Cucurbit Powdery Mildew Research Articles

Atmospheric moisture influences on conidia development in Podosphaera xanthii through host-plant morphological responses

This study investigated the effects of atmospheric moisture on conidia development in cucurbit powdery mildew fungus (Podosphaera xanthii) through host-plant responses. Cucumber (Cucumis sativus L.) seedlings were grown under a high vapour pressure deficit (VPD) (3.8 kPa) or a low VPD (0.4 kPa). When the cotyledons had expanded, spores of P. xanthii were inoculated onto the adaxial surface. Inoculated seedlings for both treatments were then placed in a growth chamber maintained at a VPD of 2.1 kPa. The density of visible P. xanthii colonies on the high-VPD-acclimated cotyledons was 0.46 to 0.85 times that of the low-VPD-acclimated cotyledons 7 days after inoculation. It is likely the post-germination behaviour of P. xanthii such as the infection and consequent hyphal development was affected because spore germination did not differ between the treatments. The percentage of adaxial epidermal leaf cells with haustoria was also lower in the seedlings which had acclimated to a higher VPD. The high-VPD-acclimated cotyledons were thicker and had greater dry mass per area. The water potential of cotyledons did not differ between the treatments, although the stomatal conductance of high-VPD-acclimated cotyledons was lower than that of low-VPD-acclimated cotyledons. From these results, we conclude that the inhibition of P. xanthii conidia development on high-VPD-acclimated cotyledons was mainly caused by changes in leaf morphological properties.

Efficacy of Fungicides and Rotational Programs for Management of Powdery Mildew on Cantaloupe.

Powdery mildew of cucurbits, caused by Podosphaera xanthii (syn. Sphaerotheca fuliginia auct. p.p. (Schltdl.) Pollacci), is a common and often severe disease in most areas of the world. Field trials were conducted with cantaloupe to compare disease management success provided by conventional fungicides and biofungicides having different inherent efficacies and modes of action, when applied alone throughout the treatment period or as components of fungicide application programs. Additionally, the portion of total disease control provided by each component fungicide within selected rotational programs was determined. When applied alone throughout the treatment period, disease severity in 2008 and 2009 compared with nontreated plants was reduced by values of 100, 99.3, and 98.1% by wettable sulfur (Microthiol Disperss), triflumizole (Procure), and quinoxyfen (Quintec), respectively; 83.9, 76.4, and 57.4% by trifloxystrobin (Flint), pyraclostrobin (Cabrio), or azoxystrobin (Quadris), respectively; and 39.8, 31.1, 30.0, and 28.6% by thiophanate-methyl (Topsin M), potassium bicarbonate (Kaligreen), kresoxim-methyl (Sovran), and Bacillus subtilis (Serenade), respectively. Rotational application programs composed of Microthiol Disperss, Procure, and Quintec reduced powdery mildew severity on cantaloupe by 97.5 to 100% in both trials. In comparison, disease reduction of 86.0 to 100% was achieved when the first and third fungicide applications were Quintec or Procure and the second and fourth applications were Cabrio, Flint, Kaligreen, Quadris, Serenade, Sovran, or Topsin M. In field trials designed to elucidate the portion of total disease control provided by each component fungicide within a rotational program, application sequences of Procure, Streptomyces lydicus (Actinovate), Procure, and Actinovate or Procure, Kaligreen, Procure, and Kaligreen resulted in reductions in powdery mildew severity of 69.1 and 78.7%, respectively. In comparison, inclusion of only the two Procure applications brought about a mean disease reduction of 85%, whereas inclusion of only the two Actinovate or Kaligreen applications reduced the level of powdery mildew control to 17.6 and 12.9%, respectively. The usefulness of fungicides with low inherent efficacy as resistance management partners when applied with highly efficacious at-risk fungicides may be questioned; however, the importance of any fungicide as a resistance management partner should be measured by its ability to dilute the selection pressure of the at-risk fungicide and to inhibit the growth of any resistant biotypes that may arise, factors which may not be reflected by its inherent efficacy in controlling disease. Because less efficacious fungicides did not increase the overall level of disease control, encouraging growers and pest control advisors to incorporate them into powdery mildew treatment programs may be a challenge, because resistance management is not their top priority. An ongoing educational effort emphasizing the benefits of resistance management programs with respect to prolonging the effectiveness of single-site mode of action fungicides is essential.

