|Aggressiveness of Powdery Mildew Isolates on Cucurbita maxima
Research Institute of Crop Production Praha, Division of Genetic and Plant Breeding, Department of Gene Bank, Šlechtitelů 11, 783 71 Olomouc, Czech Republic
Palacký University, Faculty of Science, Department of Botany, Šlechtitelů 11, 783 71 Olomouc, Czech Republic
Introduction. Pathogenicity of Czech isolates of cucurbit powdery mildew (Erysiphe cichoracearum (Ec) and Sphaerotheca fuliginea (Sf)) on cucurbitaceous vegetables have been studied both on the level of their virulence and aggressivenes. Several Ec and Sf pathotypes previously described by Bertrand (2) were identified and new reaction patterns were recognized (4). Moreover some isolates were virulent also to watermelon (5) and one Ec isolate was virulent to C. melo MR-1 (6). The purpose of this study was to evaluate the aggressiveness of Ec and Sf isolates on Cucurbita maxima and to consider the influence of leaf side of this host genotype on the development of powdery mildew infection under controlled conditions.
Materials and Methods. A total of 15 E. cichoracearum and 8 S. fuliginea isolates collected from field cultures of cucurbits (Cucurbita pepo, C. maxima, Cucumis sativus) at seven distinct regions of the Czech Republic in 1997 were used for this study. Isolates were obtained from single spore lesions on leaves. Isolates were maintained in-vitro on the cotyledons of Cucumis sativus cv. Marketer according to methods described by Bertrand (2). For the determination of pathotypes the methods and differential plant genotypes proposed and kindly provided by Bertrand (2) were used. Czech C. maxima cv. Goliáš (accession number 09-H39-0137) originated from Gene Bank RICP in Olomouc (Czech Republic)
For the aggressiveness tests host material was prepared and inoculated separately from assays for pathopype determination. Leaf discs of 1.5 mm in diameter were cut out by a cork borer from well developed leaves of C. maxima cv. Goliáš plants 6-9 weeks old and placed either abaxial and adaxial sides on agar medium in Petri dishes. Aggressiveness of each isolate was evaluated on five discs in two replications for each side position. The inoculation was performed by spraying inoculum on discs. The inoculum was prepared by washing cucumber cotyledons with mycelium and conidia in water with addition of Tween. The number of conidia was approx. 55x103 in 1 ml of suspension.
Intensity of sporulation on each disc was assessed visually 7, 10, 14 and 17 days after inoculation on a scale of 0 (no sporulation) to 4 (more than 75% of disc surface covered by mycelium) (7). The value of infection degree (ID) was counted for each isolate separately on the upper- (ID-u) and lower- (ID-l) leaf sides and a value of total infection degree (%TID) was counted from all subsequent evaluations as a percentage of maximum potential score. Data were treated statistically by one-way analyses of variance and Sheffe multiple range analyses in the programme Statgraphics (3).
Results and Discussion. Within isolates under study a total of three Ec pathotypes and two Sf pathotypes have been identified (Table 1). The average values of infection degree for C. maxima response to Ec were on the upper leaf side %TID-u = 34.3 (ab) and on the lower leaf side %TID-l = 22.3 (a), average values characterizing response of host genotype to Sf were on the upper leaf side %TID-u = 57.1 (b) and on the lower leaf side %TID-l = 37.2 (ab). Multiple range analyses (99%, Sheffe) proved differences between obtained values. Generally, the species Ec was less aggressive to C. maxima than Sf on both sides of leaf discs and the infection of both powdery mildew species (Ec,Sf) was more severe on the upper leaf disc side as compared to the response of the lower disc side. The curves characterizing infection development on each leaf side were similar for both powdery mildew species (Figure 1). During first seven days after inoculation the infection development of both Ec and Sf was faster on the upper leaf side, than during next three days this process accelerated on the lower side.
A large variability of aggressiveness within individual isolates of both fungi was found. Similar phenomenon was mentionned also by Bardin et al. (1). Ec isolates with different virulence (e.g. 25/97 and 38/97) expressed very similar aggressiveness on both leaf sides of C. maxima and, on contrary the aggressiveness of Ec isolates virulent to the same spectrum of differential plant species (e,g, 38/97 and 40/97) was quite different. Sf isolates 34/97 and 37/97 with the same reaction pattern on differential genotypes proved different aggressiveness on both sides of C. maxima leaf discs. Three isolates of Ec and two isolates of Sf were more aggressive on the lower leaf side than on the upper one (Table 1).
Differences in aggressiveness within individual isolates were not associated with original host plant species, region of their collecting and/or their virulence (pathotype). Only one Ec isolate was not virulent on the lower leaf side. Such reaction would be further studied and considered to be used as a marker for partial resistance of Cucurbita maxima cv. Goliáš as proposed by Leibovich et al. (8) for some other Cucurbita species.
Explanation of the role of specific morphological, biochemical and physiological features of each leaf side and co-evolution with powdery mildews could provide general conclusions about specific host - pathogen interactions. Aggressiveness of powdery mildew isolates is further studied on a broader spectrum of host genotypes.
Till now, Ec is a predominating powdery mildew species on cucurbits in the Czech Republic (4). A very high aggressiveness of Sf isolates on C. maxima can influence a broader geographic distribution of this powdery mildew species on this territory.
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Table 1. Aggressiveness of powdery mildew isolates on Cucurbita maxima cv. Goliáš
Isolate number Pathotypex TID(%)y Host plantz Region upper lower
nd 7.5 3.8 C. maxima Olomouc
40/97 AB1B2CCm 15.0 3.8 C. pepo SC Prostějov
15/97 nd 20.0 0.0 C. pepo ZU Prostějov
30/97 nd 25.0 1.3 C. pepo ZU Prostějov
41/97 AB1B2CCm 25.0 25.0 C. pepo ZU Prostějov
19/97 AB1B2CCm 27.5 13.8 C. maxima Praha
17/97 ACm 36.3 26.3 C. pepo PU Prostějov
44/97 AB1B2CCm 36.3 42.5 C. pepo VM Olomouc
29/97 AB1B2CCm 45.0 6.3 C. pepo ZU Olomouc
23/97 AB1B2CCm 46.3 54.7 C. maxima Prostějov
20/97 AB1B2CCm 67.5 33.7 C. sativus Olomouc
38/97 AB1B2CCm 68.8 28.8 C. pepo ZU Brno
25/97 ACCm 70.0 35.0 C. pepo ZU Brno
average 34.3 ab1 22.3 a1
13/97 AB1CCm 40.0 17.5 C. pepo
36/97 AB1-CCm 45.0 42.5 C. maxima Šumperk
32/97 nd 48.4 26.3 C. pepo ZU Olomouc
34/97 AB1CCm 55.6 71.3 C. pepo Olomouc
16/97 nd 58.8 12.5 C. pepo ZU Olomouc
26/97 AB1CCm 60.0 77.5 C. maxima Olomouc
37/97 AB1CCm 70.0 10.0 C. pepo ZU Kroměříž
10/97 AB1Cm 78.8 40.0 C. maxima Olomouc
average 57.1b1 37.2ab1
compatible reaction on: A C. sativus
cv. Marketer C C. pepo
cv. Diamant F1
B1 C. melo Védrantais Cm C. maxima cv. Goliáš
B2 C. melo
PMR 45 D C. lanatus
cv. Sugar Baby
Total infection degree on lower and upper leaf side of C. maxima
z C. pepo
morphotype according to Paris (9): PU pumpkin, SC
scallop, VM vegetable marrow, ZU zucchini
Homogeneous groups (Multiple range analyses
, 99%, Sheffe)