Causal Agent Of Downy Mildew Research Articles

Antisense Expression of a NBS-LRR Sequence in Sunflower (Helianthus annuus L.) and Tobacco (Nicotiana tabacum L.): Evidence for a Dual Role in Plant Development and Fungal Resistance

A partial sunflower cDNA clone, PLFOR48, segregating with a resistance marker to Plasmopara halstedii, the causal agent of downy mildew, has been cloned from the mildew resistant sunflower line, RHA 266. PLFOR48 encodes a putative protein with a nucleotide-binding site and a leucine-rich repeat domain, showing significant homology with previously cloned resistance genes belonging to the TIR-NBS-LRR family. Southern blot analysis of non-transgenic sunflower suggests that PLFOR48 is part of a multigenic family. The potential role of PLFOR48 sequence in sunflower resistance to mildew was studied, by assessing loss of function, using expression of the antisense cDNA in RHA 266 sunflower line. Quite unexpectedly, transgenic sunflower lines displayed severe developmental abnormalities, and in particular, on the main meristems of homozygote T2 progeny, thus hampering any further challenge inoculation with Plasmopara halstedii. The presence of homologous sequences to PLFOR48 in Nicotiana tabacum var Samsun NN, as demonstrated by Southern blotting, drove us to consider tobacco as an additional model to investigate the potential role of this sequence in fungal resistance. Expression of the same antisense cDNA in transgenic tobacco lines gave rise to higher degree of susceptibility to Phytophthora parasitica, as well as to severe alterations in seed development. These results suggest that PLFOR48 and homologous sequences could be involved in both regulating developmental pathways and controlling resistance to fungal pathogens.

Microsatellite DNA markers for Plasmopara viticola, the causal agent of downy mildew of grapes

AbstractMicrosatellite loci were isolated from Plasmopara viticola (Oomycetes), the causal agent of downy mildew of grape, one of the most damaging fungal diseases of grapevine worldwide. Seven polymorphic loci were obtained from an enriched partial genomic library. A low genetic diversity was observed at all loci, with a mean observed allele number of 3.75 and an observed heterozygosity ranging from 0.074 to 0.547. Cross‐amplification tests on three closely related taxa indicated that two loci could be used in other Oomycetes species. These microsatellite loci were proved to be useful for population genetic analysis.

Identification of sources of resistance in sorghum to Peronosclerospora sorghi

The main objective of this work was to identify sources of resistance in sorghum (Sorghum bicolor) to Peronosclerospora sorghi, the causal agent of downy mildew, through the evaluation of 42 sorghum genotypes under natural infection in the field. Genotypes were planted in single row plots between two rows of the susceptible line SC283, planted 30 days before, to act as spreader rows, in two separate nurseries. The experimental design was a completely randomized block design with three replications. Sorghum genotypes CMSXS156, CMSXS157, CMSXS243, TxARG-1, 8902, 9902054, 9910032, 9910296, Tx430, QL-3, SC170-6-17, CMSXS762 and BR304 were classified as highly resistant in both nurseries. Among these, SC170-6-17 and 9910296 showed 0% systemic infection. Results indicated the possible occurrence of different pathotypes of P. sorghi in the two nurseries.

Resistance of sunflower to the biotrophic oomycete Plasmopara halstedii is associated with a delayed hypersensitive response within the hypocotyls

The biotrophic oomycete Plasmopara halstedii is the causal agent of downy mildew in sunflower. It penetrates the roots of both susceptible and resistant sunflower lines and grows through the hypocotyls towards the upper part of the seedling. RT-PCR analysis has shown that resistance is associated with the activation of a hsr203J-like gene, which is a molecular marker of the hypersensitive reaction in tobacco. Activation of this gene was specifically observed during the incompatible interaction and coincided with cell collapse in the hypocotyls. This HR was also associated with the early and local activation of the NPR1 gene which is a key component in the establishment of the SAR. No such HR or a significant activation of the hsr203J-like gene were observed during the compatible combination. These results suggest that the resistance of sunflower to P. halstedii is associated with an HR which fails to halt the parasite. By contrast, this HR triggers a SAR which takes places in the upper part of the hypocotyls and eventually leads to the arrest of parasite growth. A model describing the resistance of plants to root-infecting oomycetes is proposed.

First Report of Peronospora parasitica on Wild Rocket (Diplotaxis tenuifolia) in Italy.

