PtlKrankh. Z. PflSchutz, Sonderh. XVI, 445-453 (1998)
Field efficacy of Phomopsis cokvolvulus for control of Convolvulus arvensis
S. VOGELGSANG', A. K. WATSON', A. D ~ M M A S O ' ,K. HURLE* Faculty of Agricultural and Environmental Sciences, McGill University, Macdonald Campus, 2 1. 1 1 1 Lakeshore, Ste-Anne-de-Bellevue, QuCbec, Canada H9X 3V9 Institut fiir Phytomedizin, Universitiit Hohenheim, D-70593 Stuttgart Email: svogel @ po-box.mcgill.ca
'
Summary The efficacy of the fungus, Phomopsis convolvulus Ormeno, applied pre-emergence for Convolvulus arverzsis L. (field bindweed) control, was tested in field trials. A granular barley formulation of the fungal inoculum was applied onto a soil, seeded with either pre-germinated seeds or root stocks obtained from established stands. Dramatic biomass reductions for plants originating from either seed (up to 100%) or root stocks (98%) were obtained. In a second experiment, the efficacy of P. convolvulus in pure stands of C. nrvensis was conipared with application under competitive cropping situations. In mixture with spring wheat (Triticum aestivum L.), above-ground biomass of inoculated C. arvensis plants was reduced by 98% compared with inoculated C. arvensis plants grown in pure stand. However, no significant differences in spring wheat yield were found.
Key words: Biological control, competition, established plants, granules, mycoherbicide, pre-emergence, seedlings
Zusammenfassung Wirksamkeit von Phomopsis convolvulus auf Convolvulus arvensis unter Feldbedingungen Die Effektivitat des Pilzes Phomopsis convolvulus Ormeno zur Bekampfung von Convolvulus arvensis L. (Ackerwinde) wurde im Vorauflaufverfahren in Freilandversuchen getestet. Um die Wirksamkeit gegeniiber Keimlingen und etablierten Pflanzen zu iiberpriifen, wurde Pilzinokulum, formuliert als Gerstengranulat, auf den Boden ausgebracht, der vorgekeimte C arvensis-Samen bzw. Wurzeln von etablierten, . gemaehten C. arvensis-Bestaenden enthielt. Durch eine Pilzapplikation wurde die oberirdische Trockenmasse beider Pflanzenstadien mit bis zu 100 bzw. 98% drastisch reduziert. In einem weiteren Versuch wurde die Effektivitat von P. convolvulus in gem3hten C. arvensis-Monokulturen oder in Konkurrenz mit einem Kulturpflanzenbestand bewertet. Irn Vergleich zu einer Vorauflaufbehandlung in Monokultur wurde die Trockenmasse von C.arvensis in Mischbestiinden mit Sommerweizen um 98% reduziert, was sich jedoch nicht signifikant auf den Weizenemag auswirkte. Stichworter: Biologische Bekiimpfung, etablierte Bestiinde, Granulat, Keimlinge, Konkurrenz, Mykoherbizid, Vorauflauf
Introduction Convolvulus arvensis L. is a serious perennial weed in several crops found throughout many regions of the world (HOLMet al. 1977). Effective control of this weed using current methods, such as cultivation,
crop rotation, and chemical herbicides (DERSCHEID et al. 1970) is often not possible due to its extensive root system, high competitiveness, and variable susceptibility to several important herbicides (DEGENNARO WELLER 1984; KOSINSKI WELLER 1989; YERKES WELLER 1996). In addition, and and and the move towards reduced cultivation or zero tillage has led to an increased prevalence of C. arvensis (PHILLIPS et al. 1980). The occurrence of a foliar pathogen, Phomopsis convolvulus Ormeno on C. arvensir plants, was first reported by Ormeno-Nuiiez in 1988 (ORMENO-NUREZal. 