Scientific critique of the paper “Climatic distribution of citrus black spot caused by <Emphasis Type="Italic">Phyllosticta citricarpa</Emphasis>. A historical analysis of disease spread in South Africa” by Martínez-Minaya et al. (2015)

The global distribution of citrus black spot (CBS) disease, caused by Phyllosticta citricarpa, is climatically constrained, which is evident from its occurrence in citrus growing areas with warm, summer rainfall and its absence from areas with cooler, Mediterranean-type winter rainfall. Various epidemiological and modelling studies have supported this observation, predominantly estimating unsuitability for P. citricarpa in Mediterranean type climates, with no more than marginal suitability estimated at a few localities within some regions with Mediterranean type climates. The study by Martínez-Minaya et al. (European Journal of Plant Pathology, 143, 69–83, 2015), describes an historic sequence of recorded CBS occurrence in parts of South Africa, conducts an autocorrelation analysis and a correlative analysis with Köppen-Geiger climate zones and makes observations about the occurrence of certain Köppen-Geiger climate zones in the European Union. The study suggests that significant portions of the European Union and the broader Mediterranean basin are climatically similar to warm, summer rainfall areas in South Africa where P. citricarpa persists and causes CBS disease and concludes that the potential distribution of P. citricarpa is less constrained by climatic factors than spatial contagion. However, in this critique we expose methodological shortcomings in the Martínez-Minaya et al. (European Journal of Plant Pathology, 143, 69–83, 2015) study and conclude that the study grossly overestimated the extent of the geographical area that could support P. citricarpa, thereby rendering the findings scientifically unreliable.     

Transgenic citrus expressing the antimicrobial gene <Emphasis Type="Italic">Attacin E</Emphasis> (<Emphasis Type="Italic">attE</Emphasis>) reduces the susceptibility of ‘Duncan’ grapefruit to the citrus scab caused by <Emphasis Type="Italic">Elsinoë fawcettii</Emphasis>

Citrus scab, caused by Elsinoë fawcettii (anamorph Sphaceloma fawcettii), is a common foliar fungal disease affecting many citrus cultivars, including grapefruit. No commercial grapefruit cultivar is resistant to scab, and the disease results in severely blemished fruit which reduces its marketability. Transgenic ‘Duncan’ grapefruit trees expressing the antimicrobial attE gene were produced via Agrobacterium-mediated transformation. In in vitro leaf and greenhouse assays, several transgenic-lines had significantly lower susceptibility to E. fawcettii compared to the non-transformed control (P?P?P?attE mRNA was inversely related to the number of copies detected by Southern blot. The least susceptible line had a single inserted copy of the attE transgene whereas more susceptible lines had multiple copies. Since the attacin mode of action was thought to be specific to Gram-negative bacteria, it was unexpected to find that there was a significant activity against E. fawcettii.     

Activity of <Emphasis Type="Italic">ß</Emphasis>-1,3-glucanase and <Emphasis Type="Italic">ß</Emphasis>-1,4-glucanase in two potato cultivars following challenge by the fungal pathogen <Emphasis Type="Italic">Alternaria solani</Emphasis>

Early blight of potato, caused by Alternaria solani, is a ubiquitous disease in many countries around the world. Our previous screening of several Iranian potato cultivars found that variation in resistance exists between two cultivars: ‘Diamond’ and ‘Granula’. Cultivar Diamond is more resistant to multiple isolates of A. solani when compared to cv. Granula. Furthermore, we have found that different pathogen isolates have varying degrees of infection. We monitored the activities of two pathogen-related (PR) glucanase proteins in Diamond and Granula in response to two isolates of A. solani with different degrees of virulence. ß-1,3-glucanase and ß-1,4-glucanase activities were recorded in healthy and diseased leaves of potatoes up to 10 days after inoculation. Their activities were found to be higher in diseased leaves when compared to those of uninfected leaves. Our data suggest that significantly reduced activities of theses enzymes in potato could be related to a lower degree of resistance or an increased ability of a more aggressive isolate to suppress PR protein expression.     