Several species of Diplotaxis (D. tenuifolia, D. erucoides, and D. muralis), known as wild rocket, are now widely cultivated in Italy. Wild rocket is used in Mediterranean cuisine as salad, a component of packaged salad products, and as a garnish for food. During the fall of 2002, a foliar disease of D. tenuifolia was observed in the field or greenhouse on several commercial farms in the Liguria Region of northern Italy. Symptoms appeared as small, irregular, dark brown-to-black speckling on the adaxial surfaces of leaves. The speckled areas sometimes expanded into larger spots. These symptoms were followed by leaf yellowing and the appearance of sporangiophores and sporangia on the lower and upper leaf surfaces. Sporangiophores were dichotomously branched with slender curved tips. Sporangia were ovoid, measuring 20 to 28 (average 22) μm long and 15 to 25 (average 19) μm wide. The causal agent of the disease was identified as Peronospora parasitica (3). Pathogenicity was established by inoculating 10 30-day-old plants of D. tenuifolia grown in pots in a peat/pumice/clay/composted bark mix (60:20:10:10), with a conidial suspension (102 conidia per ml). Ten noninoculated plants maintained under the same conditions served as the control. Plants were maintained in a glasshouse at air temperatures ranging between 10 and 26°C (average 16°C) and relative humidity at 85%. The pathogenicity test was done twice. Downy mildew symptoms developed within 12 days, and the same fungus was observed on inoculated plants. Noninoculated plants did not develop symptoms. To our knowledge, this is the first report of P. parasitica on D. tenuifolia in Italy. P. parasitca has been reported as the causal agent of downy mildew on D. muralis in England (1) and on cultivated rocket (Eruca sativa) in California (2).

First Report of Downy Mildew Caused by Plasmopara obducens on Impatiens in California.

During February 2004, diseased double impatiens (Impatiens walleriana) plants were received from a commercial grower in southern California. The upper surfaces of symptomatic leaves were pale yellow with no distinct lesions. Diseased leaves later wilted, and severely affected leaves abscised from the stem. At the nursery, only double impatiens plants in the Fiesta series were infected, and some cultivars were more heavily infected than others. Disease incidence in cv. Sparkler Hot pink was nearly 100%. The interior of infected leaves was colonized by coenocytic mycelium. A conspicuous white growth was observed only on the underside of leaves. Sporangiophores were hyaline, thin walled, emergent from stomata, and had slightly swollen bases. Sporangiophore branching was distinctly monopodial. Smaller sporangiophore branches were arranged at right angles to the supporting branches, and tips of branches measured 8 to 14 μm long. Sporangia were ovoid and hyaline with a single pore on the distal ends. Distal ends of sporangia were predominantly flat but occasionally had a slight papilla. Short pedicels were present on the attached ends. Sporangia measured 19.4 to 22.2 (-25.0) μm × 13.9 to 16.7 (-19.4) μm. Oospores were not observed in leaf tissue. On the basis of symptoms and morphology of the organism, the pathogen was identified as Plasmopara obducens J. Schröt. Pathogenicity tests were done on double type cvs. Fiesta, Tioga Red, and Tioga Cherry Red and on single type cvs. Cajun Watermelon and Accent Lilac. Plants were spray inoculated with sporangiospore suspensions (1 × 104 sporangiospores per milliliter), incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C). Symptoms and signs of downy mildew developed after 12 days only on inoculated cv. Fiesta plants, and the pathogen morphology matched that of the originally observed pathogen. Nontreated control plants did not develop downy mildew. To our knowledge, this is the first report of downy mildew on impatiens in California. P. obducens is one of two causal agents of downy mildew of impatiens (2,4). The other pathogen, Bremiella sphaerosperma, has dichotomous sporangiophore branching and causes lesions with well-defined margins (2,4). In the United States, the disease has been recorded in the eastern and northeastern states and in Indiana, Minnesota, Mississippi, Montana, and Wisconsin (3). In Canada, the disease has been recorded in Manitoba and Quebec (1).

First Report of Peronospora arborescens as the Causal Agent of Downy Mildew on Papaver nudicaule in Italy.

