Studies on red light-induced resistance of broad bean to Botrytis cinerea: I. Possible production of suppressor and elicitor by germinating spores of pathogen

When detached broad bean leaves were preinoculated with virulent strain B304 of Botrytis cinerea 24 h before a challenge inoculation with strain B304, lesion formation by B304 was significantly inhibited in red light but not in the dark. In leaves that were preinoculated with avirulent strain 021 and then challenged by B304, however, lesion formation was not inhibited even under red light. Such differences in lesion formation after the challenge inoculation with an avirulent strain were also observed with lesions caused by Alternaria alternata, a nonpathogen of broad bean and by avirulent strain 021R in the presence of germination fluid from spores of strains B304 and 021R. These results suggest the possibility that virulent B. cinerea produced a suppressor involved in induced susceptibility and an elicitor involved in resistance induced by red light during spore germination.     

Effects of barley genotype on induced resistance to powdery mildew

Infection of barley by an avirulent isolate of Erysiphe graminis f.sp. hordei reduces the development of powdery mildew disease by a virulent isolate, inoculated subsequently. This phenomenon, known as induced resistance, was studied in three sets of near-isogenic lines of barley, each of which included varieties with four different race-specific resistance genes. The extent of induced resistance, expressed as a reduction in the number of colonies formed, differed in lines carrying different race-specific resistance genes, being most effective in lines with Mla7 , followed by Mth , then Mla6 and Mla13. These differences may be related to the extent of the hypersensitive response to infection by avirulent spores. A further effect of induced resistance in reducing sporulation, taking the number of colonies into account, was least effective in Mth lines, with the other lines ranking as they did when induced resistance was expressed as colony numbers. By contrast, the genetic backgrounds of the three near-isogenic sets had similar effects on the fractional reduction of colony number by induced resistance. The effect of induced resistance on sporulation was strongest in the most resistant background, and weakest in the most susceptible.     

Resistance in leaf blades assessed by counting conidia correlates with whole-plant-specific resistance in leaf sheaths in a compatible rice–Magnaporthe oryzae interaction

Resistance in the leaf blades of rice plants against a virulent race of the rice blast fungus Magnaporthe oryzae was quantitatively examined using a modified spot inoculation method. Numbers of conidia produced in the lesions were affected by plant age and paralleled the frequency of resistance infection types, which is indicative of whole-plant-specific resistance (WPSR), in the inoculated leaf sheaths of the corresponding plants. Exogenous abscisic acid treatment, which suppresses WPSR, also increased the susceptibility of the leaf blades. These results indicate a correlation between the resistance of the leaf blades and the WPSR in the leaf sheaths.     

Virulent and avirulent isolates of Pseudomonas syringae subsp. savastanoi as colonizers of olive leaves: evaluation of possible biological control of the olive knot pathogen1

The epiphytic survival of virulent and avirulent (indoleacetic acid-deficient mutants) isolates of Pseudomonas syringae subsp. savastanoi, and their ability to colonize the olive phylloplane, was monitored for up to 30 days after artificial inoculation of several olive cultivars. After an initial decrease in numbers, the virulent bacteria multiplied and established a potential inoculum on olive leaf surfaces. In contrast, the numbers of avirulent bacteria rapidly diminished, so that they were eventually no longer detectable. Under the conditions used in this study, none of the avirulent mutants (producers or non-producers of bacteriocins) were able to compete with or successfully to inhibit the virulent isolate in vivo. This was probably due to the lack of epiphytic multiplication of the avirulent mutants.     

稻梨孢菌与其他寄主梨孢菌在水稻植株上的交互作用

作者研究了1个稻梨孢菌株与4个稻以外寄主梨孢菌株在混合接种和间隔接种条件下,在水稻植株上的交互作用。结果表明:弱致病菌与强致病菌间交互作用较强,非致病菌与强致病菌间较弱。弱致病菌可明显降低强致病菌的致病性,非致病菌对强致病菌的作用,多数组合表现为减轻病害,但有少数组合表现为促进发病。弱致病菌先接种;2天后接种强致病菌,比两者混合接种具有更强烈的互作效应,病斑数减少达35.7%-38.1%。先接种菌的孢子浓度对后接种菌的致病性也有影响。     

