<Emphasis Type="Italic">Ulocladium atrum</Emphasis> as a biological control agent for white mold of bean caused by<Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>

Field studies were conducted near Lethbridge, Alberta, Canada, in 2001, 2004 and 2005 to determine the efficacy of the antagonistic fungusUlocladium atrum for control of white mold of bean caused bySclerotinia sclerotiorum. Results of the 3 years of field trials showed that, compared with the untreated control, foliar application of a spore suspension ofU. atrum (300 ml m⁻² of 10⁶ spores ml⁻¹ suspension) significantly reduced incidence and severity of white mold, increased seed yield and reduced contamination of bean seed by sclerotia ofS. sclerotiorum. The level of control of white mold observed in the treatment ofU. atrum was similar to that of the mycoparasitic fungusConiothyrium minitans, but lower than the fungicide treatments of Ronilan (vinclozolin) at the rate of 1200 g ha⁻¹ per application in 2001, or Lance (boscalid) at the rate of 750 g ha⁻¹ per application in 2004 and 2005. The potential for use ofU. atrum as a biological control agent for sclerotinia diseases is discussed. http://www.phytoparasitica.org posting Nov. 12, 2006.     

核盘菌自噬相关基因SsATG5和SsATG8的功能分析

为探究自噬在核盘菌Sclerotinia sclerotiorum致病过程中的作用,利用酵母Saccharomyces自噬相关基因（autophagy-related gene,ATG）编码的蛋白序列比对核盘菌基因组,获得核盘菌假定ATG,并以核盘菌1980菌株为出发菌株,基于同源重组的原理对假定ATG进行敲除和回补,并测定不同突变体的生长表型和致病能力。结果表明,从核盘菌基因组中比对到2个ATG,分别命名为SsATG5和SsATG8,两者在核盘菌致病过程中均上调表达。SsATG5和SsATG8敲除突变体在菌丝生长、产草酸和侵染垫形成方面与野生型菌株无明显差异,但SsATG5敲除突变体在离体拟南芥Arabidopsis thaliana叶片上的致病力显著下降了约40%,在活体拟南芥植株上的致病力显著下降了约80%,同时SsATG5回补突变体恢复了正常的致病力。表明SsATG5参与了核盘菌的致病过程,证实自噬在核盘菌致病过程中发挥着重要作用。     

Effectiveness of Coniothyrium minitans and Trichoderma atroviride in suppression of sclerotinia blossom blight of alfalfa

The effects of the mycoparasites Coniothyrium minitans and Trichoderma atroviride on the suppression of alfalfa blossom blight caused by Sclerotinia sclerotiorum were evaluated under indoor and field conditions. When T. atroviride (9·0 × 10⁴ conidia/floret) + S. sclerotiorum (6·0 × 10³ ascospores/floret) or C. minitans (9·0 × 10⁴ conidia/floret) + S. sclerotiorum (6·0 × 10³ ascospores/floret) were applied to detached young alfalfa florets, T. atroviride effectively inhibited saprophytic growth of S. sclerotiorum, whereas C. minitans showed no inhibition under the same conditions. When T. atroviride (6·9 × 10⁴ conidia/floret) + S. sclerotiorum (6·0 × 10³ ascospores/floret) or C. minitans (6·9 × 10⁴ conidia/floret) + S. sclerotiorum (6·0 × 10³ ascospores/floret) was applied to young alfalfa petals in vivo just after pollination, the percentage of pod formation was higher for T. atroviride+S. sclerotiorum than that for C. minitans+S. sclerotiorum, and the percentage of pod rot was lower for T. atroviride+S. sclerotiorum than that for C. minitans+S. sclerotiorum. However, when they were applied to senescent petals attached to developing pods of alfalfa at 9·2 × 10⁴ conidia/floret together with S. sclerotiorum at 4·5 × 10³ ascospores/floret at 14 days after pollination, C. minitans was more effective than T. atroviride in suppressing sclerotinia pod rot and seed rot of alfalfa. Field experiments showed that three applications of C. minitans (5·4 × 10⁶ conidia mL⁻¹) or T. atroviride (5·4 × 10⁶ conidia mL⁻¹) at a 7-day interval to blossoms of alfalfa effectively suppressed sclerotinia pod rot in two out of three annual trials. Coniothyrium minitans effectively suppressed sclerotinia seed rot in all three years, whereas T. atroviride was not effective against seed rot in any of the trial years. The efficacy of C. minitans was not significantly different (P > 0·05) from benomyl (250 µg ai mL⁻¹). This study suggests that C. minitans has potential as a biocontrol agent to control blossom blight of alfalfa caused by S. sclerotiorum.     

