Root and stem rot of chrysanthemum caused by five <Emphasis Type="Italic">Pythium</Emphasis> species in Japan

Root and stem rot with wilt of above ground parts of cultivated chrysanthemums was first found in Ibaraki, Toyama and Kagawa prefectures, Japan in 2002 and 2003. Pythium species were isolated from the diseased tissues and identified as P. dissotocum, P. oedochilum, P. sylvaticum, P. ultimum var. ultimum and asexual strains of P. helicoides based on their morphologies and sequences of rDNA-ITS region. All the Pythium species were strongly pathogenic to chrysanthemums in pot conditions and were reisolated from the inoculated plants. Because Pythium root and stem rot of chrysanthemum has never been reported in Japan, we propose that this is a new disease that can be caused by the five Pythium species.     

Pythium rot of chingensai ( Brassica campestris L. chinensis group) caused by Pythium ultimum var. ultimum and Pythium aphanidermatum

Severe rot was found at the base of leaves and stems of chingensai (Brassica campestris L. chinensis group) in Okayama Prefecture in 2000. The causal fungi were morphologically identified as Pythium ultimum Trow var. ultimum and P. aphanidermatum (Edson) Fitzpatrick. This is the first report of rot caused by Pythium species on chingensai. We named this disease Pythium rot of chingensai.     

Damping-off of cabbage plug seedlings caused by Pythium ultimum var. ultimum in Japan

In October 2004, Pythium ultimum var. ultimum was isolated from rotten stems of cabbage plug seedlings in a commercial nursery in Mie Prefecture (Japan). The isolated fungus was then used to inoculate seedlings and subsequently reisolated from the seedlings with the damping-off disease, showing that P. ultimum var. ultimum is a new pathogen causing cabbage seedling disease.     

Pathogenicity ofPythium species on cucumber in peat-sand,rockwool and hydroponics

Pythium spp. that cause damping-off of seedlings also can cause root rot of older plants and lead to yield reductions. This can be especially severe in soilless cultures where the fungus can spread easily with the nutrient solution. 39Pythium isolates obtained from discolored roots were assayed for their ability to cause damping-off on cucumber seedlings in sand-peat and for their pathogenicity in soilless culture of cucumber in rockwool or hydroponic solution. Isolates ofPythium aphanidermatum, P. irregulare, P. sylvaticum andP. ultimum were highly pathogenic in sand-peat, but onlyP. aphanidermatum strains were pathogenic in soilless conditions and led to root decay, plant death in rockwool culture and growth reduction in hydroponic culture. One strain ofP. aphanidermatum significantly reduced the yield of cucumber grown in rockwool under conditions similar to those of commercial cultures.     

Biological control of damping-off of sugar beet by Pseudomonas putida applied to seed pellets

Pseudomonas putida 40RNF applied to seed pellets reduced the occurrence of Pythium damping-off of sugar beet. A density of 6 × 10⁷ 40RNF per pellet reduced Pythium damping-off from 70 to 26% when seeds were sown in artificially infested soil (250 propagules Pythium ultimum per g dry soil). The efficacy of 40RNF was dependent on its density in the seed pellet (in the range 2 × 10⁴–6 × 10⁸ per pellet) and on the number of propagules of Pythium in soil. 40RNF declined to or stabilized at approximately 1 × 10⁶ per pellet 3 days after planting, and this was independent of the inoculum density. This indicated that the crucial steps resulting in damping-off of sugar beet caused by Pythium ultimum must occur within 3–4 days of sowing. 40RNF reduced pericarp colonization by P. ultimum by 43% 48 h after planting and caused a 68% decrease in the number of sporangia of P. ultimum in the surrounding soil (0.0–5.0 mm). P. putida 40RNF also reduced pre and post-emergence damping-off (from 69.5 to 37.5%) caused by indigenous populations of Pythium species in an infested soil and this was as effective as the fungicide hymexazol (69.5 to 40%).     

Biological control of damping-off caused byPythium ultimum andRhizoctonia solani usingTrichoderma spp. applied as industrial film coatings on seeds

Conidia of sevenTrichoderma strains were applied on cucumber or radish seeds as a simple methyl cellulose coating or through an industrial film coating process. The seeds were sown in a peat-based soil artificially infested byR. solani orP. ultimum. Four strains controlled damping-off caused byR. solani when applied as a simple coating or as an industrial film-coating. Also, four strains significantly reduced damping-off caused byP. ultimum in cucumber. A correlation was found between production of volatile antibiotics in vitro and control ofP. ultimum. Survival during storage varied according to the strain. Better survival was observed for two strains, with a decrease in conidial viability of one order of magnitude after storage for three and five months at 15 ° C and 4 ° C, respectively. The results show the feasibility of biocontrol of seedling diseases by some antagonists applied onto seeds through an industrial film-coating process.     