The antagonistic strain Bacillus subtilis UMAF6639 also confers protection to melon plants against cucurbit powdery mildew by activation of jasmonate- and salicylic acid-dependent defence responses.

Biological control of plant diseases has gained acceptance in recent years. Bacillus subtilis UMAF6639 is an antagonistic strain specifically selected for the efficient control of the cucurbit powdery mildew fungus Podosphaera fusca, which is a major threat to cucurbits worldwide. The antagonistic activity relies on the production of the antifungal compounds iturin and fengycin. In a previous study, we found that UMAF6639 was able to induce systemic resistance (ISR) in melon and provide additional protection against powdery mildew. In the present work, we further investigated in detail this second mechanism of biocontrol by UMAF6639. First, we examined the signalling pathways elicited by UMAF6639 in melon plants, as well as the defence mechanisms activated in response to P. fusca. Second, we analysed the role of the lipopeptides produced by UMAF6639 as potential determinants for ISR activation. Our results demonstrated that UMAF6639 confers protection against cucurbit powdery mildew by activation of jasmonate- and salicylic acid-dependent defence responses, which include the production of reactive oxygen species and cell wall reinforcement. We also showed that surfactin lipopeptide is a major determinant for stimulation of the immune response. These results reinforce the biotechnological potential of UMAF6639 as a biological control agent.

ウリ科うどんこ病菌及びすす病菌を与えたスワルスキーカブリダニの発育と産卵

The ability of the predatory mite Amblyseius swirskii Athias-Henriot to feed on some species of crop fungi was investigated to determine the effect of plant fungi on mite development and reproduction. Larval and nymphal survival, developmental duration, total oviposition, and female longevity of the mite were compared among different dietary regimes: cucurbit powdery mildew on pumpkin leaf, Pinus thunbergii pollen, Ephestia kuehniella eggs, pumpkin leaf, mycelia of sooty mold, mycelia and conidia of sooty mold, Potato sucrose agar medium, and distilled water. Mites completed development and laid eggs only on cucurbit powdery mildew, P. thunbergii pollen, and E. kuehniella eggs. Mites fed mildew took 1.5 days longer to complete their development than those on the other two diets, and adults oviposited significantly fewer eggs than those fed E. kuehniella eggs, but similar numbers as those fed P. thunbergii pollen. These results demonstrated the possibility that the plant fungus cucurbit powdery mildew facilitates mite colonization and reproduction on crops.

Outbreak of Cucurbit Powdery Mildew on Watermelon Fruit Caused by Podosphaera xanthii in Southwest Florida.