Biological Control of Sclerotinia Diseases of Rapeseed by Aerial Applications of the Mycoparasite Coniothyrium minitans

Indoor and field experiments were conducted to evaluate the efficacy of applying the mycoparasite Coniothyrium minitans to the aerial parts of rapeseed plants at the flowering stage to control sclerotinia diseases caused by Sclerotinia sclerotiorum. Under controlled conditions, a petal inoculation technique was used to determine the effect of conidial suspensions of C. minitans on suppression of sclerotinia leaf blight. Results showed that C. minitans was effective in inhibiting infection initiated by ascospores of S. sclerotiorum on flower petals by restricting mycelial growth of the pathogen. Suppression of lesion development was related to the conidial concentration of C. minitans, with larger lesions at low concentration (5×10³conidia ml⁻¹), but smaller lesions at high concentration (5×10⁴ conidia ml⁻¹ or higher). When C. minitans-treated rapeseed leaves were inoculated with mycelia of S. sclerotiorum, C. minitans failed to prevent infection of leaves, but caused a significant reduction in number of sclerotia produced on the diseased leaves. No significant difference in efficacy was detected between the two isolates of C. minitans, LRC 2137 and Chy-1, on the two rapeseed cultivars, Westar (spring type) and Zhongyou 821 (winter type). Results of field trials showed a significant reduction of stem rot of rapeseed in four (1997, 1999, 2003 and 2004) out of five years by aerial application of C. minitans, compared with controls. No significant difference in suppressive efficacy was observed between the treatments of C. minitans (10⁶ conidia ml⁻¹), C. minitans (10⁶ conidia ml⁻¹) + benomyl (50 μg ml⁻¹) and benomyl (100 μg ml⁻¹) in 2003, and between the treatments of C. minitans (10⁶ conidia ml⁻¹), C. minitans (10⁶ conidia ml⁻¹) + vinclozolin (100 μg ml⁻¹) and vinclozolin (500 μg ml⁻¹) in 2004. Sclerotia of S. sclerotiorum collected from diseased plants in plots treated with C. minitans in 1999, 2000 and 2003, or with C. minitans + benomyl in 2003 were infected by C. minitans at frequencies ranging from 21.3 to 54.5%. This study concludes that aerial spraying of C. minitans is an effective method for controlling sclerotinia diseases of rapeseed.     

Efficiency of isolates ofConiothyrium minitans as mycoparasites ofSclerotinia sclerotiorum,Sclerotium cepivorum andBotrytis cinerea on tomato stem pieces

Summary Twenty five isolates ofConiothyrium minitans were screened for antagonism toSclerotinia sclerotiorum in a Petri dish bioassay using tomato stem segments placed on sterile sand. The antagonistic activity of 23 isolates was quite uniform and only two less antagonistic isolates were identified. Antagonism, expressed as a reduction in the rate of tissue colonization byS. sclerotiorum, occurred, whetherC. minitans was co-inoculated at the same time, one day before or one day afterS. sclerotiorum, but was slightly restricted whenS. sclerotiorum was given a lead of one day. On average, 50–80% of sclerotia of S.sclerotiorum formed on the stem pieces were infected byC. minitans two weeks after inoculation. Excluding the less antagonistic isolates,Coniothyrium minitans was recovered from over 80% ofS. sclerotiorum-infected stem segments when co-inoculated but from a maximum of only 7% of stem pieces when exposed toC. minitans alone. When the experiments were carried out on non-sterile soil instead of sterile sand, infection of stem pieces byS. sclerotiorum was reduced and recovery ofS. sclerotiorum andC. minitans from stem segments was decreased. SevenC. minitans isolates were also screened againstSclerotium cepivorum andBotrytis cinerea and, whereas the effect ofC. minitans onS. cepivorum-infected tissue and sclerotia was essentially similar to that observed withS. sclerotiorum, B. cinerea infected tissue and sclerotia were not invaded by the antagonist.     