Host-mediated interactions between pathogen genotypes

Barley c ltivars with different resistance genes were inoculated with various sequences of virulent and non-virulent isolates of Erysiphe graminis f.sp. hordei. The resistance induced in incompatible reactions decreased infection by virulent isolates while, conversely, the degree of susceptibility induced in compatible reactions was sufficient to increase the ability of normally non-virulent isolates to infect a resistant host. The net effect of interactions was always overall reduction of conidial production when compared with inoculation of a virulent isolate alone.
Each interaction between isolates was dependent on the host cultivar, the pathogen isolates used and on the inoculation sequence. Often, induced resistance was most obvious when a nonvirulent isolate was inoculated prior to a virulent isolate. Induced susceptibility was usually more apparent when the virulent isolate was inoculated first. However, on the cultivar Wing, induced susceptibility was more apparent following prior inoculation of a non-virulent isolate. On the cultivar Hassan, prior inoculation with a virulent isolate sometimes induced susceptibility, but in some cases more susceptibility was induced by prior inoculation with a non-virulent isolate. In the latter instances, although relatively little infection developed, the majority of the spores produced were those of the normally non-virulent isolate.
There was some evidence of non-virulent isolates acquiring the ability to infect normally incompatible hosts. This effect disappeared after several generations in the absence of the virulent isolate.     

Vertical distribution of leaf blast lesions in mixtures of rice cultivar Sasanishiki and its resistant near-isogenic line

The vertical distribution of leaf blast lesions caused by the fungus Pyricularia grisea was studied to estimate the degree of leaf blast suppression in rice multilines in experimental paddy fields for 4 years. Leaf blast in 1 : 1 and 1 : 3 mixtures of susceptible rice cultivar Sasanishiki and its resistant near-isogenic line, Sasanishiki BL7, developed slower than that in pure stands of Sasanishiki. The average distance of lesions on leaves from the ground in the 1 : 3 mixtures was significantly lower than that in the pure stands at the end of leaf blast epidemics (at booting stage). This result shows that the distribution of leaf blast lesions in the upper layer differs between the susceptible pure stands and the 1 : 3 mixtures at the end of leaf blast epidemics.     

Induction of a defense response in strawberry mediated by an avirulent strain of <Emphasis Type="Italic">Colletotrichum</Emphasis>

In the strawberry crop area of Tucumán (north-west Argentina) the three species of Colletotrichum causing anthracnose disease (C. acutatum, C. fragariae and C. gloeosporioides) were detected. Among all isolates characterized, one of them identified as C. acutatum (M11) and another as C. fragariae (F7) were selected due to their conspicuous interaction with the strawberry cultivar Pájaro. Whereas isolate M11 produced a strong compatible interaction in cv. Pájaro with clear disease symptoms (DSR = 5.0), the isolate F7 brought about a typical incompatible interaction (DSR = 1.0). When plants of cv. Pájaro were inoculated with F7 prior to the inoculation with M11, the former avirulent strain prevented the growth of the latter virulent pathogen. Experimental evidence indicated that the time elapsed between the first inoculation with the avirulent pathogen and the second inoculation with the virulent one was crucial to inhibit the growth of the latter. The growth of F7 on the plant without provoking damage and the fact that there was no in vitro antagonistic effect between the pathogens, suggests that the avirulent strain triggers a plant defensive response against M11. The defense response was further confirmed by the detection of an early oxidative burst occurring within 4 h after the first inoculation and by the observation of anatomical changes associated with defense mechanisms that lasted 50 days after the inoculation with F7. Results obtained support the hypothesis that the plant resistance against the virulent strain M11 is elicited by one or more diffusible(s) compound(s) produced by the avirulent strain F7.     

Avirulent strain of Colletotrichum induces a systemic resistance in strawberry

Strawberry plants exposed to an avirulent isolate of Colletotrichum fragariae acquired strong resistance against a virulent strain of C. acutatum. Biochemical, morphological and molecular markers indicated that the strong defence response was associated with an oxidative burst and a transient accumulation of salicylic acid (SA). A maximum accumulation of H₂O₂ and O₂ ^? was observed 8 h after inoculation (hai), callose was detected 48 hai, and a peak of SA was observed 48 hai. Biochemical and phytopathogenic analyses carried out in non-treated tissues revealed that the defence response was systemic and remained fully active 60 days after the first inoculation. Experiments also showed that the resistance acquired by mother plants after the inoculation with the avirulent isolate could be passed to daughter plants through runners. Further characterization of the induced systemic resistance showed that the resistance was not only effective against a virulent strain of C. acutatum but also against Botrytis cinerea.     