First report of pileus rot disease on cultivated <Emphasis Type="Italic">Morchella importuna</Emphasis> caused by <Emphasis Type="Italic">Diploöspora longispora</Emphasis> in China

In March 2013, pileus rot disease was first observed on cultivated Morchella importuna. Infected ascomata were covered by white, velvety mycelia mainly on the pileus, and the infection resulted in malformed fruiting bodies. The causal pathogen was identified as Diploöspora longispora based on its morphology and the internal transcribed spacer of its ribosomal DNA sequences. After inoculation of young ascomata with the isolates, the original symptoms were reproduced, and the same pathogen was reisolated from diseased ascomata. This is the first report of pileus rot disease on morels caused by D. longisporain China.     

<Emphasis Type="Italic">Amaranthus</Emphasis> spp.: a new host of “<Emphasis Type="Italic">Candidatus</Emphasis> Phytoplasma aurantifolia”

Mexico is considered to be one of the centers of origin of grain amaranth species. Recently, plants with abnormal anatomical features were observed in experimental fields established in Central Mexico. The most noticeable symptoms, which consisted of excessive stem and bud proliferation, mosaics and unusual coloration, suggested that they might be phytoplasma-induced disorders. Thus, different accessions of grain amaranth (Amaranthus hypochondriacus and A. cruentus) plants were subjected to polymerase chain reaction (PCR) analysis specifically designed to detect these pathogens. Two universal phytoplasma-specific primer pairs were tested in a nested PCR assay, with primer pair P1/tint (followed by primer pair R16F2/R16R2). Further DNA sequencing analysis of the resulting amplicons indicated that these phytoplasmas may be related to others already affecting important agricultural crops in Mexico, such as soybean. Data are presented that disclose the etiology of these syndromes by the use of molecular techniques. To the best of our knowledge, this finding constitutes the first report of a phytoplasma-related disease in grain amaranth.     

Effect of polyploidisation on the response of apple (<Emphasis Type="Italic">Malus</Emphasis> × <Emphasis Type="Italic">domestica</Emphasis> Borkh.) to <Emphasis Type="Italic">Venturia inaequalis</Emphasis> infection

Spinach is one of the most nutritious green-leaf vegetables. In the spinach production, diseases cause a significant loss in both yield and quality. Improving disease resistance is one of the major challenges in spinach breeding. Arabidopsis nucleoporin CONSTITUTIVE EXPRESSER OF PATHOGENESIS-RELATED GENES 5 (CPR5) functions as a negative regulator of plant cell death and immunity as cpr5 mutant exhibits spontaneous cell death and heightened immunity. In addition, CPR5 play a role in trichome development as the majority of cpr5 mutant trichomes are branchless whereas wild type trichomes are often three-branched. In the spinach genome, we identified a homolog of Arabidopsis CPR5, referred to as Spinacia oleracea CPR5 (SoCPR5). To investigate the function of SoCPR5, we introduced SoCPR5 into Arabidopsis cpr5 mutant. Our data showed that both spontaneous cell death and heightened immunity were suppressed in the SoCPR5-transgenic cpr5 mutants, verifying that SoCPR5 functions as its Arabidopsis counterpart in plant cell death and immunity. SoCPR5 also fully restored wild type trichome phenotype of the cpr5 mutant. Our study therefore indicates that the function of SoCPR5 is conserved between plant species and SoCPR5 can be applied for genetic manipulation of plant immunity in spinach.     

PCR-mediated Detection of <Emphasis Type="Italic">Xanthomonas oryzae </Emphasis>pv. <Emphasis Type="Italic">oryzae </Emphasis>by Amplification of the 16S–23S rDNA Spacer Region Sequence

A detection method specific for Xanthomonas oryzae pv. oryzae, the pathogen responsible for bacterial blight of rice, was based on the polymerase chain reaction (PCR) and designed by amplifying the 16S–23S rDNA spacer region from this bacterium. The nucleotide sequence of the spacer region between the 16S and 23S rDNA, consisting of approximately 580-bp, from X. oryzae pv. oryzae, X. campestris pv. alfalfae, X. campestris pv. campestris, X. campestris pv. cannabis, X. campestris pv. citri, X. campestris pv. cucurbitae, X. campestris pv. pisi, X. campestris pv. pruni and X. campestris pv. vitians, was determined. The determined sequences had more than 95% identity. Therefore, a pair of primers, XOR-F (5′-GCATGACGTCATCGTCCTGT-3′) and XOR-R2 (5′-CTCGGAGCTATATGCCGTGC-3′) was designed and found to specifically amplify a 470-bp fragment from all strains of X. oryzae pv. oryzae isolated from diverse regions in Japan. No PCR product was amplified from X. campestris pathovars alfalfae, campestris, cannabis, carotae, cucurbitae, dieffenbachiae, glycines, pisi, pruni, vitians or zantedeschiae, except for pathovars citri, incanae and zinniae. The method could also detect the pathogen in infected rice leaves within 3 hr, at a detection limit of 4×10¹ cfu/ml. Received 17 December 1999/ Accepted in revised form 10 April 2000     