Enhancement of sphaeropsis canker of aleppo pine by the Israeli pine bast scale

An interaction between the fungal pathogenSphaeropsis sapinea (Fr.) Dyko & Sutton, causal organism of Sphaeropsis canker, and the Israeli pine bast scaleMatsucoccus josephi (Homoptera: Margarodidae), causal agent of pine decline, was studied, since both were prevalent on Aleppo pine (Pinus halepensis) stands.Sphaeropsis sapinea was isolated from larvae (stage II) and molts of the Israeli pine bast scale collected in four Aleppo pine stands. When Aleppo pine saplings were infested with pine bast scale and subsequently inoculated withS. sapinea most plants died, whereas mortality of seedlings inoculated withS. sapinea alone without previous scale infestation was negligible. Scale infestation of saplings without subsequent fungal inoculation caused the death of one third of the plants. Wounding of pine shoot apices by removal of needle fascicles enhanced infection byS. sapinea. The results support the assumption thatM. josephi can play an important role in transmitting the fungal inoculum to the host tree as well as enabling its penetration to the shoots through wounds in the bark. http://www.phytoparasitica.org posting Dec. 20, 2004.     

Comparative resistance to foliar fungal pathogens in transgenic carrot plants expressing genes encoding for chitinase, β-1,3-glucanase and peroxidise

Genes encoding an acidic wheat class IV chitinase (383), an acidic wheat β 1,3-glucanase (638) and a rice cationic peroxidase (POC1) were introduced into ‘Nantes Coreless’ carrot (Daucus carota) by Agrobacterium-mediated transformation. The genes were introduced singly or in various combinations followed by selection imposed by the herbicide phosphinothricin. Regenerated plantlets were screened for presence and expression of the three transgenes using PCR, Southern and Northern hybridisations. Eighteen transgenic lines expressing a single transgene and 2 lines each co-expressing 638/383 and 383/POC1 were assessed for resistance to the necrotrophic fungal pathogens Botrytis cinerea and Sclerotinia sclerotiorum. Percentage leaf area diseased was measured 4 and 7 days after inoculation (dai) and compared to non-transformed control plants. Six lines expressing β-1,3-glucanase 638 alone had no enhanced resistance to B. cinerea at 4 dai and only slight resistance to S. sclerotiorum; there was no effect at 7 dai. Two out of the six lines expressing 383 alone had enhanced tolerance to both pathogens with a 20–50% reduction in disease development at 7 dai. Two lines co-expressing 638/383 had slight reductions in disease by (10–20%) similar to that of the lines expressing chitinase 383 alone. Highest levels of disease resistance were seen in transgenic lines expressing POC1, alone or in combination with chitinase 383. Disease symptoms were slower to develop and symptoms were reduced by up to 90% for B. cinerea and 70% for S. sclerotiorum. The 383/POC1 co-expressing plants developed disease at levels similar to that of POC1 alone. Petioles of plants over-expressing POC1 had higher levels of lignin accumulation constitutively compared to control plants, which was greatly enhanced following inoculation with S. sclerotiorum. These results indicate that peroxidase over-expression can lead to significant disease reduction against necrotrophic pathogens in transgenic carrot plants.     

Soybean plants expressing an active oligomeric oxalate oxidase from the wheat gf-2.8 (germin) gene are resistant to the oxalate-secreting pathogen Sclerotina sclerotiorum

Sclerotinia stem rot (SSR) caused by Sclerotinia sclerotiorum (Lib.) De Bary is a serious fungal disease of soybean. Senescing petals provide a starting nutrient source for the invasion of healthy tissue by the advancing oxalic acid secreting fungal hyphae. Since oxalic acid is a major pathogenicity factor of SSR, transgenic soybean capable of degrading oxalic acid may be resistant to the pathogen. Transgenic soybean plants were produced byAgrobacterium -mediated transformation with the wheat germin gene (gf-2.8) encoding an oligomeric protein, oxalate oxidase (OxO), which oxidizes oxalic acid to carbon dioxide and hydrogen peroxide (H₂O₂). Transgenic soybean homozygous for 35S- gf-2.8 produced an approx. 130 kDa protein indistinguishable from wheat germin, and with OxO activity. OxO activity was prominent in cell walls proximal to the site of pathogen attack. The transgenics had greatly reduced disease progression and lesion length following cotyledon and stem inoculation with S. sclerotiorum indicating that the germin gene product conferred resistance to SSR. This is the first report of plant resistance to the fungal pathogen S. sclerotiorum in transgenic plants expressing OxO.     