Lesion-based analysis of leaf blast suppression in mixture of rice cultivar and a resistant near-isogenic line

Leaf blast suppression in multilines was evaluated based on the number of susceptible lesions observed in a pure stand of susceptible rice cultivar Sasanishiki, and in 1 : 1 and 1 : 3 mixtures of Sasanishiki and a resistant near-isogenic line, Sasanishiki BL4 or BL7, from 1998 to 2001. The number of lesions first observed in fields in the 1 : 1 and 1 : 3 mixtures were close to theoretical numbers calculated using the number of lesions observed in the pure stands and the ratios of the susceptible Sasanishiki in the mixtures. The ratio of the number of lesions in the 1 : 1 and 1 : 3 mixtures to the number in the pure stand was 0.29 and 0.09, respectively. The relationship between these ratios and the ratios of susceptible Sasanishiki in mixtures was defined in an equation to estimate the degree of leaf blast suppression. Validation studies for the ratios of the number of lesions in the 1 : 1 and 1 : 3 mixtures to the number in the pure stand were conducted in two different locations and showed that the ratios are almost acceptable. The calculated autoinfection to alloinfection ratio was 1.3 and 1.4 in the 1 : 1 and 1 : 3 mixtures, respectively, suggesting that the calculated ratio will affect the degree of leaf blast suppression. Thus, predictors were obtained to estimate leaf blast suppression for effective blast control in multilines.     

Comparative colonisation by virulent versus avirulent <Emphasis Type="Italic">Pyricularia oryzae</Emphasis> on wild <Emphasis Type="Italic">Oryza australiensis</Emphasis>

Pyricularia oryzae (rice blast) conidial development at pre-penetration stage determines success or otherwise of infection inside the rice host plants. Studies on conidial germination and growth on the leaf surface in commercial rice (Oryza sativa) report differently, dependent upon host type and level of blast resistance. Although wild rice (O. australiensis) is known to be an alternative host of blast, the interaction between P. oryzae conidia and wild O. australiensis on its leaf surface has not been previously studied. We found significant (P?<?0.001) differences in conidial development between two blast isolates with different virulence in terms of conidial germination, germ tube growth and appressoria formation on both wild and cultivated rice. Conidial germination at 6 h post-inoculation (hpi) for the virulent isolate was significantly (P?<?0.001) delayed. Germ tubes of the avirulent isolate conidia grew significantly (P?<?0.001) faster and with significantly (P?<?0.001) longer germ tubes than from virulent conidia. Appressoria development for the virulent isolate was significantly (P?<?0.001) faster at its later growth stages of 12 and 18 hpi when approximately 100% of germ tubes formed appressoria. In contrast, formation rate of appressoria for the avirulent isolate was significantly (P?<?0.001) slower and only reached 76% of germ tubes forming appressoria. Appressoria formation on O. australiensis was significantly (P?<?0.001) greater than the formation on O. sativa for both virulent and avirulent P. oryzae at 12 hpi, a clear indication that host type influences the extent of appressoria formation.     

水稻品种抗瘟性诱导研究Ⅰ.非生物诱导物的筛选及诱导方法初探

 采用经反复筛选的化学药剂亚硒酸钠、放线菌酮、氯化钾、硫酸锰和中药材前胡、白芷的提取液浸泡水稻种子或进行叶面喷雾,可诱导稻株产生对稻瘟病的抗性。在人工喷雾接种条件下,对感病品种抗叶瘟的诱导效果为43%~93%;在注射接种条件下,抗穗瘟的诱导效果为29%~78%。供试药剂对感病品种和弱致病菌株的诱导效果优于对抗病品种和强致病菌株;药液叶面喷雾的诱导效果优于药液浸种。     

Possible involvement of salicylic acid in systemic acquired resistance ofCucumis sativus againstSphaerotheca fuliginea

The possible involvement of salicylic acid in systemic acquired resistance ofCucumis sativus againstSphaerotheca fuliginea was studied. Cucumber plants were inoculated with tobacco necrosis virus on the cotyledons and the level of endogenous salicylic acid in the first true leaf was determined by gas chromatography. Salicylic acid increased continously from the second day after virus inoculation to the fifth day, when the same leaf was inoculated withSphaerotheca fuliginea. In healthy plants, the efficiency of exogenous salicylic acid in inducing resistance was assayed by applying aqueous solutions at different times beforeSphaerotheca fuliginea inoculation. To evaluate the level of induced resistance, the following parameters were examined by light microscopy: percentage of conidial germination, length of the hyphae derived from single conidia, number of haustoria, percentage of epidermal cells with lignified walls and of necrotic cells underlying fungal hyphae. In treated plants conidial germination was reduced, the total length of the hyphae was shorter, the number of haustoria was lower and the haustorium-containing epidermal cells had more frequently lignified walls. Moreover, an evident increase in callose deposition was observed leading to the formation of oversized papillae around the penetration pegs. These results indicate that the application of salicylic acid before inoculation withSphaerotheca fuliginea reduces the intensity of the infectious process and that salicylic acid is involved in the expression of systemic resistance in cucumber challenged by the biotrophic pathogenSphaerotheca fuliginea.     