The first detection of ‘<Emphasis Type="Italic">Candidatus</Emphasis> Phytoplasma trifolii’ in <Emphasis Type="Italic">Rhododendron</Emphasis> hybridum

Four Rhododendron hybridum plants (from cvs Moravanka and Don Juan), all exhibited symptoms of shortened axillary shoots, reduced leaves with vein clearing and yellowing, undeveloped flowers, and general stunting in a rhododendron nursery garden in southern Bohemia in 2007. Electron microscopy examination of ultra-thin sections revealed the presence of numerous polymorphic phytoplasma-like bodies in the phloem tissue of leaf midribs and petioles. The phytoplasma etiology of this disease was further confirmed by polymerase chain reaction (PCR) using universal phytoplasma primers. Restriction fragment length polymorphism (RFLP) analysis of amplification products obtained with a R16F2/R16R2 primer pair from all symptomatic plants indicated the presence of phytoplasma from the 16SrVI-A subgroup. A detailed comparison of the amplified sequences and phylogenetic analysis confirmed that the phytoplasma belonged to the subgroup 16SrVI-A (clover proliferation phytoplasma group). This is the first report of the natural occurrence of ‘Candidatus Phytoplasma trifolii’ in plants of Rhododendron hybridum.     

<Emphasis Type="Italic">In planta</Emphasis> multiplication and graft transmission of ‘<Emphasis Type="Italic">Candidatus</Emphasis> Liberibacter asiaticus’ revealed by Real-Time PCR

The bacterium Candidatus Liberibacter asiaticus (CLas) is associated with huanglongbing (HLB) in citrus in many countries. Despite the fact that many characteristics of the disease are known, the rate of multiplication of the bacterium within an infected tree is still poorly understood. To study this feature, we used the quantitative Real-Time polymerase chain reaction (Q-PCR) assay to follow and to quantify the multiplication of CLas in grafted infected young sweet orange plants. The rate of infection by grafting reached 100% at 120 days post-inoculation (dpi) showing that grafting could easily transmit CLas. A well-adjusted linear regression equation describing the bacterial growth in planta was obtained independently with measurements taken using repeated sampling in the same plant or different plants through the analysed period. The bacterial population, measured as copy number (CN) of the 16S rDNA target gene g⁻¹ of tissue, increased 10,000 times from 10³ at 30 dpi to approximately 10⁸ CN at 240 dpi indicating that CLas multiplication was fastest in young citrus plants. We observed a direct relationship between the concentration of pathogen and the expression of symptoms. Yellowed leaves or shoots, are commonly the first observed symptom of HLB, and were present in trees with a low amount of bacteria (10⁵ CN g⁻¹). Blotchy mottle symptoms were observed in trees with 10⁷ CN g⁻¹ of bacteria after 180 dpi. Buds taken from infected, but non-symptomatic branches were grafted on Rangpur lime and resulted in transmission rates ranging from 10 to 60%.     

Virulence structure of the <Emphasis Type="Italic">Blumeria graminis</Emphasis> DC.f. sp. <Emphasis Type="Italic">avenae</Emphasis> populations occurring in Poland across 2010–2013

The aim of the present study was to determine the virulence structure of powdery mildew of oats (Blumeria graminis DC.f. sp. avena) in Poland in the years 2010–2013. For this purpose, powdery mildew isolates were collected from three experimental stations in Poland. To assess the virulence of the isolates, eight oat varieties with different responses to the pathogen were used. The results showed that a significant proportion of powdery mildew isolates found in Poland overcame the resistance genes of varieties Bruno (Pm6), Jumbo (Pm1) and Mostyn (Pm3). In contrast, lines Av1860 (Pm4), Am27 (Pm5) and Cc3678 (Pm2) were completely resistant to all pathogen isolates involved in the experiment. Changes constantly occurring in the powdery mildew population perfectly reflect diversity indexes, which were the smallest in the first year of observation, where in the following years these parameters were significantly higher. It is worth noting that the presence of powdery mildew is seasonal and local, which is reflected in the prevalence of the disease in a defined area of the country.     