Bait method to detect Pythium species that grow at high temperatures in hydroponic solutions

Pythium helicoides, P. aphanidermatum and P. myriotylum are important pathogens that cause root rot of several crops in hydroponic culture and in ebb-and-flow irrigation systems. These species belong to a group of Pythium species that can grow at temperatures higher than 40°C. We developed a method for baiting these high-temperature Pythium species and evaluated its practicality to monitor their presence in nutrient solutions. Seeds of cucumber, tomato, radish, hemp, perilla and millet and leaves of bent grass and rose were tested as baits in hydroponic systems. Hemp, perilla and radish seeds and bent grass and rose leaves were more effective than the other baits for Pythium zoospores, and bent grass leaves were the most effective. In a sensitivity test, bent grass leaf traps (BLTs) detected three Pythium species after only a 1 day exposure to suspensions of 40 zoospores per liter of water, and the frequency of detection increased with zoospore density and with baiting period. A temperature of 38°C was optimum for the selective reisolation of the high-temperature Pythium species from the BLTs. The BLT was also tested with inoculated and noninoculated miniature roses that shared a recirculating nutrient solution. The pathogen was detected in the nutrient solution 23 days before the disease spread to the noninoculated roses. In addition, P. helicoides was detected 30 days before the disease was evident in a commercial greenhouse. The baiting method described here will be useful for monitoring high-temperature Pythium species in recirculating hydroponic culture systems.     

Effect of indole-acetic acid (IAA) on the development of symptoms caused by <Emphasis Type="Italic">Pythium ultimum</Emphasis> on tomato plants

The effect of indole-acetic acid (IAA) on the development of symptoms caused by Pythium ultimum on tomato plants was investigated using different bioassays. Application of IAA (5 μg ml⁻¹) on tomato seedlings inoculated with P. ultimum did not affect their emergence suggesting that IAA did not affect the severity of Pythium damping-off. However, IAA was shown to influence the development of P. ultimum symptoms on tomato plantlets. Low concentrations of IAA (0–0.1 μg ml⁻¹) within the rhizosphere of plantlets increased the severity of the symptoms caused by P. ultimum, while higher concentrations (10 μg ml⁻¹), applied either by drenching to the growing medium or by spraying on the shoot, reduced the symptoms caused by this pathogen. In addition, the study demonstrated that P. ultimum produces IAA in liquid culture amended with L-tryptophan, tryptamine or tryptophol (200 μg ml⁻¹) and in unamended culture.     

<Emphasis Type="Italic">Pythium</Emphasis> and <Emphasis Type="Italic">Phytophthora</Emphasis> species associated with root and stem rots of kalanchoe

Pythium and Phytophthora species were isolated from kalanchoe plants with root and stem rots. Phytophthora isolates were identified as Phytophthora nicotianae on the basis of morphological characteristics and restriction fragment length polymorphism (RFLP) analysis of the rDNA-internal transcribed spacer regions. Similarly, the Pythium isolates were identified as Pythium myriotylum and Pythium helicoides. In pathogenicity tests, isolates of the three species caused root and stem rots. Disease severity caused by the Pythium spp. and Ph. nicotianae was the greatest at 35°–40°C and 30°–40°C, respectively. Ph. nicotianae induced stem rot at two different relative humidities (60% and >95%) at 30°C. P. myriotylum and P. helicoides caused root and stem rots at high humidity (>95%), but only root rot at low humidity (60%).     