1988b). Since then, studet ies on host specificity, conidia mass production, storage, and efficacy of foliar treatments have been carried out (ORMENO-NU~~EZ1988a. b, MORM al. 1989a,b, MORNet al. 1990). In an attempt to overet a]. et come the pathogens' requirement for long dew periods in order for sufficient disease to occur, a granular pre-emergence formulation of P. convolvulus has been developed and tested. Inoculum produced on pot barley grains and applied onto the soil surface has effectively controlled C. arvensis seedlings under both controlled environment and field conditions (VOGELGSANG et al. 1994). However, within most cropping systems, the largest proportion of C. arvensis biomass develops from existing intact or fragmented roots and not seedlings (WEAVER and RILEY1982). Subsequently, experiments were conducted to determine the efficacy of P. convolvulus on both seedlings and established plants (VOGELGSANG al. 1998b). Moreoet ver, previous studies assessing the efficacy of P, convolvulus to suppress C. arvensis have only been carried out in pure stand of the host plant. However, as has recently been cautioned (DITOMMASO al. et 1996), assessing the efficacy of potential biocontrol agents in host monoculture populations may result in significantly different responses than might be obtained when the target weed is grown in the presence of a crop. Indeed, a number of studies have demonstrated the more detrimental effect of fungal pathogens on their hosts when grown in mixture with a crop than when grown in pure stand (GROVES and WILLIAMS 1975, PAULand AYRES 1987, DITOMMASO al. 1996). Hence, the objectives of this paper were to: (I) et highlight results from experiments that evaluated the pre-emergence activity of P. convolvulus on seedling and established plant growth of C. arvensis, and (2) evaluate the impact of P. convolvulus on C. arvensis under competitive cropping situations.
Materials and methods
Inoculum and plant production From 3-week-old P. convolvulus colonies produced on potato dextrose agar (PDA), 5 x 107 conidia were used to inoculate 1 L screw cap jars containing 100 g of autoclaved pot barley and 80 ml deionized water. Barley cultures were incubated for 3 weeks at room temperature with 12 h day-' near-ultraviolet-light (F40 BLAB Blacklight, General Electric Lighting, Cleveland. OH, USA) and shaken every second day. Colonized barley grains were milled and air dried as described previously (VOGELGSANGal. 1998b). et C. arvensis seeds (Valley Seed Co. Fresno, CA, USA) were soaked for 20 seconds in near boiling water and incubated on moist paper towels in a plastic tray in the dark for 24-36 hours at 30 f 1C. Unless otherwise indicated, 200 imbibed seeds were sown in 0.25 m2 field plots at a depth of approx. 6 cm. Experiment 1 - Seedlings versus established plants A two-factor experiment was carried out involving two plant growth stages and four application treatments (three inoculation dates and one uninoculated control). To produce established stands of C. arvensis from seeds, plants were grown for 6 weeks before they were cut at the soil line. On the same day established plants were cut (26125 June 199511996). seedling material was produced by sowing pregerminated seeds in plots not containing established plant material. At 3, 5, and 7 days after sowing (DAS), 30 g of fungal inoculurn was uniformly spread by hand onto the soil surface. Experiment 2 - Eficacy under a competitive cropping situation A three-factor experiment was carried out examining the effect of the presence or absence of crop (spring wheat, Triticum aestivum), weed (C. arvensis), and pathogen (P, convolvulus). The following treatments (TRT) were evaluated:
Phonlopsis convolvulus for control of Convolvulus arvensis
TRT
(1)
447
Crop
Weed
(2) (3) (4) (5)
+ + + +
+ +
Pathogen
+
On 8 May 1997, spring wheat (cultivar 'SS Blomidon') was sown at a rate of approximately 120 kgha in five wheat rows 20 cm apart using a mechanical seeder. The experimental plot size was I x 2 m (i.e. consisting of 5 wheat rows). Two days after planting, 400 imbibed C. arvensis seeds were sown as evenly as possible by hand in between spring wheat rows in 0.64 m2 (80 cm x 80 cm) subplots. Outside the 1 x 2 m mixture plots or within C. arvensis monoculture plots, emerged wheat seedlings were removed by hand. Five DAS, 65 g of fungal inoculum was spread onto the soil surface of subplots.