Histological study of giant cells formed by the root-knot nematodeMeloidogyne artiellia as compared withM. hapla andM. javanica in cabbage, turnip and barley

Feeding sites induced by the root-knot nematodeMeloidogyne artiellia in turnip (Brassica rapa), cabbage (Brassica oleracea) and barley (Hordeum vulgare) were examined by light and electron transmission microscopy, and compared with those formed byM. javanica andM. hapla. The feeding cells ofM. artiellia in turnip and cabbage showed hypertrophy, hyperplasia and vacuolization, and became multinucleate giant cells. In contrast to the typical simple-shaped giant cells ofM. javanica andM. hapla, those ofM. artiellia had an amoeboid structure containing ‘protuberances’, which are distinct from previously reported ‘projects’ of giant cells induced byM. incognita in other plants. Protuberances expanding between vascular cells were observed in young and developed giant cells. Since no cell-wall fragments were found at the bases of the protuberances, either by light or electron microscopy, it is strongly indicated that these structures were caused by local expansion of the giant cells rather than by fusion with adjacent cells. The giant cells ofM. artiellia in barley had regular shapes of giant cells without protuberances, and resembled those induced byM. javanica orM. hapla in turnip and cabbage. http://www.phytoparasitica.org posting Sept. 17, 2006.     

Reactive oxygen intermediates and oxalic acid in the pathogenesis of the necrotrophic fungus Sclerotinia sclerotiorum

An effective colonization of the host plant tissue by the necrotrophic fungus Sclerotinia sclerotiorum requires the secretion of the non-host specific toxin oxalic acid (OA), which is known to suppress the generation of reactive oxygen intermediates (ROI). A full-length cDNA coding for an oxalate decarboxylase (TOXDC), which converts OA into CO₂ and formate, was isolated from the basidiomycete Trametes versicolor. It was overexpressed in tobacco plants to study the role of ROI and OA in the interaction between tobacco and S. sclerotiorum. The transgenic plants contained less OA and showed a delayed colonization of S. sclerotiorum; furthermore a strong ROI accumulation and nearly no catalase activity compared to the wild type (WT) plants could be detected. In addition, inoculation experiments with transgenic catalase-deficient plants (CAT1AS) and in vitro studies showed that S. sclerotiorum copes with strong ROI stress. Our results indicate that OA supports the infection process caused by S. sclerotiorum and the fungus itself is able to tolerate high ROI concentrations. The nucleotide sequence data is available from the NCBI Genbank nucleotide-sequence database under the number AY370675     

Fire blight of pears in Israel: infection, prevalence, intensity and efficacy of management actions

The pear production area in Israel is 1500 ha, most of which(ca 1200 ha) is located in the northern part of the country. Fire blight (caused by the bacteriumErwinia amylovora (Burrill) Winslowet al.) was first observed in Israel in that region (in 1985) and the disease has prevailed there since then. In a comprehensive survey conducted in Israel in 1996–1999, data were collected and observations were made yearly in one-third to one-half of the pear production area. The aim was to document the prevalence and intensity of fire blight in commercial orchards and to use the data to evaluate the efficacy of management measures employed for its suppression. Regionwise, a severe fire blight epidemic developed in 1996, moderate epidemics developed in 1998 and 1999, and a mild epidemic developed in 1997. The intensity of fire blight in the preceding season in a specific orchard was more influential on current season severity in a season with a mild epidemic than in a season with a moderate epidemic. Analysis of disease onset records and weather data revealed that only a few (1– 3) infection episodes occurred in individual orchards each year. Comparison of fire blight intensity in orchard-plots treated before green tip with copper hydroxide with nontreated plots revealed that the treatment had no effect on disease intensity during bloom. The efficacy of bactericide sprays applied during bloom was not related to the number of sprays applied but to the timing of spraying. Adequate control was achieved in orchard-plots sprayed soon before or after the occurrence of infection episodes. Contribution no. 508/00 from the Inst. of Plant Protection, ARO, Bet Dagan, Israel.     

A Polymerase Chain Reaction (PCR) Assay for the Detection of Inoculum of Sclerotinia sclerotiorum

The development of a polymerase chain reaction (PCR) assay for the detection of inoculum of the plant pathogenic fungus Sclerotinia sclerotiorum is described. The PCR primers were designed using nuclear ribosomal DNA internal transcribed spacer sequences. Specific detection of DNA from S. sclerotiorum was possible even in the presence of a 40-fold excess of DNA from the closely related fungus Botrytis cinerea. PCR products were obtained from suspensions of untreated S. sclerotiorum ascospores alone, but DNA purification was required for detection in the presence of large numbers of B. cinerea conidiospores. Specific detection of inoculum of S. sclerotiorum was possible in field-based air-samples, using a Burkard spore trap, and from inoculated oilseed rape petals. The assay has potential for incorporation into a risk management system for S. sclerotiorum in oilseed rape crops.     