Iceland poppy (Papaver nudicaule L.) is increasingly grown on the Italian Riviera for export as a cut flower. During the spring of 2003, leaves with irregular, brown, angular spots were collected from a commercial crop grown outdoors near Ventimiglia (northern Italy) with temperatures ranging from 3 to 14°C. Leaves of infected plants appeared curled and blistered; the infected portions of the leaves turned chlorotic. On both surfaces of infected leaves, a characteristic gray, furry growth was evident, particularly at the center of the necrotic areas. Infected leaves eventually died without dropping. Basal leaves with poor air circulation were the most severely affected by the disease. Microscopic observations revealed conidiophores branching dichotomically at least five times. Conidiophores ended with sterigmata bearing single conidia. Conidia measured 19 to 24 × 16 to 18 μm and were elliptical to near spherical and hyaline. Oospores were not present. The pathogen was identified as Peronospora arborescens based on the morphological characteristics (2). Pathogenicity was confirmed by inoculating 60-day-old healthy P. nudicaule plants with a conidial suspension (1 × 105 conidia per ml). Five plants were used as replicates. Inoculated and noninoculated plants were maintained in a growth chamber at 15°C and 90 to 95% relative humidity. After 7 to 10 days, typical symptoms of downy mildew developed oninoculated plants. Peronospora arborescens was observed on infected leaves. Noninoculated plants did not show symptoms. To our knowledge, this is the first report of Peronospora arborescens on P. nudicaule in Italy. Peronospora arborescens was previously reported on P. nudicaule and on many other species of Papaver (P. somniferum, P. dubium, P. caucasicum, P. rhoeas, P. setigerum, and P. argemone) in everal countries (1,2).

Ultrastructural and Immunochemical Changes During the In Vitro Development of Plasmopara halstedii

ABSTRACT The asexual phase of the life cycle of Plasmopara halstedii, the causal agent of downy mildew of sunflower, plays a key role in the propagation of the disease. We investigated the morphological and ultrastructural changes that occur during the asexual development of the pathogen. Direct examination of infected cotyledons confirmed the presence of sporangiophores. In contact with water, important ultrastructural changes occurred, affecting the surface of zoosporangia, which became smoother, and their cytoplasm, which differentiated into flagellate zoospores. The subsequent encystment of zoospores was characterized by the synthesis of a cell wall and the loss of the flagella. In addition, two monoclonal antibodies (MAbs) specific for P. halstedii were used to analyze the immunochemical changes associated with these modifications. MAb 16A6, which bound to a 48-kDa glycoprotein, mainly labeled the surface of mobile or encysted zoospores and of mother cells of germ tubes. Conversely, MAb 2F9, which recognized highly glycosylated antigens, labeled the surface of zoosporangia and of flagellate zoospores, but not the encysted zoospores. These results provide new insights into the morphological and ultrastructural changes associated with the release and the encystment of zoospores which may be interesting targets for the development of new antimicrobial products.

Occurrence of Pseudoperonospora cubensis Pathotype 5 on Squash in Italy

During the period of May to August 2002, typical symptoms of downy mildew were observed on squash (Cucurbita pepo L.) in both tunnel and open field cultivation in central Italy (Latium and Umbria). The disease spread rapidly and because the control measures used were not effective, growers suffered severe yield losses. Infected plants showed yellow spots on the upper leaf surface. Based on morphological features observed at ×10 to ×40 magnification, the pathogen was identified as Pseudoperonospora cubensis (Berk. & M.A. Curtis) Rostovzev. To verify the pathogenicity of the fungus, a sporangial suspension (1 × 104 sporangia per ml) was sprayed on leaves of squash plants with two expanded leaves, which were held in dark moist chambers at 20 ± 2°C for 48 h. Control plants were sprayed with sterile water. Inoculated and control plants were kept in a growth chamber at 24 ± 2°C with 14/10 h day/night cycles. Chlorotic spots and sporulation were observed on inoculated plants. The morphological features of the fungus obtained from the inoculated plants were identical to those from the original diseased plant. Previous observations of downy mildew in Italy suggested that squash was not susceptible (1). The physiological specialization among isolates of P. cubensis based on the compatibility of different cucurbit hosts (2) was not tested previously in Italy. To determine the pathotype of four fungal isolates obtained from squash cultivated in different localities, 10 plants per species of the cucurbit species (2) were inoculated with each isolate using the same procedure described for the pathogenicity test. Disease symptoms were detected on all inoculated hosts, including squash, suggesting that all the fungal isolates obtained from squash are pathotype 5. Only pathotype 5 is a common causal agent of downy mildew of squash and other cucurbit hosts. During the period of our observations, climatic conditions were unusually wet because frequent storms occurred during the summer providing favorable environmental conditions for the development of secondary spread. Today, it appears there are no commercially acceptable cultivars of squash resistant to downy mildew available to growers in Italy.