Study sites and experimental design Experiment 1 was carried out at the Horticulture Research Centre of Macdonald Campus, Ste-Anne-deBellevue, QC, Canada. The soil type was a Chicot fine sandy-loam with a pH of 5.3 and 3% organic matter. Experiment 2 was carried out at the Emile Lods Agronomy Research Centre of Macdonald Campus. The soil type was a St. Bernard fine sandy-loam (orthic melanic brunisol) with a pH of 6.8 and 3% organic matter. For experiment 1, a randomized complete block design (RCBD) with five blocks was used and a total of 40 0.25 mZplots was established. The distance between plots and blocks was 0.5 m and 1 m, respectively. For experiment 2, a RCBD with 4 blocks and a total of 20 2 m2 plots was established. Each plot contained a 0.64 m2 subplot where planting of C. arvensis, application of fungal inoculum, and harvest of both, C. antensis and spring wheat was carried out. Spacing between plots and blocks was 2 m. Air temperature and precipitation data were obtained from the McGill Meteorological Observation Centre. Assessment of eficacy Foliar necrosis for experiment 2 was evaluated 34 DAS, using a rating system, with 0 = no visible symptoms, 1 = less than 25% necrosis, 2 = 26-50% necrosis, 3 = 51-7595 necrosis and 4 = greater than 75% et necrosis (ORMENO-NUNEZ al. 1988a). Mortality was assessed 41 DAS by counting the number of seedlings with completely necrotic hypocotyls. Disease rating and mortality determination were performed for each plant and results were pooled and averaged for each plot. For experiment 1, plant coverage was determined 25 DAS. Margins of each plot were delineated with white string, and black and white photographic pictures were taken. Contours of living plant material were traced on the photograph, and percentage coverage of healthy tissues was assessed using an image analyzer ( ~ e c o @ Image Analysis Sys2001 tem, Instruments LA&, Longueuil, QC, Canada). For experiment 1 and 2, C. arvensis above-ground dry biomass was determined 25 and 104 DAS, respectively. Plants were cut at the soil level, and living tissues dried in paper bags for 4 days at 60C. and weighed. Biomass was recorded on a per plot basis. In experiment 2, spring wheat was harvested from subplots 1 day after harvest of C.arvensis. (i.e. 107 days after sowing of spring wheat). Both spring wheat above-ground biomass and grain weight per plot was recorded. Moreover, the number of fully developed heads per subplot was also determined. Statistical analysis Experiment 1 was performed twice and experiment 2 once. Percentage coverage, mortality, and biomass data were arcsin or loglo (x+l) transformed as appropriate prior to analysis of variance and differences between treatment means were determined using Tukey's W test (a 0.05) (STEEL & TORRIE = 1980). Disease ratings were compared using the Friedman Test, followed by a multiplecomparison procedure to evaluate differences between treatment means (DANIEL 1978). Results for the two trials of experiment 1 were not pooled due to heterogeneity of variances as determined by Levene's test (DUFNER al. 1992). et However, similar trends were observed, hence, the results of one trial are presented.
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Results
Experimenr I - Seedlings versus established plants Most seedlings and shoots emerged 5 to 6 DAS, however emergence of established plants continued until the end of the experiment. Throughout the application period, weather conditions were warm and moist with heavy precipitation 2 days after the last granular application (Fig. 1). Air temperatures reached 2SC, respectively. Disease symptoms developed on plants originating from both source materials (i.e. seeds and roots) and for all fungal applications. Hence, infection resulted in significant above-ground biomass reductions for both inoculated seedlings and established plants (Fig. Seedlings 2). above-ground biomass was reduced by nearly 100% while biomass reductions for established plants were as high as 96 to 98 % compared with uninoculated controls. The substantial differences in biomass for inoculated versus control plants occurred despite substantial cross-infection of plants within control plots. In general, plot coverage data closely paralleled above-ground biomass results (Fig. 2).
3o r p e r a t u r e ("C)
Precipitation (mm)
,
, .
Time
Fig. 1: Precipitation, air and soil temperatures during field experiment 1. Temperatures are daily means. Abb. 1: Niederschlag, LA&- und Bodentemperaturen i Feldversuch I . Temperaturen sind Tagesm mittelwerte.