Potential inhibitors against <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>, produced by the fungus <Emphasis Type="Italic">Myrothecium</Emphasis> sp. associated with the marine sponge <Emphasis Type="Italic">Axinella</Emphasis> sp.

Sclerotinia sclerotiorum is a worldwide ascomycete fungal plant pathogen, which causes enormous yield losses on major economic crops such as crucifers, grain legumes and several other plant families. The objective of this research was to isolate and characterise some bioactive products from cultures of fungi associated with the marine sponge Axinella sp. In total, nine fungal isolates were obtained from the marine sponge Axinella sp. collected from the South China Sea. A group of test strains, including two G⁺ strains (Bacillus subtilis and Staphylococcus aureus), two G⁻ strains (Escherichia coli and Pseudomonas aeruginosa) and three fungi including two plant pathogenic fungi Sclerotinia sclerotiorum and Magnaporthe grisea and Saccharomyces cerevisiae, were employed as the indicator organisms for bioactivity screening. Using antagonistic tests and bioactive screening of the ethyl acetate (EtOAc) extracts of the corresponding cultures, fungal isolate JS9 showed the stronger efficacy against the test indicator strains, especially the indicator fungal pathogens. Isolate JS9 was further identified as Myrothecium sp. by a combination of morphological features and 18S rDNA BLAST on GenBank. Two macrocyclic trichothecenes, roridin A (compound 1) and roridin D (compound 2) were purified by tracking the activity of the EtOAc extract fractions and characterised with spectral analyses including MS, ¹H-NMR, ¹³C-NMR and disortionless enhancement by polarization transfer (DEPT). In vitro antifungal tests showed that the two macrocyclic trichothecenes were bioactive against S. cerevisiae, M. grisea and S. sclerotiorum with minimal inhibitory concentrations of 31.25, 125 and 31.25 μg ml⁻¹ for roridin A, and 62.5, 250 and 31.25 μg ml⁻¹ for roridin D, respectively. The present investigation demonstrated that two antifungal trichothecenes including roridin A and roridin D produced by the fungus Myrothecium sp. isolated from the marine sponge Axinella sp. could be potential inhibitors against the plant pathogen S. sclerotiorum. Lian Wu Xie and Shu Mei Jiang contributed equally to this work.     

The interacting effects of temperature,duration of wetness and inoculum size on the infection of pear blossoms by<Emphasis Type="Italic">Erwinia amylovora</Emphasis>, the causal agent of fire blight

Pear blossoms are the plant tissue that is most vulnerable to infection byErwinia amylovora (Burrill) Winslowet al., the causal agent of fire blight. The interacting effects of temperature, wetness duration and inoculum size on the development of fire blight symptoms in detached pear blossoms were determined in three sets of experiments conducted under controlled conditions. It was expected that this information would facilitate the improvement of a warning system used in fire blight management. Results of the ANOVA tests of the data revealed highly significant interactions among the factors tested. The factors that contributed most to disease incidence were temperature and inoculum size; effects of wetness duration were significant in some cases, but that effect was small. It was further demonstrated that the effects of the interaction of these factors on the incidence of blossom infection may be understood in terms of the general concept of compensation. According to this concept, conditions highly favorable for one of the factors essential for pathogen development may compensate for other factors, for which the conditions are less favorable. As a result of the complex interactions observed between the biotic and abiotic factors, because of compensation relationships and because some of the factors cannot be estimated adequately (for example, inoculum level), it was concluded that it is not yet possible to improve fire blight management by using data on the quantitative relationships between biotic and abiotic factors. Contribution No. 532/02 from the Agricultural Research Organization. http://www.phytoparasitica.org posting Feb. 20, 2003.     

Population structure,races, and host range of <Emphasis Type="Italic">Aphanomyces euteiches</Emphasis> from alfalfa production fields in the central USA