First Report of Bremia lactucae Causing Downy Mildew on Helichrysum bracteatum in Italy.

Helichrysum bracteatum, also known as strawflower, is commonly grown for the production of dried flowers and, more recently, as a potted plant. This latter cultivation system is becoming increasingly important on the Liguria Coast in northern Italy. During the spring of 2002, severe oubreaks of a previously unknown disease were observed in commercial farms in the area of Albenga (northern Italy) on several cultivars of H. bracteatum. Leaves of infected plants appeared curled and blistered; the infected portions of leaves turned chlorotic. On the lower leaf surface of chlorotic areas, a dense, whitish growth was evident. Infected leaves eventually wilted without dropping. Basal leaves with poor air circulation were the most severely affected. Certain cultivars of H. bracteatum (such as 'Florabella Pink') were most seriously affected, while others ('Florabella Gold' and 'Florabella White') had less disease. Microscopic observations revealed sporangiophores emerging from the stomata that were dichotomically branched, ending with 4 to 7 sterigmata. The sporangia were globose and measured 15.5 to 16.8 μm in diameter. The pathogen was identified as Bremia lactucae based on the morphological characteristics. Pathogenicity was confirmed by inoculating healthy H. bracteatum (100-day-old 'Florabella Gold') as well as Lactuca sativa (25-day-old 'Salad bowl') plants with a sporangial suspension (1 × 105 sporangia/ml). Five plants of H. bracteatum and 10 of lettuce were used as replicates. Noninoculated plants served as controls. Inoculated and uninoculated plants were maintained in a growth chamber at 20°C and 90 to 95% relative humidity. After 7 to 10 days, typical symptoms of downy mildew developed on H. bracteatum and lettuce plants artificially inoculated. Bremia lactucae was observed on infected leaves. Uninoculated plants did not show symptoms. To our knowledge, this is the first report of Bremia lactucae on H. bracteatum in Italy. B lactucae was previously reported as the causal agent of downy mildew on H. bracteatum in several countries including the United Kingdom (3), the United States (1), and Egypt (2).

First Report of Blue Mold or Downy Mildew of Pepper from Nurseries in Southeastern Spain.

During October and November 2001, four nurseries reported severe losses in production of pepper seedlings (Capsicum annuum). Plants were affected with the following symptoms: chlorotic spots on upper leaf surfaces along with a dark brown felt and violet reflections on the undersurface of leaves. Spots became necrotic and expanded to include almost the entire blade prompting defoliation that made the plants worthless. These disease symptoms had not been observed in Spain previously. At least four pepper seedbeds were affected and 1.6 million plants (>40% total production) suffered severe defoliation. California type cultivars that produce yellow fruit (Capino, Vélea, and Fiesta) exhibited more severe symptoms compared with cultivars that produce red fruit (Orlando, Haban, Barbadillo, Ribera, and Requena). Lamuyo type cultivars were not severely diseased. Identification of the parasitic fungus from leaves revealed that Peronospora tabacina was the causal agent of downy mildew in pepper, the same pathogen known as the causal agent of tobacco blue mold. Sexual reproductive structures were not found on pepper leaves. Sporangia and sporangiophores corresponded with those described for P. tabacina (synonym P. hyoscyami f. sp. tabacina) (3). The shape of sporangia was spherical in the youngest sporangia and oval to elliptical in mature sporangia (23 × 16 μm). Sporangia were borne on dendritic, dichotomously branched sporangiophores that branched four to eight times and terminated in curved, acute apices. Sporangiophores occurred singly or in small groups. Pathogenicity tests were conducted on California and Lamuyo type pepper cultivars. An inoculum suspension prepared by washing leaves with distilled water was sprayed on seedlings with four true leaves. Inoculated seedlings were maintained at temperatures of 15 to 25°C (night/day). P. tabacina exhibiting the same morphological features as those described above was observed 15 days later on pepper leaves. This disease on pepper was described first in the United States (1,2) and subsequently reported in Greece and Australia (2). The fungus caused disease in nurseries producing pepper seedlings following production of tobacco seedlings or close to other tobacco plants (1). In Murcia, this downy mildew in pepper appeared in pepper nurseries with supplemental heating and did not appear in those without heating. However, the disease spread when diseased pepper seedlings were moved to nonheated nurseries greenhouses. The inoculum may originate from tobacco plants introduced in the greenhouses for the purpose of propagating parasites of whitefly (Bemisia tabaci). Otherwise, tobacco is not cultivated in the Murcia region.