Mechanisms of induced resistance in lettuce against <Emphasis Type="Italic">Bremia lactucae</Emphasis> by DL<Emphasis Type="Bold">-</Emphasis>β<Emphasis Type="Bold">-</Emphasis>amino<Emphasis Type="Bold">-</Emphasis>butyric acid (BABA)

BABA induced local and systemic resistance in lettuce (Lactuca sativa) against the Oomycete Bremia lactucae. Structure-activity analysis showed no induced resistance by related amino-butanoic acids or β-alanine. The R-enantiomer of BABA induced resistance whereas the S-enantiomer did not, suggesting binding to a specific receptor. Other compounds known to be involved in SAR signaling, including abscisic acid, methyl-jasmonate, ethylene, sodium-salicylate and Bion® (BTH) did not induce resistance. Systemic translocation of ¹⁴C-BABA and systemic protection against downy mildew were tightly correlated. BABA did not affect spore germination, appressorium formation, or penetration of B. lactucae into the host. Epifluorescence and confocal microscopy revealed that BABA induced rapid encasement with callose of the primary infection structures of the pathogen, thus preventing it from further developing intercellular hyphae and haustoria. Invaded host cells treated with BABA did not accumulate phenolics, callose or lignin, or express HR. In contrast, cells of genetically-resistant cultivars accumulated phenolics, callose and lignin and exhibited HR within one day after inoculation. The callose synthesis inhibitor DDG did not inhibit callose encasement nor compromised the resistance induced by BABA. PR-proteins accumulated too late to be responsible for the induced resistance. DAB staining indicated that BABA induced a rapid accumulation of H₂O₂ in the penetrated epidermal host cells. Whether H₂O₂ stops the pathogen directly or via another metabolic route is not known.     

<Emphasis Type="Italic">Spiraea salicifolia:</Emphasis> a new plant host of “<Emphasis Type="Italic">Candidatus</Emphasis> Phytoplasma ziziphi”-related phytoplasma

Spiraea salicifolia is widely grown in China as an ornamental plant, and its roots and young leaves have many medical uses. On the campus of Northwest A&F University, we observed diseased S. salicifolia plants that had yellowed, dwarfed, deformed leaves and other symptoms resembling diseases caused by phytoplasma. This study was aimed at determining the causal agent of the disease. On the basis of phytoplasma-specific DNA amplification by PCR, a phytoplasma infection of S. salicifolia was confirmed. The phytoplasma was related to “Ca. Phytoplasma ziziphi” according to RFLP and phylogenetic analyses. This report is the first of phytoplasma infection of S. salicifolia in China.     

Molecular diversity of ‘<Emphasis Type="Italic">Candidatus</Emphasis> Phytoplasma mali’ and ‘<Emphasis Type="Italic">Ca</Emphasis>. P. prunorum’ in orchards in Slovenia

The plant parasitic nematode Longidorus poessneckensis found in Austria, the Czech Republic, Germany, Poland and the Slovak Republic was molecularly characterized. Mitochondrial genes encoding cytochrome c oxidase subunit I (COI) and nicotinamide dehydrogenase subunit 4 (nad4), the D2 and D3 expansion segments of 28S rRNA and Internal transcribed spacer 1 (ITS1) rRNA were sequenced for 16 L. poessneckensis populations. Six haplotypes of COI and five haplotypes of nad4 were determined. Nucleotide intraspecific variation was up to 17.1% for the partial sequenced COI gene and up to 17.7% for the partial sequenced nad4 gene, the latter being the highest up to date known intraspecific variation in this genus. The analyses of multiple amino acid sequence alignments of mitochondrial genes revealed low variability (0–2.4%) for COI gene and high divergence (0–7.6%) for nad4 gene. Intraspecific sequence diversity for the D2-D3 of 28S rRNA gene was up to 1.2% and for ITS1 rRNA gene was up to 1.6%. It has been hypothesized, that during the Last Glacial Maximum, L. poessneckensis populations probably persisted in refuge areas in the Carpathian Mountains and subsequently expanding from these areas and colonizing other European regions.     