Aphanomyces euteiches (races 1 and 2) causes root rot of alfalfa; however, its population biology and distribution are poorly understood where alfalfa is a major crop. The objectives of this study were to (1) characterise the distribution and frequency of races of A. euteiches in Illinois alfalfa fields, (2) determine host range of A. euteiches on cultivated and native legumes, and (iii) to describe genetic diversity and population genetic structure of A. euteiches in alfalfa fields. To accomplish this, soil samples (n = 103) were collected from 30 alfalfa fields in 18 Illinois counties. Using the susceptible cv. ‘Saranac’, 148 isolates of A. euteiches were baited from the soil. The virulence phenotype of isolates representing all 18 counties was tested, and 54% were R1 and 46% were R2. Both races were detected in 61% of the counties, whereas only R1 was detected in 22% and R2 in 17%. Thirteen legume hosts for isolates from alfalfa fields were identified based on symptoms and/or production of oospores in roots. In addition to six previously known hosts, seven species were susceptible to infection: kura clover, purple prairie clover, white prairie clover, ladino clover, hairy vetch, Canadian milk vetch, and Illinois tick trefoil. AFLP analysis revealed high levels of genetic diversity among the isolates from different fields and counties and a lack of genetic structuring of populations based on race or geographical origin. The results suggest that populations of A. euteiches in alfalfa fields are diverse, often composed of races 1 and 2, and create risk for alfalfa and to multiple cultivated and native legume species.     

Use of <Emphasis Type="Italic">Coniothyrium minitans</Emphasis> as a biocontrol agent and some molecular aspects of sclerotial mycoparasitism

The use of the sclerotial mycoparasite Coniothyrium minitans as a biological control agent of diseases caused by sclerotium-forming pathogens especially Sclerotinia sclerotiorum is briefly reviewed. A number of studies have examined production and application methods, integrated control, ecology, and modes of action in order to understand the biology of the mycoparasite and enhance activity and reproducibility of use. Recently, development of a number of molecular-based techniques has begun to allow the examination of genes involved in mycoparasitism. Some of these procedures have been applied to identify pathogenicity genes involved in the infection of sclerotia of S. sclerotiorum by C. minitans and this work is discussed.     

Tolerance ofsclerotinia sclerotiorum to glyceollin i,a soybean phytoalexin

Glyceollin I was fungistatic rather than fungicidal towardSclerotinia sclerotiorum. Within the mycelial mat apical cells were more vulnerable than mature cells.S. sclerotiorum removed large amounts of glyceollin from solution by a non-energy-requiring process. Contribution from Ministero dell’Università e della Ricerca Scientifica e Tecnologica.     

Sclerotinia rot of blueberry caused by <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>

In 2002, rotted flower clusters and blighted shoot tips and leaves were observed on highbush blueberry (Vaccinium corymbosum L.) and rabbiteye blueberry (V. ashei Reade) plants in Chiba, Japan. The causal fungus isolated from the diseased plants was morphologically identified as Sclerotinia sclerotiorum (Libert) de Bary. The fungus reproduced natural symptoms after inoculation, then reisolated from the symptomatic parts. This is the first report of blueberry sclerotinia rot caused by S. sclerotiorum. The nucleotide sequence data reported are available in the DDBJ/EMBL/GenBank databases under accession numbers AB269903#(020501) and AB233346 (020505).     

Epidemiology of Northern leaf blight on sweet corn

The epidemiology of northern leaf blight of corn, caused byExserohilum turcicum (Pass.) Leonard and Suggs, is reviewed. The minimal dew period required for infection is temperature-dependent. At 25°C, 1 h of dew is sufficient to cause infection and at this temperature the minimal dew period for sporulation is 14 h. Under natural conditions when one dew night is not long enough for conidia to develop, the dew period on the following night enables the completion of conidial formation. The amount of conidia formed is dependent on temperature, light, plant age, leaf position and plant susceptibility. Both qualitative and quantitative types of resistance were identified in several hybrids. Subsequently, there developed additional biotypes ofE. turcicum which are aggressive to plants containing qualitative monogenic resistance. Within the same physiological race, a significant variation in aggressiveness between isolates from various locations is observed. The pathogen overwinters as mycelia and conidia in infected leaves, husks and other plant parts, or onSorghum halepense L. Reduction in yield due to northern leaf blight is associated with the level of resistance of the host plant, with disease severity, plant age during infection, and position of infected leaves.     

Interaction betweenSclerotinia sclerotiorum andConiothyrium minitans strains with different aggressiveness

The effects ofSclerotinia sclerotiorum live and autoclaved sclerotia, and sclerotial exudates, and commercial oxalic acid were testedin vitro on sevenConiothyrium minitans strains differing in aggressiveness towardsS. sclerotiorum. Only sclerotial exudates and autoclaved sclerotia affected the mycelial growth rate of almost all the strains tested, whereas a change in theC. minitans mycelial growth pattern was observed in the presence of autoclaved sclerotia and live sclerotia germinating by the myceliogenic eruptive germination. In addition, sclerotial exudates had a stimulatory effect on spore germination. These findings indicate that the various treatments could influence theC. minitans strains regardless of their aggressiveness.