Chitin synthesis during in planta growth and asexual propagation of the cellulosic oomycete and obligate biotrophic grapevine pathogenPlasmopara viticola

PCR amplification of two CHS gene fragments of the obligate biotroph Plasmopara viticola, the causal agent of downy mildew of grapevine, is described. While one fragment shows homology to fungal class IV chitin synthases, the other fragment groups with other oomycete chitin synthases to form a novel class of chitin synthases most closely related to class I-III. RT-PCR experiments indicate that PvCHS1 is constitutively expressed, whereas PvCHS2 is specifically transcribed in sporangiophores and sporangia. Analyses of wheat germ agglutinin labeling patterns by confocal laser scanning microscopy show that chitin is present on the surface of hyphal cell walls during in planta growth, and of sporangiophores and sporangia.

Chitin synthesis during in planta growth and asexual propagation of the cellulosic oomycete and obligate biotrophic grapevine pathogen Plasmopara viticola

PCR amplification of two CHS gene fragments of the obligate biotroph Plasmopara viticola, the causal agent of downy mildew of grapevine, is described. While one fragment shows homology to fungal class IV chitin synthases, the other fragment groups with other oomycete chitin synthases to form a novel class of chitin synthases most closely related to class I–III. RT-PCR experiments indicate that PvCHS1 is constitutively expressed, whereas PvCHS2 is specifically transcribed in sporangiophores and sporangia. Analyses of wheat germ agglutinin labeling patterns by confocal laser scanning microscopy show that chitin is present on the surface of hyphal cell walls during in planta growth, and of sporangiophores and sporangia.

Induction of resistance to downy mildew (Peronospora parasitica) in cauliflower by DL‐β‐amino‐n‐butanoic acid (BABA)

Of the three isomers of aminobutyric acid, only the β isomer (BABA) was effective in inducing resistance against Peronospora parasitica, the causal agent of downy mildew, in cauliflower (Brassica oleracea var. botrytis). A single foliar spray applied to 7‐day‐old seedlings protected the plants against Peronospora parasitica for at least 15 days. Of the enantiomers (R and S), only the R was effective. Resistance was accompanied by a hypersensitive‐like reaction (necrotic spots) which was evident before inoculation. BABA was systemically effective when applied to the roots, but failed to protect cotyledons adjacent to treated ones. Unlike other chemical inducers, BABA was effective when applied several hours postinoculation. It had no effect on P. parasitica spore germination. In cauliflower seedlings, BABA did not induce the accumulation of the pathogenesis‐related protein PR‐1, PR‐2, PR‐3, PR‐5 and PR‐9. Only treated and challenged seedlings accumulated PR‐2.

Heterothallism in Peronospora farinosa f.sp. chenopodii, the causal agent of downy mildew of quinoa (Chenopodium quinoa).

Heterothallism in Peronospora farinosa f.sp. chenopodii, the causal agent of downy mildew of quinoa (Chenopodium quinoa) is reported for the first time. Downy mildew is the most important disease of this crop in the Andean region. Eight single-lesion isolates from different regions in Peru and Bolivia were crossed in all possible combinations using a detached leaf assay, to determine the mating system of the downy mildew pathogen. The presence of two mating types, P1 and P2, was revealed showing that P. farinosa f.sp. chenopodii is heterothallic. It is suggested that frequent sexual reproduction is an important evolutionary force in this pathogen in South America.

RPP13 is a simple locus in Arabidopsis thaliana for alleles that specify downy mildew resistance to different avirulence determinants in Peronospora parasitica.

Disease resistance (R) genes are found in plants as either simple (single allelic series) loci, or more frequently as complex loci of tandemly repeated genes. These different loci are likely to be under similar evolutionary forces from pathogens, but the contrast between them suggests important differences in mechanisms associated with DNA structure and recombination that generate and maintain R gene diversity. The RPP13 locus in Arabidopsis represents an important paradigm for studying the evolution of an R gene at a simple locus. The RPP13 allele from the accession Nd-1, designated RPP13-Nd, confers resistance to five different isolates of the biotrophic oomycete, Peronospora parasitica (causal agent of downy mildew), and encodes an NBS-LRR type R protein with a putative amino-terminal leucine zipper. The RPP13-Rld allele, cloned from the accession Rld-2, encodes a different specificity. Comparison of three RPP13 alleles revealed a high rate of amino acid divergence within the LRR domain, less than 80% identity overall, compared to the remainder of the protein (> 95% identity). We also found evidence for positive selection in the LRR domain for amino acid diversification outside the core conserved beta-strand/beta-turn motif, suggesting that more of the LRR structure is available for interaction with target molecules than has previously been reported for other R gene products. Furthermore, an amino acid sequence (LLRVLDL) identical in an LRR among RPP13 alleles is conserved in other LZ NBS-LRR type R proteins, suggesting functional significance.