Cucurbit powdery mildew caused by the obligate parasite Podosphaera xanthii occurs commonly on foliage, petioles, and stems of most cucurbit crops grown in the United States. (3). However, in the field, fruit infection on cucurbits including watermelon (Citrullus lanatus), is rarely, if ever, observed (2). Consequently, it was atypical when severe powdery mildew-like symptoms were observed on seedless and seeded watermelon fruit on several commercial farms in southwestern Florida during November and December 2010. Severe powdery mildew was also observed on 'Tri-X 313' and 'Mickey Lee' fruit grown at SWFREC, Immokalee, FL. Infected fruit developed poorly and were not marketable. Powdery mildew symptoms were mainly observed on young immature fruit, but not on mature older fruit. Abundant powdery mildew conidia occurred on fruit surface, but not on the leaves. Conidia were produced in chains and averaged 35 × 21 μm. Observation of conidia in 3% KOH indicated the presence of fibrosin bodies commonly found in the cucurbit powdery mildew genus Podosphaera (3). Orange-to-dark brown chasmothecia (formerly cleisthothecia) containing a single ascus were detected on the surface of some fruit samples. Conidial DNA was subjected to PCR using specific primers designed to amplify the internal transcribed spacer (ITS) region of Podosphaera (4). The resulting amplicons were sequenced and found to be 100% identical to the ITS sequences of P. xanthii in the NCBI database (D84387, EU367960, AY450961, AB040322, AB040315). Sequences from the watermelon fruit isolate were also identical to several P. fusca (synonym P. xanthii), P. phaseoli (GQ927253), and P. balsaminae (AB462803) sequences. On the basis of morphological characteristics and ITS sequence analysis, the pathogen infecting watermelon fruit can be considered as P. xanthii (1,3,4). The powdery mildew isolate from watermelon fruit was maintained on cotyledons of squash (Cucurbita pepo, 'Early Prolific Straight Neck'). Cotyledons and leaves of five plants each of various cucurbits and beans were inoculated with 10 μl of a conidial suspension (105conidia/ml) in water (0.02% Tween 20). Two weeks after inoculation, abundant conidia were observed on cucumber (Cucumis sativus, 'SMR-58') and melon (Cucumis melo) powdery mildew race differentials 'Iran H' and 'Vedrantais'. However, no growth was observed on melon differentials 'PI 414723', 'Edisto 47', 'PMR 5', 'PMR 45', 'MR 1', and 'WMR 29' (2,3). The powdery mildew isolate from watermelon fruit behaved as melon race 1 (3). Mycelium and conidia were also observed on fruit surface of watermelon 'Sugar Baby' and a susceptible U.S. plant introduction (PI 538888) 3 weeks after inoculation. However, the disease was not as severe as what was observed in the fields in fall 2010. The pathogen did not grow on plants of Impatiens balsamina or on select bean (Phaseolus vulgaris) cultivars ('Red Kidney', 'Kentucky Blue', and 'Derby Bush'), but did grow and produce abundant conidia on 'Pinto bush bean'. Powdery mildew on watermelon fruit in production fields can be considered as a potentially new and serious threat requiring further studies to develop management strategies.

The Iturin-like Lipopeptides Are Essential Components in the Biological Control Arsenal of Bacillus subtilis Against Bacterial Diseases of Cucurbits

The antibacterial potential of four strains of Bacillus subtilis, UMAF6614, UMAF6619, UMAF6639, and UMAF8561, previously selected on the basis of their antifungal activity and efficacy against cucurbit powdery mildew, was examined. Among these strains, UMAF6614 and UMAF6639 showed the highest antibacterial activity in vitro, especially against Xanthomonas campestris pv. cucurbitae and Pectobacterium carotovorum subsp. carotovorum. These strains produced the three families of lipopeptide antibiotics known in Bacillus spp.: surfactins, iturins, and fengycins. Using thin-layer chromatography analysis and direct bioautography, the antibacterial activity could be associated with iturin lipopeptides. This result was confirmed by mutagenesis analysis using lipopeptide-defective mutants. The antibacterial activity was practically abolished in iturin-deficient mutants, whereas the fengycin mutants retained certain inhibitory capabilities. Analyses by fluorescence and transmission electron microscopy revealed the cytotoxic effect of these compounds at the bacterial plasma membrane level. Finally, biological control assays on detached melon leaves demonstrated the ability of UMAF6614 and UMAF6639 to suppress bacterial leaf spot and soft rot; accordingly, the biocontrol activity was practically abolished in mutants deficient in iturin biosynthesis. Taken together, our results highlight the potential of these B. subtilis strains as biocontrol agents against fungal and bacterial diseases of cucurbits and the versatility of iturins as antifungal and antibacterial compounds.

Sources of Resistance to Race 2WF Powdery Mildew in U.S. Watermelon Plant Introductions

Powdery mildew of cucurbits, incited by Podosphaera xanthii (Castagne) Braun & Shishkoff (syn. Sphaerotheca fuliginea auct. p.p.), is an economically important foliar disease. which is now common in watermelon [Citrullus lanatus (Thunb.) Mastum. & Nakai]. This disease occurs in all watermelon-growing areas and can reduce yields by up to 30%. Finding and breeding for resistance to this disease is important to reduce dependence on fungicides and to use in combination with fungicides to limit the spread of fungicide-resistant P. xanthii. This is the first English report that race 2WF of P. xanthii can infect watermelon. It is the prevalent race of watermelon powdery mildew in Beijing.