Effect of Combined Treatment of Pasteurisation and <Emphasis Type="Italic">Coniothyrium minitans</Emphasis> on Sclerotia of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in Soil

Integrated control of soil-borne plant pathogens such as Sclerotinia sclerotiorum is becoming more important as the soil fumigant methyl bromide is being phased out of use. Two alternative methods of control that have been found to reduce viability of sclerotia are steam sterilisation (pasteurisation) of soil or the application of the mycoparasite Coniothyrium minitans. This work investigated the possibility of integrating these two control measures. Soil was pasteurised in an autoclave, using a temperature of 80 °C for 3 min to simulate the possible temperatures reached by soil steaming machines for field use. Coniothyrium minitans was subsequently applied to the pasteurised soil to assess the effects of the combination of control measures in reducing sclerotial viability of S. sclerotiorum. Similar results were found in two soil types. Either method used individually was effective in decreasing the number of viable sclerotia, but no further reduction in sclerotial viability was seen when the two methods were combined. Coniothyrium minitans was found to colonise pasteurised sclerotia significantly quicker than untreated sclerotia, and it was seen that there was an increase in number of C. minitans in pasteurised soil in the presence of sclerotia. Experiments were also conducted to investigate the effect of application timing of the biocontrol agent to soil following pasteurisation, in relation to sclerotial infection. Here, two different isolates of S. sclerotiorum were used, with similar results. Application of C. minitans to soil immediately following pasteurisation resulted in sclerotial infection by the mycoparasite, but application 7 days or more after soil pasteurisation resulted in low recovery of the biocontrol agent from sclerotia, possibly due to the mycoparasite being masked by the presence of other fungi which colonised the sclerotia first.     

Identification of a novel point mutation in the <Emphasis Type="Italic">β</Emphasis>-tubulin gene of <Emphasis Type="Italic">Botrytis cinerea</Emphasis> and detection of benzimidazole resistance by a diagnostic PCR-RFLP assay

The molecular basis of resistance to benzimidazole fungicides with laboratory and field mutant isolates of Botrytis cinerea was investigated. After chemical mutagenesis with N-methyl-N-nitrosogouanidine (NMNG) two different benzimidazole-resistant phenotypes were isolated on media containing carbendazim or a mixture of carbendazim and diethofencarb. The mutant isolates from the fungicide-mixture-containing medium were moderately resistant to carbendazim with wild-type tolerance to diethofencarb while mutant isolates from carbendazim-containing medium were highly resistant to carbendazim but sensitive to diethofencarb. The studied field isolates were highly resistant to benzimidazoles and sensitive to diethofencarb. Study of fitness characteristics of benzimidazole highly-resistant isolates showed that the resistance mutation(s) had no apparent effect on fitness-determining parameters. Contrary to this, the moderately benzimidazole-resistant strains, with no increased diethofencarb sensitivity, had a significant reduction in certain ecological fitness-determining characteristics. Analysis of the sequence of the β-tubulin gene revealed two amino acid replacements in the highly benzimidazole-resistant mutants compared to that of the wild-type parent strain. One was the glutamic acid (GAG) to alanine (GCG) change at position 198 (E198A), identified in both laboratory and field highly benzimidazole-resistant isolates, a mutation previously implicated in benzimidazole resistance. The second was a novel benzimidazole resistance mutation of glutamic acid (GAG) to glycine (GGG) substitution at the same position 198 (E198G), identified in a highly benzimidazole-resistant laboratory mutant strain. Molecular analysis of the moderately benzimidazole-resistant strains revealed no mutations at the β-tubulin gene. A novel diagnostic PCR-RFLP assay utilising a BsaI restriction site present in the benzimidazole-sensitive (E198) but absent in both resistant genotypes (E198G and E198A) was developed for the detection of both amino acid replacements at the β-tubulin gene.     