Virulence Characterization of Single-Zoospore Isolates of Sclerospora graminicola from Pearl Millet.

Sclerospora graminicola, the causal agent of downy mildew in pearl millet, is well-known for variation in its virulence pattern. Nine single-zoospore isolates (Sg 026-Z-1 to Sg 026-Z-9) derived from an oosporic isolate Sg 026 from a pearl millet F1 hybrid cultivar Nath 4209 grown in a farmer's field in a village, Veelad, in Maharashtra state, India, and three controls (Sg 026, Field-1, and Field-2) were evaluated for their virulence in two experimental runs in a greenhouse. The isolates were maintained on pot-grown seedlings of a highly susceptible pearl millet line, 7042S, in a greenhouse through asexual (sporangial) generations. Pot-grown seedlings of six pearl millet potential differential lines/cultivars (7042S, NHB 3, MBH 110, ICMH 451, 843B, and 852B) were spray-inoculated with a sporangial suspension (5 × 105 sporangia ml-1) and maintained in a greenhouse at 25 ± 2°C. Data were recorded for latent period (days) and disease incidence (%), from which a virulence index (incidence × latent period-1) was calculated to quantify disease-causing potential of isolates. Results indicated significant variation in latent period, incidence, and virulence index among isolates. The isolates were classified into four distinct pathotype groups based on their virulence indices on six pearl millet lines. Because of the significant variation for virulence in the S. graminicola population infecting Nath 4209, it is recommended that the hybrid be regularly monitored for downy mildew infection in farmers' fields, and be replaced by a resistant cultivar that is genetically unrelated to the parental lines of Nath 4209. This will help delay or avoid development of downy mildew epidemics and the resulting heavy loss to pearl millet farmers in the region.

New Races of Sunflower Downy Mildew (Plasmopara Halstedii) in Germany

AbstractPlasmopara halstedii, the causal agent of downy mildew of cultivated sunflower (Helianthus annuus), was documented in Germany for the first time in commercial fields. The pathogen was first observed in the Württemberg area, where races 1 and 4 were identified using a set of differential lines. Later, commercial fields near Baden were found to be infected by race 5, which is the first occurrence of that race outside of North America. With the discovery of race 5, there are now eight races of the sunflower downy mildew fungus that have been found in Europe. The sunflower cultivars most frequently grown in Germany were investigated for resistance to race 1, 4 and 5; while all were resistant to race 1, none were resistant to either race 4 or 5.

Temperature-dependent resistance to downy mildew in muskmelon: Structural responses

Pseudoperonospora cubensis (Berk. et Curt.) Rost. the causal agent of downy mildew in cucurbits, produced large (10-15 mm) sporulating lesions in the susceptible muskmelon ( Cucumis melo L.) cultivar Hemed (He) but small (1-2 mm), chlorotic, water-soaked lesions with negligible sporulation in the resistant inbred line PI 124111F (PI). The resistance of the inbred line was found to be temperature-dependent: it was maximal at 21-25 °C and minimal at 12-15 °C. At 12 °C large lesions with conspicuous sporulation developed in both the susceptible and in the resistant plants. Autofluorescence, callose and lignin-like deposits were observed in infected PI but not in He incubated at 21 °C. These structural responses were not observed in either PI or He incubated at 12 °C. Peroxidase activity and autofluorescence were higher in the resistant inbreds PI and F 10 (near isogenic line) as compared to He at 21 °C but not at 12 °C. We suggest that the resistance genes Pc-1 and Pc-2, which confer resistance to downy mildew in PI 124111F, are specifically inactivated at the lower temperature.

A tandemly repeated sequence from the Plasmopara halstedii genome

A 747-bp tandem repeat element from the genome of the fungus Plasmopara halstedii, the causal agent of downy mildew of sunflower, was cloned and analyzed. The clone can be used as a probe to distinguish races of the pathogen. Sequence analysis of a copy of this element revealed the presence of 103 direct repeats of 6 bp or greater and two potential ORFs. This tandemly repeated element consists of four subunits that may have evolved as a result of several unequal crossing-over events.