Impact of Fungicide Program and Powdery Mildew Resistance in Three Varieties of Pumpkin

The effects of fungicide program and varietal resistance on the development of cucurbit powdery mildew [PM (Podosphaera xanthii)] in pumpkin (Cucurbita pepo) were tested in 2009 and 2010 in Lexington, KY. Three fungicide programs consisted of a no-fungicide control, low input (chlorothalonil), and high input (chlorothalonil alternated with myclobutanil) were applied to three varieties of pumpkin with different levels of cucurbit PM resistance. Varieties tested were Howden, Aladdin, and Camaro, which had no, moderate, and high levels of cucurbit PM resistance, respectively. There were no significant variety by fungicide program interactions for yield, number fruit per acre, and average fruit weight in both years. In 2009 and 2010, variety significantly affected yield. Yield increases corresponded to the level of cucurbit PM resistance. The high-resistance variety, Camaro, had the greatest yields in both years, while Howden, with no PM resistance, had the lowest yields. In 2010, yield significantly increased with the intensity of fungicide program. With the exception of cull percentage in 2009, there were no fungicide program by variety interactions for yield parameters. In both years, PM disease severity, characterized by the area under the disease progress curve (AUDPC), was lowest on ‘Camaro’ and highest on ‘Howden’. In 2009, unsprayed ‘Camaro’ had similar AUDPC values as ‘Aladdin’ and ‘Howden’ treated with low- or high-input fungicide programs. In 2010, unsprayed ‘Camaro’ had less disease than ‘Aladdin’ or ‘Howden’ that had received the high-intensity fungicide program. These results suggest that yields can be maintained with a minimal fungicide program when using a variety with high cucurbit PM resistant in some environments. A high-input fungicide program may be necessary to maintain yields of varieties with low cucurbit PM resistance.

Distribution, Host Range and Disease Severity of Pseudoperonospora cubensis on Cucurbits in the Czech Republic

Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is a major cucumber disease in the Czech Republic. Disease prevalence, host range and disease severity were evaluated from 2001 to 2009. The geographical distribution of P. cubensis was assessed on ca 80–100 locations per year in two main regions of the Czech Republic (central and southern Moravia, and eastern, northern and central Bohemia). Infection by P. cubensis was observed primarily on cucumber (Cucumis sativus) but only on the leaves. During the study, disease prevalence ranged from 66 to 100%. The majority of C. sativus crops were heavily infected at the end of the growing season (second half of August). Generally, P. cubensis was present at high or very high disease severity. The loss of foliage results in the reduction in the quality and quantity of marketable yield of fruit. Pseudoperonospora cubensis was widespread across the whole area of the Czech Republic studied. Very rarely, infection was recorded in muskmelon (Cucumis melo) and Cucurbita moschata. Of other pathogens, the most frequently recorded was the cucurbit powdery mildew (Golovinomyces cichoracearum and Podosphaera xanthii).

Inheritance of Resistance in Melon PI 313970 to Cucurbit Powdery Mildew Incited by Podosphaera xanthii Race S

A new strain of cucurbit powdery mildew incited by Podosphaera xanthii (Castagne) Braun & Shishkoff (formerly Sphaerotheca fuliginea), designated race S, is virulent on all the commonly used melon (Cucumis melo L.) powdery mildew race differentials and first appeared on melon in Yuma, AZ, and Imperial Valley, CA, in 2003. Melon PI 313970 (C. melo var. acidulus) was resistant to P. xanthii race S in 2003 and subsequent years. Inheritance of resistance to P. xanthii race S was studied in two naturally infected, replicated field tests in Imperial Valley in 2005 and 2009 using the parents and F1, F2, and reciprocal backcross generations from crosses of PI 313970 with powdery mildew-susceptible ‘Top Mark’. Resistance to race S was recessive: all F1 and BCTM individuals were susceptible. One recessive gene, designated pm-S, conditioned resistance to race S in the F2 and BCPI. The relationship of pm-S to the previously reported recessive and codominant genes in PI 313970 for resistance to P. xanthii races 1, 2, 2U.S., 3, 3.5, 4.5, and 5 remains to be determined.

