Phytophthora cinnamomi. By 24 h the magnitude of the increase ranged from 2—159% above that of the uninoculated controls and was evidently not related to host susceptibility. A time sequence study of lesion extension and the associated increased respiration rates for both susceptible and tolerant eucalypts demonstrated a difference in response. The rate of respiration in the tolerant species increased 2 % and only at the site of inoculation, whereas in the susceptible species the respiration rate increased in a wave which began at the inoculation site and continued along the root with the advancing fungal invasion. Respiration rate only increased in regions of the root actually inhabited by the pathogen. The fungal contribution to the total respiration of infected roots was less than 1 % and was determined by measuring respiration of inoculated killed roots. Respiration rates were measured in the presence of potassium cyanide (KCN) and salicylhydroxamic acid (SHAM). Both KCN-sensitive and SHAM-sensitive respiration occurred in normal uninfected E. marginate seedlings. A large proportion of the increase in total respiration rate of infected seedlings compared with uninoculated controls was due to the alternate, SHAM-sensitive pathway. The physiological implications of these results are discussed.     

Potential for dissemination of Phytophthora cinnamomi by feral pigs via ingestion of infected plant material

Feral pigs have long been implicated as potential vectors in the spread of the devastating plant pathogen Phytophthora cinnamomi due to their rooting and wallowing activities which may predispose them as vectors of infested soil. In this study, we aim to determine whether feral pigs have the potential to act as vectors of plant pathogens such as P. cinnamomi through their feeding activity. The typically omnivorous diet of feral pigs may also lead to the passage of P. cinnamomi infected plant material through their digestive system. This study investigates the potential for feral pigs to pass viable P. cinnamomi in their faeces following the ingestion of millet seeds, pine plugs and Banksia leptophilia roots inoculated with P. cinnamomi. Recovery rates of P. cinnamomi from the millet seeds, pine plugs and B. leptophilia roots following a single ingested bolus were 33.2, 94.9 and 10.4 %, respectively supported by quantitative PCR analysis. These results demonstrate that P. cinnamomi remain viable within infected plant material following passage through the pig digestive tract, although the digestive processes reduce the pathogen’s viability. An inverse relationship was observed between the viability of infected material and passage time, suggesting that partially digested plant material provides protection for P. cinnamomi against the adverse environmental conditions of the pig digestive tract. Phytophthora cinnamomi remained viable for up to 7 days in larger pieces of colonised woody plant material such as the pine plugs. A plant infection trial using passaged P. cinnamomi colonised pine plugs showed that even material that remained in the digestive tract for 7 days was capable of infecting and killing healthy plants, susceptible to P. cinnamomi. This study provides compelling evidence that feral pigs have the ability to transport viable P. cinnamomi in their digestive tract.     

Effects of cover crops, weeds and plant debris on development of Mycocentrospora acerina in caraway

This paper reports on the search for inoculum sources of Mycocentrospora acerina on caraway (Carum carvi L.). Obvious suspects are cover crops of biennial caraway and preceding crops of annual caraway. Other suspects are weeds in or alongside the field. Finally, survival structures of the fungus, chlamydospore chains, packed in plant debris or naked, are suspected. M. acerina is able to infect many plant species, including cover crops of caraway such as spinach for seed production and peas. However, the agronomical suitability of a crop to serve as a cover crop of biennial caraway proved to be a more important factor in determining caraway yield than the susceptibility of the cover crop to M. acerina. This finding was corroborated by the fact that spinach and peas as preceding crops had no significant effects on M. acerina development in spring caraway sown the next year. Dill, barley and four weed species were found as new hosts of M. acerina. The role of weed hosts, susceptible crops and plant debris in the survival of the fungus in years without caraway is discussed. Caraway sown on soil containing infested caraway straw, infested debris of other plant species or chlamydospores grown in pure culture, became infected by M. acerina. Only high inoculum densities of chlamydospores in the soil caused severe damping-off of caraway seedlings. The opportunity for disease management by agronomical means is quite limited.     

Benzoic acid induces tolerance to biotic stress caused by Phytophthora cinnamomi in Banksia attenuata

Banksia attenuata plants were treated with soil drenches or foliar sprays of benzoic acid (BZA) to determine induced resistance to Phytophthora cinnamomi. Stems of B. attenuata were inoculated with the pathogen 1 week after treatment with BZA. Resistance was estimated by measuring P. cinnamomi lesions on stems. Treatment with 0.10 mM, 0.25 mM or 0.50 mM BZA caused a reduction in lesion size with 0.50 mM BZA applied as a soil drench being the most effective treatment at suppressing the development of lesions. This is the first report of BZA induced host resistance in any plant species to any pathogen.