Variation for virulence in Erysiphe fischeri from Senecio vulgaris

Variation for virulence was investigated in two populations of Erysiphe fischeri , the cause of powdery mildew of the common annual weed Senecio vulgaris (groundsel). Infection types were recorded on detached leaf segments from 50 inbred lines of S. vulgaris following inoculation with 24 single-conidial-chain isolates of E. fischeri (12 each from two UK sites; one, at Glasgow, located about 480 km north of the other, at Wellesbourne). Mean infection types for each isolate/line combination were categorized by applying several upper limits below which combinations were considered to be incompatible. Regardless of the limit applied, numerous specific virulence and resistance phenotypes could be discriminated. Virulence phenotypes were complex and all isolates were capable of colonizing and reproducing on the majority of groundsel lines. However, all isolates were completely avirulent (no reproduction) on at least one groundsel line, so no isolate was universally virulent. Plants of several groundsel lines exhibiting different resistance phenotypes were exposed to natural infection at Wellesbourne to act as mildew traps, to examine the virulence characteristics of the pathogen population. Common components of the fungus population colonized groundsel lines exhibiting resistance to only a few or none of the single-conidial-chain isolates. Rarer components of the pathogen population colonized groundsel lines with resistance to the majority or all of the isolates previously tested. These latter components of the fungus population also detected resistance in groundsel lines previously recorded as susceptible to all isolates.     

稻梨孢菌与其他寄主梨孢菌在水稻植株上的交互作用

作者研究了1个稻梨孢菌株与4个稻以外寄主梨孢菌株在混合接种和间隔接种条件下,在水稻植株上的交互作用。结果表明:弱致病菌与强致病菌间交互作用较强,非致病菌与强致病菌间较弱。弱致病菌可明显降低强致病菌的致病性,非致病菌对强致病菌的作用,多数组合表现为减轻病害,但有少数组合表现为促进发病。弱致病菌先接种;2天后接种强致病菌,比两者混合接种具有更强烈的互作效应,病斑数减少达35.7％—38.1％。先接种菌的孢子浓度对后接种菌的致病性也有影响。     

Pine Wilt Disease Caused by the Pine Wood Nematode: The Induced Resistance of Pine Trees by the Avirulent Isolates of Nematode

Since the beginning of the 20th century, pine trees in Japan have been seriously damaged by the pine wilt disease. This disease is caused by the pine wood nematode, Bursaphelenchus xylophilus, which is transmitted by the Japanese pine sawyer, Monochamus alternatus. The control of disease depends to a large extent on chemicals, but the public is now demanding environmentally friendly control methods. The virulence of B. xylophilus varies very widely. Pre-inoculation of young pine trees in a nursery with avirulent B. xylophilus has induced systemic resistance of trees against a subsequent inoculation with virulent B. xylophilus. This induced resistance was considered a hopeful means for developing a biological control for the disease. The induced resistance by the avirulent nematodes was also expressed in mature pine trees in a forest where the disease was naturally epidemic. However, the effects of induced resistance were not satisfactory for practical biological control. Since the inoculation with higher concentrations of the avirulent B. xylophilus induced the resistance more effectively, the pre-inoculation method will need to be improved to develop the biological control. The induced resistance of pine trees by avirulent B. xylophilus should be one of the candidate biological control methods against pine wilt disease. This induced resistance also provides an experimental system to clarify physiological interactions between the nematodes and pine trees.     

Efficacy of Hypovirulent Binucleate Rhizoctonia sp. to Control Banded Leaf and Sheath Blight in Corn

Water agar, artificially infested soil and leaf sheath inoculation methods were used to assess the suitable time of application, varietal host response and persistence of Rhv7, a hypovirulent, binucleate Rhizoctonia collected from soil in the Philippines, to effectively control virulent Rhizoctonia solani AG1-IA isolate RS35 on corn. With the water agar method, prior inoculation (2 to 3 days) with the biocontrol agent is essential to achieve maximum control of the pathogen. Disease protection was increased from 23 to 70% as the pre-incubation time of Rhv7 prior to the challenge inoculation with virulent isolate was lengthened from 0 to 3 days. Disease severity and incidence of banded leaf and sheath blight (BLSB) on the three corn hybrids were also suppressed in artificially infested soil. In leaf sheath inoculation, the suppressive ability of Rhv7 against BLSB on corn persisted during lesion expansion. This suppression was expressed as slower disease progress in plants with Rhv7 than in plants without Rhv7. Early and timely brace root formation that detached infected sheaths, also aided in reducing the number of diseased plants in Rhv7-treated plots at the final observation. Mycelial growth activity of RS35 was reduced when corn plants were pre-inoculated with Rhv7 before challenge even if there was no contact detected between Rhv7 and RS35, which suggests that Rhv7 protects corn against BLSB by induced resistance. The hypovirulent, binucleate Rhizoctonia Rhv7 strain effectively controlled R. solani AG1-IA isolate RS-35 in corn. Received 30 August 1990/ Accepted in revised form 25 October 1999     