Gaps and perspectives of pathotype and race determination in <i>Golovinomyces cichoracearum</i> and <i>Podosphaera xanthii</i>

Golovinomyces cichoracearum and Podosphaera xanthii (family Erysiphaceae) are the most important species causing cucurbit powdery mildew (CPM), a serious disease of field and greenhouse cucurbits. Both species are highly variable in their pathogenicity and virulence, as indicated by the existence of large number of different pathotypes and races. Various independent systems of CPM pathotype and race determinations and denominations are used worldwide. CPM pathotype identification is based on intergeneric and interspecific differences in host-CPM interactions. The most commonly used set of CPM pathotype differentials includes one genotype from four species representing three agriculturally important cucurbit genera plus two genotypes from a fifth species, melon Cucumis melo L. CPM races are characterized by specialization on different cultivars or lines of one host species and have, to date, been differentiated only on melon (C. melo L.). The most frequently used set of melon differentials includes 11 genotypes that can differentiate CPM races originating from melon and other cucurbits, e.g., cucumber, Cucurbita spp., and watermelon. In this paper, we critically review the current state, gaps, and perspectives in our understanding of pathogenicity variation in these two CPM pathogens at the pathotype and race levels.

A Sequence-amplified Characterized Region Marker for a Single, Dominant Gene in Melon PI 134198 that Confers Resistance to a Unique Race of Podosphaera xanthii in China

Melon production in China is threatened by cucurbit powdery mildew incited by Podosphaera xanthii, which can cause severe yield losses. Disease reactions of 10 melon P. xanthii race differentials indicated the presence of a unique race of P. xanthii, denoted pxCh1, in the Shanghai area. Resistance to P. xanthii race pxCh1 in melon PI 134198, which originated from India, was controlled by a single, dominant gene, designated Pm-8, in a cross with susceptible melon inbred S717. A search for a molecular marker linked to Pm-8 was initiated using the same cross. Bulk segregant analysis of their F2 progeny revealed one sequence-related amplified polymorphism (SRAP) marker of 148 bp linked to Pm-8, at 3.9 cM. The SRAP segment was recovered, cloned, and sequenced and converted into a sequence-amplified characterized region marker for marker-assisted selection of Pm-8 in breeding programs to develop melon cultivars resistant to P. xanthii race pxCh1.

VARIATION FOR FUNGICIDE RESISTANCE AMONG CUCURBIT POWDERY MILDEW POPULATIONS IN THE CZECH REPUBLIC

A total of 88 cucurbit powdery mildew (CPM) isolates, 52 of Golovinomyces cichoracearum (Gc) and 36 of Podosphaera xanthii (Px), collected in the period 2005-2007 in the Czech Republic (CR), were screened for tolerance and/or resistance to the three freauentlv used fungicides (fenarimol, dinocap, thiophanate-methyl). Benomvl served as ineffective control. Bv employing benomyl, which used to be the most frequently used fungicide for CPM protection in the CR, we could monitor and compare the development and changes in variation of tolerance/resistance among Czech CPM populations. A total of 36 (21 Gc, 14 Px) CPM isolates from 2007 were also tested for azoxystrobin tolerance and/or resistance. Fungicide sensitivity was determined by a modified leaf-disc bioassay with five concentrations. Significant differences among fungicides and even between the years 2005 and 2006-2007 were observed. Occurrence of resistant and/or tolerant isolates of both powdery mildew species in different locations was observed. Fenarimol (Rubigan 12 EC) and dinocap (Karathane LC) expressed decreasing efficacy in the year 2005, however in 2006-2007 these two fungicides showed a high level of effectiveness and all isolates of both CPMs were controlled by the recommended concentration (36 μg a.i./ml for fenarimol, 105 μg a.i./ml for dinocap). Benomyl (Fundazol 50 WP) and thiophanate-methyl (Topsin M 70 WP) were found to be ineffective, because the majority of screened isolates (96%) belonged to the highly resistant strains, with a resistant reaction at the recommended concentration (250 μg a.i./ml for benomyl, 525 μg a.i./ml for thiophanate-methyl) and tolerance or resistance at a higher concentration than recommended (500 μg a.i./mi for benomyl and 1050 μg a.i./ml for thiophanate-methyl). Some 89% of CPM isolates (except the year 2005 only 22% Px isolates tested on thiophanate-methyl) showed profuse or limited sporulation at a concentration 2× higher than recommended (1000 μg a.i./ml for benomyl and 2100 μg a.i./ml for thiophanate-methyl). Azoxystrobin (Ortiva) expressed decreasing efficacy, with 37% of isolates (Gc and Px) tolerant or resistant to the recommended concentration (500 μg a.i./ml) and also to higher concentrations (1000 μg a.i./ml and 2000 μg a.i./ml).

