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Differential Susceptibility of Russian Thistle Accessions to Colletotrichum Gloeosporioides

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Differential Susceptibility of Russian Thistle Accessions to Colletotrichum Gloeosporioides Differential susceptibility of Russian thistle accessions to Colletotrichum gloeosporioides William Bruckart , , a, Craig Cavina, Laszlo Vajnab, Ildiko Schwarczingerb and Frederick J. Ryanc a USDA-ARS-FDWSRU, 1301 Ditto Ave., Ft. Detrick, MD 21702, USA b Plant Protection Institute, Hungarian Academy of Sciences, P.O. Box 102, Budapest 1525, Hungary c USDA-ARS-EIDP, 9611 South Riverbend Ave., Parlier, CA 93648, USA Received 16 May 2003; accepted 1 December 2003. Available online 30 December 2003. Abstract Molecular information suggests that Russian thistle (Salsola tragus L.) in the US may consist of more than one genetic entity. This genetic variation needs to be taken into account when developing agents for biological control of this important weed. Preliminary evidence suggests that there are differences in susceptibility of Salsola sp. to infection by fungal pathogens. In the present study, an isolate of Colletotrichum gloeosporioides, a pathogen of Russian thistle collected in Hungary was tested for its ability to infect and damage California accessions of S. tragus (referred to as Type A) and the related S. tragus, Type B. The minimum dew period and temperature required for infection of S. tragus with C. gloeosporioides was determined to be 12–16 h and 25 °C. Both Type A and Type B were susceptible, but C. gloeosporioides caused greater damage and reductions in biomass of Type A than of Type B. Fresh weights of Type A and Type B were reduced from controls by 60 and 9%, respectively, after inoculations with C. gloeosporioides. Results from this study illustrate the importance of understanding target plant taxonomy in biological control evaluations. Author Keywords: Salsola tragus; Salsola australis; Salsola iberica; Salsola kali; Colletotrichum gloeosporioides; Chenopodiaceae; Uredinales; Biological control; Weeds; Rust fungus; Plant taxonomy; Russian thistle; Biotypes Article Outline 1. Introduction 2. Materials and methods 2.1. The pathogen and inoculation procedures 2.2. Salsola acquisitions tested 2.3. Experiments 2.4. Data collection 2.5. Data analysis 3. Results 3.1. Infection requirements for Colletotrichum gloeosporioides 3.2. Effect of pathogenic fungi on Russian thistle accessions 4. Discussion Acknowledgements References 1. Introduction Russian thistle, or tumbleweed, is a major weed pest of the US that displaces valuable forage and crop plants, serves as a reservoir for several important vegetable viruses and insects, generates allergenic pollen, injures farm workers and animals, poisons sheep, causes traffic hazards, and raises the potential for wild fires (Crompton and Bassett, 1985; Young and Evans, 1985). Management is difficult, largely because of the relatively high cost of conventional control options in range and pasture agriculture, but also because Russian thistle has developed resistance to the sulfonylurea herbicides in certain agricultural settings ( Saari et al., 1992; Young et al., 1995). Currently, Russian thistle is classified as one species, Salsola tragus L. Historically, there have been numerous scientific names (as different species, varieties, and subspecies) applied to this plant in North America (Crompton and Bassett, 1985; Mosyakin, 1996; Wilken, 1993), which has caused confusion in documentation of the plant as a pest. Among the names is Salsola kali L., used in the original study on Uromyces salsolae Reichardt for biological control (Hasan et al., 2001) and misapplied by the authors in that study with Salsola pestifer A. Nelson, Salsola australis R. Brown, and Salsola iberica Sennen and Pau. The latter two names also are synonyms of S. tragus (Wilken, 1993). More recently, genetic variability has been investigated within S. tragus in California (Ryan and Ayres, 2000), leading to the conclusion that there were two different genetic entities present there. These were described as “types” of S. tragus. “Type A” in that work is now considered to be the true S. tragus, but it is referred to as Type A in this paper. “Type B” is morphologically similar but genetically distinct and may be a separate species from Type A. Previous work on biological control agents demonstrated that genetic variation in weeds must be taken into account in the selection of agents (Burdon et al., 1981; Hasan, 1985; Sobhian and Andres, 1978). Two lepidopterous insects (Coleophora klimeschiella Toll and Coleophora parthenica Meyrick) were released for the control of Russian thistle in the 1960s, but they have not been effective (Goeden and Pemberton, 1995; Müller et al., 1990). Recent work has shown that Type A and Type B from California were differentially attacked by a potential biological control agent, the gall midge Desertovellum stackelbergi Mamaev (Diptera: Cecidomyiidae) in field plots in Uzbekistan (Sobhian et al., 2003). In this free-choice situation, the gall midge preferred Type A but it is still under consideration as a biological control agent since it attacked both types. Two pathogens are under consideration for biological control of Russian thistle in the United States. These are Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. in Penz., a fungus collected in Hungary that causes cankers on stems and leaves (Schwarczinger et al., 1998), and the rust fungus, U. salsolae, from Turkey (Hasan et al., 2001). Results from preliminary inoculation studies indicated variability in susceptibility between different Salsola “species” (identified by collectors) and accessions of “S.tragus” (Bruckart et al., 2000) to each of these pathogens. Objective of this study was to determine the susceptibility of Type A and Type B from California to C. gloeosporioides. 