Experiment 2 - Eficacy under a competitive cropping situation Spring wheat and C. arvensis seedlings began to emerge 9 and 6 DAS, respectively. Shortly after fungal application, limited precipitation was received for more than 2 weeks and night temperatures dropped as low as 4C (Fig. 3). Initial efficacy of the fungal inoculum was very low (Table I), however, with rising temperatures and rainfall 3 weeks after sowing, disease symptoms became more apparent, especially in mixed stands with spring wheat. In contrast, disease incidence of inoculated C. arvensis plants grown in monoculture remained insignificant. Mortality of inoculated C. arvensis in mixture reached 22% whereas only 9% mortality was recorded in monoculture. Moreover, the above-ground biomass of inoculated C. arvensis plants in mixture was reduced by 86% compared with uninoculated controls (Fig. 4A). Wheat yield of plants grown in mixture with uninoculated C. arvensis was reduced by 27% compared with a weed-free spring wheat stand (Fig. 4B). When grown in mixture with diseased C. arvensis, wheat yield reduction was only slightly lower (20%) than for the uninoculated treatment. Similarly, the number of heads and grain weight were lowest in mixture with uninoculated C. arvensis, greater in mixture with diseased C. arvensis, and highest in weed-free stands (Table 1). However, differences were not significant
Phornopsis convolvulus for control of Convolvulus arvensis
449
(a=0.05) any of the parameters measured. Application of fungal granules in pure stands of spring for wheat had no significant effect on growth and yield.
0
0
Control Day 1 Day 2
Day 3
Treatment
Fig. 2: Effect of pre-emergence field application of Phomopsis convolvulus on Convolvulus arvensis above-ground biomass (bars) and plot coverage (circles) for seedlings and established plants. Day 1,2,3: 30 g P. convolvulus granules 0.25 m2 plot-' applied 3, 5,7 days after sowing. Each vertical bar is the SEM.This figure is part of a manuscript that has been recently accepted for publication (VOGELGSANG et al. 1998a). Abb. 2: Wirkung einer Freiland-Voruu@uJbehandlung mit Phomopsis convolvulus auf Convolvulus arvensis-Trockemse (Saulen) und -Bodendeckung (Kreise) von Keimlingen und etablierten Bestiinden. Day 1, 2, 3: 30 g P. convolvulus-Granulat pro 0,25 m2 Panelle 3, 5, 7 Tage nach &r Aussaat. Vertikale Striche stellen die Standardfehler der Mittelwerte dar. Diese Abbildung ist Teil eines Manuskripts, dm kiirzlich zur Veroyentlichung angenommen wurde (VOGELGSANG et al. 1998a).
Temperature (OC)
Precipitation (mm)
,
20
June
July
August
7-day intervals
Fig. 3: Total precipitation and mean air temperatures at 7-day intervals from sowing (08-10 May) to harvest (20-2 1 August) during field experiment 2. Spring wheat and Convolvulus arvensis seedling emergence occurred 9 and 6 days after sowing, respectively. Abb. 3: Gesamtniederschlag und mittlere Lufttemperaturen in 7-Tage-IntervalLen von der Aussaat (08.-10. Mai) bis zur Ernte (20.-21. August) im Feldversuch 2. Auflauf von Sommenveizen und Convolvulus arvensis erfolgte jeweils 9 und 6 Tage nach der Aussaat.
Tab. 1:
Effect of Phomopsis convolvulus on disease severity and mortality of Convolvulus arvensis I and on spring wheat yield in pure stands or in mixture. Tab. I: Wirksamkeit von Phomopsis convolvulus auf Krankheitsentwicklung und Mortalitat von Convolvulus arvensis und auf den Sommenveizenertrag in Reinbestiinden oder in Konkurrenz. C . arvensis Spring wheat Disease 3 rating 0.20 a (0.05) 1.33 ab (0.08) 1.84 b (0.1 1) Mortality
(%)
Treatment Ta - Ca Ca - PC
2
182.8 84.6 0.46 (15.1) (11.4) (0.04) Ta - PC 187.0 102.9 0.55 (12.8) (17.6) (0.09) Ta 210.5 108.4 0.51 (15.1) (20.3) (0.09) Application of 65 g P. convolvulus granules per 0.64 m2 plot 7 days after sowing of C. arvensis. Means in each column with the same letter are not significantly different, according to the Friedman test followed by a multiple comparison procedure (P = 0.15) (disease rating) or to Tukey's grouping (a= 0.05) (mortality). Data in parentheses are the SEM. Ta, Ca, PCrefer to Triticwn aestivum, Convolvulus arvensis, Phomopsis convolvulus, respectively. Disease rating scale is 0 = no visible foliar symptoms. 1 = 1- 25% necrosis, 2 = 26-50% necrosis.