OCCURRENCE OF AMPELOMYCES QUISQUALIS ON THE CUCURBIT POWDERY MILDEW SPECIES IN THE CZECH REPUBLIC

From 2002 to 2006, in the Czech Republic (CR), 585 leaf samples with powdery mildew symptoms collected from cucurbits (mainly from Cucurbita pepo, Cucurbita maxima, Cucumis sativus) were microscopically examinated for presence of hyperparasitic fungus Ampelomyces quisqualis Ces. (Aq). Identification of cucurbit powdery mildew species (Golovinomyces cichoracearum /Gc/, Podosphaera xanthii /Px/) was carried out by microscopic examination of the morphological characters of dry conidia in a 3% KOH solution. In total of 443 samples was determined as Gc, 106 samples as mixed infection (Gc, Px) and 36 samples as Px. Aq was detected on 12% of collected samples from the CR. However, frequency of its occurrence varied among the years. In 2002 and 2006, Aq was more frequent (18-20%) as compared with the years 2003-2005 (only 6-8%). Pycnidia of Aq were recorded mainly on Gc hyphae and basal cells of conidiophores (12%) or on mixed infection (Gc, Px) (14%). Occurrence of Aq on Px was only sporadic (only 3 CPM samples/2 locations - 2/1 in 2005 and 1/1 in 2006). Its distribution was recorded in 52 locations of the CR, it was not restricted to limited areas, nevertheless the most of Aq locations originated from the South Moravia. In some locations its presence was recorded repeatedly, in some others it was detected only once. The majority of samples with presence of Aq were collected in August and September.

Sensitivities to DMI fungicides in populations of Podosphaera fusca in south central Spain

Cucurbit powdery mildew elicited by Podosphaera fusca (Fr.) U Braun & N Shishkoff limits crop production in Spain. Disease control is largely dependent on fungicides such as sterol demethylation inhibitors (DMIs). Fungicide resistance is an increasing problem in this pathogen. To overcome such risk, it is necessary to design rational control programmes based upon knowledge of field resistance. The aim of this study was to investigate the state of DMI sensitivity of Spanish P. fusca populations and provide tools for improved disease management. Using a leaf-disc assay, sensitivity to fenarimol, myclobutanil and triadimenol of 50 isolates of P. fusca was analysed to determine discriminatory concentrations between sensitive and resistant isolates. As no clearly different groups of isolates could be identified, discriminatory concentrations were established on the basis of maximum fungicide field application rate, 100 mg L(-1) for the three fungicides tested. Subsequently, a survey of DMI resistance was carried out in different provinces located in the south central area of Spain during the cucurbit growing seasons in 2002, 2003 and 2004. Examination of a collection of 250 isolates revealed that 23% were resistant to fenarimol and 7% to triadimenol, the provinces of Almería, Badajoz and Murcia being the locations with the highest frequencies of resistance. By contrast, no resistance to myclobutanil was found. Results show that fenarimol and, to a lesser extent, triadimenol have become less efficient for controlling cucurbit powdery mildew in Spain. These are important observations that should lead to reconsideration of the current disease management programmes.