Enhancing Resistance of Tomato and Hot Pepper to Pythium Diseases by Seed Treatment with Fluorescent Pseudomonads

Twenty isolates of fluorescent pseudomonads were evaluated for their ability to control damping-off in tomato (Lycopersicon esculentum) and hot pepper (Capsicum annuum). These isolates were characterized as Pseudomonas fluorescens and Pseudomonas putida. Two isolates, PFATR and KKM 1 belonged to P. putida and the remaining 18 isolates belonged to P. fluorescens. Among these isolates, P. fluorescens isolate Pf1 showed the maximum inhibition of mycelial growth of Pythium aphanidermatum and increased plant growth promotion in tomato and hot pepper. P. fluorescens isolate Pf1 was effective in reducing the damping-off incidence in tomato and hot pepper in greenhouse and field conditions. Isolate Pf1 was further tested for its ability to induce production of defense-related enzymes and chemicals in plants. Earlier and increased activities of phenylalanine ammonia lyase (PAL), peroxidase (PO) and polyphenol oxidase (PPO) were observed in P. fluorescens Pf1 pretreated tomato and hot pepper plants challenged with Pythium aphanidermatum. Moreover, higher accumulation of phenolics was noticed in plants pretreated with P. fluorescens isolate Pf1 challenged with Pythium aphanidermatum. Thus, the present study shows that in addition to direct antagonism and plant growth-promotion, induction of defense-related enzymes involved in the phenyl propanoid pathway collectively contributed to enhance resistance against invasion of Pythium in tomato and hot pepper.     

Molecular identification and pathogenicity of Rhizoctonia solani and Pythium spp. associated with damping-off disease on baby leafy vegetables in Greece

In the last years, leafy vegetables cultivated as baby leaves have been established in the market and have attracted the interest of consumers throughout the world. During the growing seasons of 2019 and 2020, 97 isolates of Rhizoctonia solani and 112 isolates of Pythium spp. were obtained from baby leaf vegetables exhibited damping-off symptoms. Representative isolates of R. solani from each surveyed plant species were characterized using sequence analysis of the internal transcribed spacer (rDNA-ITS) region. Isolates were identified as belonging to four anastomosis groups (AGs): AG2-1, AG-IB, AG4-HGI and AG4-HGIII. AG4-HGI was the most prevalent group and phylogenetic analysis showed that the isolates were distinctly separated according to their AGs. Pathogenicity among the four AGs on 23 plant species varied considerably, from not susceptible to highly susceptible, while, in general, AGs did not exhibit host specificity. Furthermore, a total of 112 Pythium spp. isolates were obtained. The ITS region and the cytochrome oxidase II (coxII) gene were amplified, and three Pythium spp. were identified (P. ultimum, P. aphanidermatum and P. sylvaticum), which were used further for maximum-likelihood phylogenetic analysis. The pathogenicity of representative isolates was assessed in vitro and in vivo on 10 plant species. In general, all three tested Pythium spp. were virulent when used in vitro, while P. ultimum was the most virulent in vivo. This is the first comprehensive study aimed at determining the occurrence of specific R. solani AGs and Pythium spp. derived from baby leafy vegetables exhibiting damping-off symptoms in Greece.     

Pathogenic fungi involved in root rot of peas in the Netherlands and their physiological specialization

Research on root rot pathogens of peas in the Netherlands has confirmed the prevalence ofFusarium solani, F. oxysporum, Pythium spp.,Mycosphaerella pinodes andPhoma medicaginis var.pinodella. Aphanomyces euteiches andThielaviopsis basicola were identified for the first time as pea pathogens in the Netherlands. Other pathogens such asRhizoctonia solani andCylindrocarpon destructans were also found on diseased parts of roots. F. solani existed in different degrees of pathogenicity, and was sometimes highly specific to pea, dwarf bean of field bean, depending on the cropping history of the field.A. euteiches was specific to peas, whereasT. basicola showed some degree of physiological specialization.     

Pythium rot of Chinese cabbage (Brassica rapa L. subsp. pekinensis) caused by Pythium aphanidermatum

Severe rot was found at the base of leaves and stems of Chinese cabbage (Brassica rapa L. subsp. pekinensis) in Ibaraki Prefecture every year in early September from 2002 through 2004. The causal fungus was identified as Pythium aphanidermatum (Edson) Fitzpatrick. This is the first report of P. aphanidermatum on Chinese cabbage. A similar disease of Chinese cabbage caused by P. ultimum Trow var. ultimum is known as Pythium rot. We propose adding P. aphanidermatum as a new pathogen of this disease.     