小麦对白粉菌的抗性和感病性的诱导

 用小麦白粉菌(Erysiphe graminis f.sp.tritici)的无毒和有毒菌株,以3种接种顺序:1.先接无毒菌株,间隔一定时间后,再接有毒菌株;2.无毒和有毒菌株同时接;3.和"1"相反,接种具有不同抗性基因的小麦品种的幼苗或叶段,以研究小麦对白粉菌的诱导抗性和诱导感病性。结果表明:先接无毒菌株诱导的抗性最强;后接无毒菌株或无毒、有毒菌株同时接,当诱导菌(inducer)的密度很高时,也可以诱导出抗性,但较微弱;诱导的抗性随诱导菌的密度增高而增强;一经诱导,抗性可在被诱导部位维持到第6天;所使用的3种接种顺序都可以诱导小麦对白粉菌产生感病性,但较之诱导的抗性弱得多;在抗病性的诱导中,存在着小麦品种与诱导菌,或诱导菌与挑战菌(challenger)之间的相互作用;一个无毒菌株可以通过诱导而获得在抗病品种上生长的毒性,但当缺少有毒菌株存在时,获得毒性即消失;没有发现所诱导的抗性具有系统传导性。     

Histochemical and chemical evidence for lignin accumulation during the expression of resistance to leaf rust fungi in wheat

Levels of individual phenolic acids were examined in primary leaves of wheat (Triticum aestivum) after inoculation with avirulent and virulent strains of the leaf rust fungus (Puccinia recondita f. sp. tritici) at stages when previous work had shown fungal and host cells to be affected by expression of the Lr20 or Lr28 alleles for resistance. The predominant phenolic acid, ferulic acid, as well as p-coumaric and syringic acids were detected in primary leaves in both unbound and bound forms. They were not detected in germinating urediniospores of either rust strain. Levels of unbound phenolic acids changed little in response to infection. In Lr28-bearing leaves inoculated with an avirulent strain, increased concentrations of bound phenolic acids and three other unidentified compounds were observed about 4 h after many single or small groups of cells had undergone hypersensitive collapse. In an Lr20-bearing cultivar, levels of bound phenolic acids fell in leaves inoculated with either a virulent or avirulent rust strain. Coniferyl aldehyde and coniferyl alcohol were not detected in healthy or inoculated leaves of either wheat cultivar. Attempts to affect expression of resistance by application of inhibitors of phenylalanine ammonia-lyase were not successful and both wheat cultivars remained resistant to avirulent rust strains. The bound phenolic acids which accumulate in cells undergoing a hypersensitive response may play a role in resistance of Lr28-bearing wheat to the leaf rust fungus.     

Comparison of resistance-inducing abilities of virulent and avirulent races of Erysiphe graminis f.sp. hordei and a race of Erysiphe graminis f.sp. tritici in barley

Resistance to powdery mildew was induced in barley by preinoculation with virulent and avirulent races of barley powdery mildew ( Erysiphe graminis f.sp. hordei ), and with a race of wheat powdery mildew ( E. graminis f.sp. tritici ). Four inducer densities were tested in 13 different induction periods between 1 and 24 h. Generally, the resistance induced by barley powdery mildew increased up to 10-12 h of induction and was maintained in longer induction periods. The inducing abilities of virulent and avirulent races could not be distinguished up to 10-12 h of induction, after which the inducing ability of avirulent races increased significantly in relation to virulent races. Wheat powdery mildew was able to induce more resistance than barley powdery mildew in induction periods up to 8 h. In a single inoculation procedure the number of haustoria developing from virulent barley powdery mildew decreased as inoculum density increased. The effect was ascribed to induction of resistance. This reduction of infection efficiency in the compatible interaction was compared to induced resistance. However, the inoculum density needed for 50% resistance induction in the double inoculation procedure was approximately 40 times higher than the inoculum density needed for 50% reduction in infection efficiency in the single inoculation procedure.