Effects of Feeding by Two Folivorous Arthropods on Susceptibility of Hybrid Poplar Clones to a Foliar Pathogen

We investigated variation in folivore-induced effects on subseque t plant suitability to a foliar pathogen. We used a leaf disk assay to expose three clones of hybrid poplar, NC11382, NE332 and NM6, to colonization by a leaf spot pathogen, Septoria musiua. Undamaged leaf disks of NE332 were the most resistant to S. musiua, followed by NM6 and NC1l382, respectively. To test the effects of prior herbivory on subsequent susceptibility to this fungal pathogen, we inoculated S. musiua on leaf disks taken from leaves which had been exposed to feeding by Tetranychus mites or cottonwood leaf beetles. Prior activity by mites and cottonwood leaf beetle affected the subsequent susceptibility of clones NC 11382 a d NE332 to S. musiua. Herbivory may have many effects on host plants. One such effect, in­ duced plant resistance to herbivores and pathogens, may result from bio­ chemical and physiological changes initi ted by contact with the invading herbivore (e.g., Clausen et a1. 1989). Although herbivores and plant pathogens may be confronted with and affected by similar induced defenses (Schultz 1983, Krischik et al. 1991, Hammerschmidt 1993, Klepzig et al. 1996), these interactions have largely been studied separately. Studies exam· ining reciprocal effects between herbivores and plant pathogens are con­ ducted even less frequently, perhaps due to the complexity of these multi­ species interactions (Wargo and H uston 1974, Karban et a1. 1987, Krischik et a1. 1991, Klepzig et al. 1996). In particular, we know little about how folio vore feeding affects subsequent susceptibility to foliar pathogens. The purpose of this study was to consider how prior herbivory by folivo­ rous arthropods an host plant clone interact to affect subsequen suitability to a foliar pathogen. Our study system consisted of two folivorous arthro­ pods-Tetranychus spp. (Acari: Tetranychidae) mites and cottonwood leaf beetles, Chrysomela scripta Fabr. (Coleoptera: Chrysomelidae), three hybrid IDepartment of Plant Pathology, University of Wisconsin-Madison, Madison, WI 53706. 2Department of Entomology, University of Wisconsin-Madison, Madison, WI 53706. 3Current address: Hardwood Research Cooperative, Jordan Hall Box 8008, North Carolina State University, Raleigh, NC 27695. Correspondence: K.D. Klepzig, USDA Forest Service, Southern Research Station, 2500 Shreveport Hwy, Pineville, LA 71360. 1 Klepzig et al.: Effects of Feeding by Two Folivorous Arthropods on Susceptibility Published by ValpoScholar, 1997 100 THE GREAT LAKES ENTOMOLOGIST Vol. 30, No.3 poplar clones (Populus spp.), and one fungal foliar pathogen-Septoria mu­ siva Peck. MATERIALS AND METHODS We grew plants from dormant hardwood cuttings from coppiced field grown hybrid Populus clones NC11382 (P. nigra X P. berolinensis), NE332 (P. simonii X P. berolinensis) and NM6 (P. nigra X P. maximowiczii) (Robison and Raffa 1994, 1996) at the University of Wisconsin Arlington Experiment Station. We stored the cuttings (5 cm long) frozen until use, and plant d them in Redi-Earth Peat-Lite® soil mix. We regularly flood irrigated and fer­ tilized with 15 g Osmocote® slow-release fertilizer (7-16-12, plus micronutri­ ents), plants and grew them in a glasshouse at 16:8 L:D photoperiod, 18-29°C and 25-80% relative humidity. We assayed leaves from three types of plants. Because, within clones, variability is greater between leaves of th same physiological age than it is between trees (Robison and Raffa 1997), we removed leaves 3 and 4 (down­ ward from the apical fully expanded leaf) from each plant for use in assays the same day. We collected leaves from control (undamaged) plants of all three clones, exhibiting no visible signs or symptoms of arthropod damage or disease. We collected leaves from clones 11382 and NE 3 that exhibited ex­ tensive mite feeding damage (approximately 30-50% damage on each leaf), or exhibited C. scripta feeding damage (approximately 10% defoliation on each plant). We followed the S. musiva colonization assays developed by Ostry and Skilling (1988). Due to our incomplete understanding of the mechanisms un­ derlying any systemic responses withi poplar, and because multiple infec­ tions may occur n the same leaf we followed the method cited above and re­ moved multiple leaf disks from each leaf and used leaf disk as our experimental unit. Leaf disks (18 mm diameter) were placed abaxial side up in wells cut into 2% water agar in glass petri plates. Each petri plate con­ tained 6 disks taken from the sa e clone and treatment (n == 4 plates per clone/treatment combination). We inoculated five leaf disks n each ch mber with 0.1 ml of a spore suspension (1 x 10 6 conidia/mI), and the sixth disk with 0.1 ml of sterile distilled water. The assay plates were then incubated in a growth chamber at 24°C under continuous light. We measured areas of the resulting necrotic lesions using a transparent dot grid. The effects of treat­ ments, clones, and time since inoculation on lesion size were analyzed using a repeated measures ANOVA, followed by Fisher's protected least squares means comparisons where appropriate (Abacus Concepts 1989).