A simple detached leaf assay provides rapid and inexpensive determination of pathogenicity of Pythium isolates to ‘all year round’ (AYR) chrysanthemum roots

A detached leaf assay was developed to determine the pathogenicity of Pythium isolates to cut‐flower chrysanthemum roots. Leaves from young plants were excised and inoculated by insertion of a plug of mycelium into a slit cut in the excised petiole. After incubation leaves were assessed for presence and extent of necrosis. Necrosis indicated pathogenicity and was consistently confirmed by comparisons with whole plant inoculations. The rate of necrosis spread also gave some indication of virulence. Isolates of Pythium sylvaticum, P. ultimum and HS group were the most virulent, with a mean rate of spread of 14·6 mm per day, significantly (P < 0·05) faster than the mean rate of spread, 1·6 mm per day, of less virulent isolates. Less virulent isolates included P. irregulare, P. oligandrum and P. aphanidermatum. The latter was unexpected, as P. aphanidermatum is an important species in pythium root rot epidemics in chrysanthemums elsewhere. The value of the detached leaf assay for screening large numbers of isolates was demonstrated in a survey of isolates from clinic samples from chrysanthemum nurseries and in a series of dilution‐plating experiments looking at numbers of Pythium propagules in commercial chrysanthemum beds showing root rot. In the survey, the predominant pathogenic species was identified as P. sylvaticum and the most likely source of infection was contaminated soil as opposed to blocking media or irrigation water, whilst in soil colonization studies the use of detached leaf assays demonstrated a relationship between pathogenic inoculum concentration in soil and the expression of root rot symptoms.     

Effect of Pythium spp. and glyphosate on phytoalexin production and exudation by bean (Phaseolus vulgaris L.) roots grown in different media

Kievitone, phaseollinisoflavan and phaseollin were detected in roots of bean seedlings (Phaseolus vulgaris L.) grown in natural soil. Comparison of phytoalexin production by roots grown in different media indicated that these phytoalexins were probably induced by microorganisms in soil. The influence of common root rot pathogens of bean, Pythium spp., on phytoalexin production was determined. Pythium ultimum elicited kievitone, phaseollinisoflavan and phaseollin in roots grown in sterilized silica sand. P. sylvaticum induced only kievitone and phaseollin in the same growth medium. Glyphosate did not significantly affect the accumulation of phytoalexins within 3 days. However, by day 5, significantly more phaseollin was detected in the roots of Pythium inoculated plants treated with glyphosate than in Pythium inoculated plants not treated with glyphosate. In a hydroponic system, both Pythium spp. elicited accumulation of kievitone and phaseollin in root tissue, and both phytoalexins were exuded into the bathing solution. Glyphosate application did not significantly affect accumulation or exudation of phytoalexins by bean roots in the hydroponic system. The results from this study illustrate the nature and extent of phytoalexin production by bean roots in the absence and presence of microbes.     

Characterisation and detection of Pythium and Phytophthora species associated with grapevines in South Africa

Replant and decline diseases of grapevines not only cause quantitative and qualitative yield losses, but also result in extra costs when vineyards have to be replanted. This study investigated the role of Pythium and Phytophthora in the decline syndrome in South Africa by determining (1) the species associated with nursery and established vines, and (2) pathogenicity of Ph. sp. niederhauserii and P. vexans relative to known grapevine pathogens. Quantitative real-time PCR (qPCR) assays were also developed for detection of the most prevalent oomycete groups. In total, 26 Pythium and three Phytophthora species were identified from grapevine nurseries and established vineyards. The most common infections in sampled nursery vines were caused by P. vexans (16.7%), followed by P. ultimum var. ultimum (15.0%) and P. irregulare (11.7%). In established vineyards, P. irregulare (18.0%) and P. vexans (6.2%) were also among the three most prevalent species, along with P. heterothallicum (7.3%). Three Phytophthora species were also identified from the sampled established vines, of which Ph. cinnamomi (5.1%) was predominant, followed by Ph. sp. niederhauserii (1.1%). In established vineyards a higher incidence and more diverse species composition was observed in spring and winter, than in summer. Pathogenicity studies showed that some Ph. sp. niederhauserii and P. vexans isolates were as aggressive as the known grapevine pathogens Ph. cinnamomi and P. irregulare. Sensitive qPCR assays were developed for the detection of P. ultimum var. ultimum, P. irregulare, P. vexans and the genus Phytophthora. These assays will be invaluable in limiting pathogen dispersal through screening of nursery material. This is especially important since pathogenic species were also isolated from healthy looking vines in nurseries.     