2. Materials and methods 2.1. The pathogen and inoculation procedures Colletotrichum gloeosporioides isolate 96-067 collected from “S. kali” in Hungary by L. Vajna (Hungarian Academy of Sciences, Budapest) was used in these tests. The isolate was maintained on V-8 Juice agar. Back-up cultures were kept either on half-strength corn meal agar slants in screw-cap tubes or as agar plugs in Nunc vials in liquid nitrogen. Inoculum consisted of a suspension of conidia harvested from 20% V-8 Juice agar cultures after 2-week growth. Plants were inoculated 4 weeks after transplanting by spraying with 106 conidia/ml until the foliage was completely wet. All plants were exposed to 12–16 h dew at 25 °C and then transferred to a greenhouse (20–25 °C) until symptoms developed. Supplemental lighting was provided from October–March to give a 16-h photoperiod. 2.2. Salsola acquisitions tested Six collections of S. tragus representing two different types (Ryan and Ayres, 2000) were inoculated in these studies ( Table 1). All accessions were from California and identified to type using isozymes (Ryan and Ayres, 2000). Table 1. Collections of Salsola tragus from California used in these studies 2.3. Experiments Optimal conditions for infection of Type A by C. gloeosporioides in the greenhouse were determined. Plants (5 per treatment combination, 100 total) of Type A were inoculated as described and subjected to dew at different temperatures (10, 15, 20, 25, and 30 °C) and time periods (4, 8, 12, and 16 h). This experiment was run twice. Data were collected and analyzed as described below. The relative susceptibility of Type A and Type B accessions to infection by was determined. Both Type A and Type B plants were included in each of five replicates (n>500). Plants were inoculated as described, given 16–20 h dew at 25 °C on the basis of results from tests with C. gloeosporioides, and monitored as described. 2.4. Data collection Disease ratings were made each week for 4 weeks in each experiment and the ratings from the fourth week were used for analysis. Ratings were based on a scale from 0 to 4, where: 0, no macroscopic symptoms; 1, small or isolated lesions, <25% of the plant infected; 2, some coalesced lesions, 25–50% of the plant infected; 3, many coalesced lesions, >50% of the plant infected; and 4, severe infection, dead plant. At the end of each experiment, plants cut off at the soil line were removed and weighed (= fresh weight, FWT), dried overnight at 100 °C, and weighed again (= dry weight, DWT). A third variable, weight change (WTCH), was calculated to standardize biomass data between individual plants; it was derived by dividing the difference between FWT and DWT by the FWT, i.e., WTCH = (FWT−DWT)/FWT. 2.5. Data analysis Data for FWT and DWT were transformed to the log10+ 0.1 equivalent (= LFWT and LDWT, respectively) and tested for normality using Proc Univariate in SAS (Statistical Analysis Systems, Cary, NC; v 6.12). LDWT data were subjected to the analysis of variance using Proc GLM in SAS. LFWT data were subjected to the analysis of covariance using Proc GLM and LDWT as the covariate. WTCH data were transformed to the arcsine equivalent before analysis of variance. Least Squares Means (LSMEANS, SAS), estimated for each treatment combination, were compared on the basis of probabilities that pairs were equivalent (H0: LSMEAN (i)=LSMEAN (j), where treatment ‘i’ is C. gloeosporioides, and treatment ‘j’ is the uninoculated control). Mean separations were made only within type between each treatment and its control. Disease rating data from the study on optimal conditions for infection by C. gloeosporioides were analyzed using Proc NPAR1WAY, the Wilcoxon option, of SAS for dew period, temperature, and treatment. The same analysis was applied by type within treatment to disease rating data on susceptibility of Salsola types. 3. Results 3.1. Infection requirements for Colletotrichum gloeosporioides Infection and damage of Type A increased with increasing dew temperatures and dew periods after inoculation with C. gloeosporioides (Table 2, Fig. 1A). Fresh weight (FWT) of inoculated plants was less than that of controls (Table 2) after 12 h dew at 25 and 30 °C (P=0.08 and <0.01, respectively), and after 16-h dew at 25 and 30 °C (P 0.01). There were no significant effects associated either with shorter dew periods (data not shown) or with temperatures cooler than 25 °C at 12 or 16 h dew (Table 2).
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