Ta-Ca-PC
0.8 (0.8) 8.5 (3-2) 22.3 (2.9)
a a b
Heads1 subplot 165.3 (10.1)
Total grain weight (g/subplot) 77.6 (13.0)
Grain weighthead (g) 0.46 (0.06)
-
'
Phomopsis convolvulus for control of Convolvul~cs clrvetrsis
45 1
C-P
T-C
T-C-P
T
T-P T-C-P T-C
Treatment
Fig. 4: Effect of pre-emergence field application of Phornopsis convolvulus on ( A ) Convolvulus arvensis above-ground biomass and (B) yield of spring wheat in monoculture or mixture. T, C, P: Triticum aestivurn, Convolvulus arvensis, Phomopsis convolvulus. 65 g P. convolvulus granules 0.64 m2 plot-' applied 4 days after sowing. Each vertical bar is the SEM. Abb. 4: Wirkung einer Freiland-Vorauflaufbehandlmgmit Phomopsis convolvulus auf (A) Convolvulus arvensis-Trockenmasse und (B) Sommerweizenertrag in Monokulturen oder Mischbestanden. T, C , P: Triticum aestivum, Convolvulus arvensis, Phomopsis convolvulus. 65 g P. convolvulusGranulat pro 0.64 m2 Parzelle 4 Tage nach der Aussaat. Vertikale Striche stellen die Standardfehler der Mittelwerte dar. Discussion
In the present study, the pre-emergence potential of the fungal pathogen, P. convolvulus, to suppress C. arvensis at two growth stages was evaluated. Furthermore, the efficacy of P. convolvulus to control C. arvensis when grown in the presence of spring wheat was also examined. Both C. arvensis seedlings and established plants developed disease symptoms following fungal inoculation. However, disease severity in established plants was slightly lower than for seedlings. The possible development of a thicker cuticle by physiologically more mature plants could result in altered de-
fence mechanisms (MARTIN 1965). and could explain why tissue from established plants was less suscept~ble fungal damage. In addition, regrowth from root stocks occurred in a staggered manner and was not to co~npletedby the time all seedlings had emerged. Thus, a considerable amount of inoculum could have lost viability lying on the soil surface. The presence of a spring wheat crop with C. arvetlsis had a strong effect on efficacy of P. convolvulus. In general, disease development in this experiment was delayed, possibly due to the cool, dry weather conditions prevailing shortly after fungal application. However, when grown in mixture with spring wheat, C. nrveruis growth was substantially suppressed. Within mixed stands of weed and crop, free moisture availability might have been prolonged due to the altered microclimate (i.e. higher humidity due to shading from taller wheat plants), possibly leading t~ greater disease severity. In addition, competition from the neighbouring crop might have provided additional stress for inoculated C. arvensis plants. In contrast, growth of C. arvensis plants in monoculture was greater by the time infection was detected, resulting in lower P. convolvulus efficacy. The susbstantial differences in disease incidence observed between inoculated and uninoculated mixture plots were not reflected in greater spring wheat yields. It is possible that intense weed competition had already occurred by the time C. arvensis plants had been infected by P. convolvulus. A repeat of this experiment as well as trials that will make use of various plant densities may provide additional information as to these weed-crop-pathogen interactions. The results of our study indicate that preemergence applications of P. convolvuhs on the soil surface are effective in controlling different C. arvensis growth stages, and that weed control may be enhanced under typical cropping conditions.
Acknowledgements
The senior author is grateful to the International Council of Canadian Studies for financial support through a postgraduate scholarship. Support from the Natural Sciences and Engineering Research Council of Canada (NSERC) through an operating grant to AKW is acknowledged.
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