三类植物卵菌的全基因组比较

为有效防治由不同类型卵菌引起的植物病害,该研究对数据库中的霜霉、疫霉和腐霉3类不同卵菌的全基因组数据进行序列特征分析、同源性分析和共线性分析以及同源差异基因富集通路分析。结果显示,霜霉、疫霉和腐霉来自独立的谱系,且疫霉与霜霉的亲缘关系较近;疫霉的致病相关基因数量最多,主要有RxLR、CRN、NPP、NF和PcF基因家族,霜霉的致病相关基因数量次之,腐霉的致病相关基因数量最少;3类卵菌共确定了13 392个同源基因,其中3类卵菌均共有的同源基因为3 786个,不同菌种的同源基因数量差异较大,整体表现为疫霉>霜霉>腐霉;同源差异基因富集程度最高的前20个通路主要是基础代谢的通路和中间代谢通路,其中抗坏血酸和藻酸盐代谢、ABC转运蛋白和果糖和甘露糖代谢相对富集程度较高,基因数目也较多。     

Induction of systemic resistance,root colonisation and biocontrol activities of the rhizospheric strain of <Emphasis Type="Italic">Serratia plymuthica</Emphasis> are dependent on N-acyl homoserine lactones

Quorum sensing regulation, mediated by N-acyl homoserine lactone signals, produced by strain Serratia plymuthica HRO-C48 isolated from the rhizosphere of oilseed rape, was found to be responsible for this strain’s ability to produce the broad spectrum antibiotic pyrrolnitrin. In this study, we have shown that some other biocontrol-related traits of strain HRO-C48, such as protection of cucumbers against Pythium apahnidermatum damping-off disease, induced systemic resistance to Botrytis cinerea grey mold in bean and tomato plants, and that colonisation of the rhizosphere also depends on AHL signalling. The results prove that quorum sensing regulation may be generally involved in interactions between plant-associated bacteria, fungal pathogens and host plants.     

Suitability of Ten Plant Baits for the Rapid Detection of Pathogenic Pythium Species in Hydroponic Crops

Ten types of plant baits were tested in the laboratory to assess their capacity to detect pathogenic Pythium species. These were orange tree leaves, tomato leaves, pepper leaves, geranium leaves, Bermuda grass leaves, pine needles, immature carnation petals, hemp-seed cotyledons, pepper and cucumber fruits. The Pythium spp. tested were P. aphanidermatum, P. irregulare and Pythium group F from hydroponic market garden crops in the Poniente region of Almería (south-east Spain). The test consisted of observing the velocity at which five baits were colonized and the day of colonization of the first bait. Results indicated that the slowest baits to be infected were immature carnation petals and pine needles. These two, together with Bermuda grass leaves, were also the baits infected in lowest number, such that practically no further infection was produced in the baits after the fifth day of contact with the inoculated water. The other plant baits tested were equally suitable for detection of Pythium spp. over the first two days, although only orange leaves and hemp-seed cotyledons were infected on the first day.     

Production of Indole-3-acetic Acid and Tryptophol by Pythium ultimum and Pythium Group F: Possible Role in Pathogenesis

Pythium group F is a minor pathogen which induces symptomless infections that occur frequently and results in yield losses in tomato soilless cultures. To elucidate the mode of action of this microorganism, the influence of culture filtrates of Pythium group F on tomato growth was investigated and compared to that of the pathogen Pythium ultimum. Depending on metabolite production by the fungus, marked differences were observed in plant response. Pythium group F crude culture filtrates or low molecular weight fractions (< 500) caused swelling behind the root tip and reduced root growth; the cohesion and adherence of cells within the cortical area were also affected. These symptoms were similar to those observed on plants treated with indole-3-acetic acid. By contrast, P. ultimum filtrates caused a marked distortion of cell shape accompanied with folding of host cell walls, particularly in the cortical area. These symptoms were characteristic of the activity of toxic compound(s) on host cells. Chemical analysis of the filtrates demonstrated that indole-3-acetic acid and tryptophol were produced by Pythium group F and P. ultimum. However, Pythium group F took up and metabolized more indole-3-acetic acid precursors, especially tryptophan, a key amino acid in the pathways leading to indole-3-acetic acid synthesis. The fact that Pythium group F and P. ultimum transformed tryptamine and indole-3-acetaldehyde into indole-3-acetic acid and tryptophol confirms the existence of a tryptamine pathway within both fungi. These results support the hypothesis that auxins facilitate Pythium group F infections. On the other hand, toxin(s) and hydrolytic enzymes are likely involved in P. ultimum pathogenesis.