Diversity of defence mechanisms in plant–oomycete interactions: a case study of <Emphasis Type="Italic">Lactuca</Emphasis> spp. and <Emphasis Type="Italic">Bremia lactucae</Emphasis>

Plant pathogenic oomycetes, including biotrophic downy mildews and hemibiotrophs/necrotrophs such as Phytophthora and Pythium, cause enormous economic losses on cultivated crops. Lettuce breeders and growers face the threat of Bremia lactucae, the causal agent of lettuce downy mildew. This pathogen damages leaf tissues and lettuce heads and is also frequent on wild Asteraceae plants. The interactions of Lactuca spp. with B. lactucae (abbr. lettuce–Bremia) display extreme variability, due to a long co-evolutionary history. For this reason, during the last 30 years, the lettuce–Bremia pathosystem has been used as a model for many studies at the population, individual, organ, tissue, cellular, physiological and molecular levels, as well as on genetic variability and the genetics of host–parasite interactions. The first part of this review summarizes recent data on host–parasite specificity, host variability, resistance mechanisms and genetics of lettuce–Bremia interactions. The second part focuses on the development infection structures. Phenotypic expression of infection, behaviour of B. lactucae on leaf surfaces, the process of penetration, development of primary infection structures, hyphae and haustoria are discussed in relation to different resistance mechanisms. In the third part, the components of host resistance and the variability of defence responses are analysed. The role of reactive oxygen species (ROS), antioxidant enzymes, nitric oxide (NO), phenolic compounds, reorganization of cytoskeleton, electrolyte leakage, membrane damage, cell wall disruption, hypersensitive reaction and plant energetics are discussed in relation to defence responses. In general, the extreme variability of interactions between lettuce and Bremia, and their phenotypic expression, results from diversity of the genetic background. Different mechanisms of resistance are conditioned by an orchestra of defence responses at the tissue, cell, and molecular levels. The various events responsible for defence involve a complex interaction of the processes and reactions mentioned above. This review also provides an overview on the timing of pathogen development, host pathological anatomy, cytology and physiology of lettuce–Bremia associations. The significance of these factors on the expression of different resistance mechanisms (non-host and host resistance, race-specific and race non-specific resistance, field resistance) is discussed.     

First report of ‘<Emphasis Type="Italic">Candidatus</Emphasis> Phytoplasma malaysianum’ associated with <Emphasis Type="Italic">Elaeocarpus</Emphasis> yellows of <Emphasis Type="Italic">Elaeocarpus zollingeri</Emphasis>

“Elaeocarpus yellows” (ELY) is a widely reported phytoplasma disease of Elaeocarpus zollingeri trees in Japan. The phytoplasma associated with ELY (ELY phytoplasma) had not been identified at the species level because its 16S rRNA sequence had yet to be reported. Here, we report the results of a sequence analysis based on 16S rRNA and secA gene sequences, which showed that the ELY phytoplasma is related to ‘Candidatus Phytoplasma malaysianum’. To our knowledge, this is the first report showing the occurrence of ‘Ca. P. malaysianum’ outside Malaysia and the infection of E. zollingeri by the phytoplasma.     

β-aminobutyric acid protects <Emphasis Type="Italic">Brassica napus</Emphasis> plants from infection by <Emphasis Type="Italic">Leptosphaeria maculans.</Emphasis> Resistance induction or a direct antifungal effect?

Resistance to infection in plants can be induced by treatment with various chemicals. One such compound is β-aminobutyric acid (BABA). Its positive effect on disease resistance has been noted in several pathosystems. Here we demonstrate that treatment with BABA protects Brassica napus plants from infection by the fungal pathogen Leptosphaeria maculans. Surprisingly, BABA also displayes in vitro antifungal activity against L. maculans with EC₅₀ similar to the fungicide tebuconazole. Both spore germination and hyphal growth were affected. The toxic effect can be reverted by addition of trypton to the culture medium. We hypothesised that BABA might inhibit inorganic nitrogen assimilation. Suppression of disease progression in plants and antifungal activity in vitro was weaker for α-aminobutyric acid and negligible for γ-aminobutyric acid. In contrast to a resistance inducer benzothiadiazole, the effect of BABA on disease development was nearly independent of the timing of treatment, indicating possible antifungal activity in planta. On the other hand, quantification of multiple hormones and an expression analysis have shown that treatment with BABA induces a synthesis of salicylic acid (SA) and expression of SA marker gene PR-1, but no evidence was observed for priming of SA responses to L. maculans. While we have not conclusively demonstrated how BABA suppresses the disease progression, our results do indicate that antifungal activity is another mechanism by which BABA can protect plants from infection.