Determining “Practical” Fungicide Resistance Development and Drift in the Control of Cucurbit Powdery Mildew in Pumpkin

In 2006 and 2007, nine fungicides were evaluated to determine if “practical” fungicide resistance could be identified and if fungicide resistance drift occurred in cucurbit powdery mildew of pumpkin. The fungicides and/or tank mixes whose active ingredient(s) were evaluated included: sulfur (FRAC code M1), chlorothalonil (M5), myclobutanil (3), pyraclostrobin (11), azoxystrobin (11), quinoxyfen (13), chlorothalonil + myclobutanil (M5 + 3), famoxadone + cymoxanil (11 + 27), pyraclostrobin + boscalid (11 + 7), and water only (control). Based on visual ratings of upper and lower leaf surfaces, a FRAC code 11 resistance cucurbit powdery mildew population was present in both years. Practical resistance and cross resistance were identified where a FRAC code 11 fungicide had not been applied season-long as well as where a FRAC code 11 fungicide was applied weekly or in rotation with another fungicide chemistry. Resistance to a FRAC code 3 fungicide was not identified where a FRAC code 3 fungicide had been applied season-long, or in rotation, or where no FRAC code 3 fungicide was applied. This study demonstrates that cucurbit powdery mildew populations resistant and/or cross resistant to FRAC code 11 fungicides can develop and have the potential to disseminate into and be detected in areas where no FRAC code 11 fungicides have been applied. Accepted for publication 11 October 2010. Published 22 November 2010.

Long-lasting changes in the species spectrum of cucurbit powdery mildew in the Czech Republic - influence of air temperature changes or random effect?

Two ectoparasite powdery mildew species <i>Golovinomyces cichoracearum</i> (<i>Gc</i>) and <i>Podosphaera xanthii</i> (<i>Px</i>) occurring on cucurbits differ, besides other characteristics, by specific ecologic requirements. While <i>Px</i> is common in subtropical and tropical areas and greenhouse crops, <i>Gc</i> occurs more frequently in temperate and cooler areas under field conditions. Their presence on cucurbit field crops (<i>Cucurbita pepo</i>, <i>C. maxima</i>, <i>Cucumis sativus</i>) was monitored in the Czechoslovakia (1979–1980) and in the Czech Republic (1995–2007). Their identification was carried out by microscopic observation of the morphological characteristics of the dry conidia on 1527 leaf samples. Data on air temperature in 1979–2007 were provided by the Czech Hydrometeorological Institute. In 1979–1980 <i>Gc</i> was identified in 86.0% of samples, <i>Px</i> in 14.0% samples, there was no mixed infection; prevalence of <i>Px</i> was recorded in South Slovakia and South Moravia, and on crops under cover. Since 1995 species <i>Px</i> was recorded each year on field crops in different locations of Bohemia and Moravia, usually in mixed infection with <i>Gc</i>. The average year temperature of 8.1°C for period 1992–2007 was higher than corresponding value of 7.4°C in 1979–1983. Similarly, average temperature in vegetation season of 16.2°C in 1992–2007 was higher than corresponding value of 15.7°C in 1979–1983. The higher air temperature can positively influence spreading of <i>Px</i> in the Czech Republic. Climate variability and effect of their changes are discussed in relationship to the geographic distribution and geographic patterns of cucurbit powdery mildews.

First Occurrence of Cucurbit Powdery Mildew Caused by Race 3-5 of Podosphaera fusca in Spain.

A new race of cucurbit powdery mildew was observed for the first time on melon (Cucumis melo) in three research greenhouses in the Axarquia area of southern Spain during the spring of 2008. Fungal growth appeared as white powdery colonies initially restricted to the upper leaf surfaces. Morphological characteristics of colonies, conidiophores, conidia, germ tubes, and appressoria indicated that the powdery mildew fungus was Podosphaera fusca (also known as P. xanthii) (3), a fungal pathogen extensively reported in the area (1). However, the fungus developed on plants of melon cv. PMR 6, which is resistant to races 1 and 2 of P. fusca, suggesting that the fungus could belong to race 3, a race of P. fusca not yet reported in Spain. Race determination was carried out by inoculating the third true leaf of a set of differential melon genotypes that were maintained in a greenhouse. Symptoms and colonization observed on cvs. Rochet, PMR 45, PMR 6, and Edisto 47 indicated that the isolates belonged to race 3-5 of P. fusca. Fungal strains of races 1, 2, and 5 of P. fusca (all present in Spain) were used as controls. Pathotype designation was determined by inoculating different cucurbit genera and species (2). In addition to melon, the isolates were pathogenic on zucchini (Cucurbita pepo) cv. Diamant F1, but failed to infect cucumber (C. sativus) cv. Marketer and watermelon (Citrullus lanatus) cv. Sugar Baby; therefore, the isolates were pathotype BC (2). Races 1, 2, 4, and 5 of P. fusca have been previously reported in the area (1). The occurrence of race 3-5 represents another challenge in the management of cucurbit powdery mildew in Spain.

Species spectra, distribution and host range of cucurbit powdery mildews in the Czech Republic, and in some other European and Middle Eastern countries

The occurrence and geographic distribution of powdery mildew on cucurbits was studied in the Czech Republic, Austria, France, Germany, Great Britain, Italy, Slovakia, Slovenia, Spain, Switzerland, the Netherlands, as well as in Turkey and Israel, during the period 1995–2000. In total, 599 leaf samples with powdery mildew symptoms were collected from cucurbits from 166 locations within the Czech Republic; an additional 69 samples were collected from 42 locations elsewhere. Two powdery mildews (Golovinomyces cichoracearum and Podosphaera xanthii) were identified. The host range included the cultivated cucurbits (Cucumis melo, C. sativus, Cucurbita pepo, C. maxima, and Citrullus lanatus) and several other species. P. xanthii was the only powdery mildew pathogen found in Spain, Israel, and Turkey. P. xanthii and G. cichoracearum were detected in the other surveyed countries, occurring in both mixed infections and separately. In the more northerly latitudes and higher elevations, G. cichoracearum is more often the single species. G. cichoracearum was the dominant powdery mildew species in the Czech Republic (detected in 98.8% of the locations there); P. xanthii was found as the lone species in 1.2% of locations. At 28.4% of locations, G. cichoracearum was found with P. xanthii as mixed infections. The hyperparasitic fungus Ampelomyces quisqualis was found in 30% of the samples from the Czech Republic and was also recorded in Austria, Italy, Slovenia, Switzerland, and Great Britain.

Powdery Mildew Control and Yield Response of Inodorus Melon

The research was carried out on melon (Cucumis melo L. var. inodorus Naud.) in 2006 and 2007 at “Pantanello” Experimental Farm (40° 24’N; 16° 48’E; 10 m a.s.l.; Metaponto, southern Italy) to evaluate the efficacy of a low environmental impact control strategy against powdery mildew of cucurbits. Winter melon was treated with a new anti-oidium formulation, called Stifenia, obtained from fenugreek seeds and stimulating the plant self-defence. The adopted experimental design included two control strategies (1. biological, using Stifenia and 2. conventional, using penconazole, myclobutanil and sulphur) and an untreated control (treated with water alone) applied to two cultivars of inodorus melon (cv ‘Amarillo’ and HF1 ‘Cocorito’, the latter a genotype resistant to powdery mildew). Stifenia applications were not effective against the disease; in fact, there were no differences in percentage of attacked plant surface between treated plots and untreated ones. The melon marketable yield was significantly higher with the conventional strategy respect to Stifenia and control. Repeated applications of Stifenia resulted in a significant decrease of marketable yield even in comparison with the untreated control. The cultivars significantly affected powdery mildew development, since the resistant one (‘Cocorito’) was attacked later and damaged always lower than the non-resistant genotype (‘Amarillo’). Laboratory analyses carried out on infected leaves always confirmed that Golovinomyces cichoracearum D.C. was responsible of the disease.