Septoria musiva Peck (teleomorph Mycosphaerella populorum G.E. Thompson) causes a leaf spot and canker disease of poplars.  The disease has been reported to be more severe on harsher sites.  To determine if water stress predisposes hybrid poplars to colonization by S. musiva, we conducted a greenhouse study on 2-month-old rooted cuttings of clones NM-6 (nigra x maximowiczii) and NC-11396 (maximowiczii x berolinensis).  Half of the trees were watered 3 times/wk; the rest were watered only when the lowest measured predawn water potential fell below -1.5 MPa.  We wounded trees by removing the fourth fully expanded leaf from the apex and inoculated them by placing a colonized agar plug on the wound.  We used two single-conidium isolates that originated from the same leafspot.  We also included wounded and nonwounded controls.  After 80 days, we measured the lengths of the cankers that resulted.  Analysis of variance indicated that the effects of inoculation treatment, water stress, and treatment by water stress interaction were significant (p < 0.01).  Mean canker length differed between isolates and was greater for water-stressed trees.  These results indicate that host condition influences canker development in hybrid poplars colonized by Septoria musiva.
 

Septoria musiva Peck (teleomorph Mycosphaerella populorum G.E. Thompson) causes a leaf spot and canker disease of poplars, reported to be more severe on harsh sites.  To determine if drought stress predisposes hybrid poplars to colonization by S. musiva, we conducted a greenhouse study of clones NM-6 (nigra x maximowiczii) and NE-308 (nigra var. charkowiensis x berolinensis).  We planted 6 rooted cuttings of each clone in 56x46x28 cm boxes containing 1:1 Fafard Mix #2:sandy loam field soil.  Boxes were watered either 3 times/wk; or after the predawn water potential fell below -1 MPa.  Removing the fourth fully expanded leaf from the apical meristem, we placed an agar plug colonized by S. musiva, or a sterile plug, over the leaf scar.  After 80 days, stressed trees developed significantly larger cankers than non-stressed trees.  We cut two 15mm diameter disks from the first or second fully expanded leaf from two trees of each clone per box, and placed them in 24 well tissue culture plates.  Each disk in a pair  was inoculated with 0.1 ml of either sterile water or 10⁴ spore/ml suspension.  We used Optimas image analysis software to assess disease severity.  In contrast to our canker data, disks from stressed trees remained less symptomatic than those from well-watered trees.  These results indicate that host condition influences S. musiva symptom development in hybrid poplar stems and excised leaf disks.
 
 

Commercially available hardware and software for image acquisition and analysis can facilitate estimation of foliage disease severity.  These methods can decrease labor, increase accuracy, and allow repeated, nondestructive measures.  We utilized the program Optimas 4.1 (BioScan, Inc., Edmonds, WA) to quantify symptom development in a leaf-disk assay used to assess susceptibility of Populus hybrids to the leafspot and canker pathogen Septoria musiva Peck.  Leaf disks (15 mm diameter) were placed in 24-well tissue culture plates with 1 ml water agar per well.  They were inoculated with 0.1 ml of either sterile water or a suspension of 10⁴ conidia per ml and incubated in the light at 20^oC for nine weeks.  Periodically after inoculation we acquired and saved JPEG compressed images of 240 leaf disks using a Sony 3CCD color video camera and a TrueVision Targa M8 frame grabber board.  Later, we analyzed these images using a macro that semi-automated the process.  Our macro made it possible to distinguish between green, healthy tissue and dark, necrotic tissue with a high degree of accuracy, and incorporated the percent area routine of the Optimas program to calculate relative disease severity.  Data was exported directly into an Excel 4.0 (Microsoft, Inc., Redmond, WA) spreadsheet.  We were able to complete image acquisition, storage, and analysis and proceed to statistical analysis in under 12 hr for 240 disks (3 min per sample).
 

The amount of necrosis developed after inoculation of excised hybrid poplar leaf disks has been proposed to reflect relative susceptibility to the leafspot and canker pathogen Septoria musiva (teleomorph Mycosphaerella populorum).  To determine if host factors such as leaf maturity influence bioassay results, we conducted a study of clones NM-6 (nigra x maximowiczii) and NE-308 (nigra var. charkowiensis x berolinensis) grown in a greenhouse.  We cut two 15mm diameter disks from the thrid or fourth and tenth or eleventh fully-expanded leaves (from the shoot apex).  Disks were placed in 24-well tissue culture plates with 1ml of water agar per well, inoculated with 0.1 ml of either sterile water (control) or a suspension of 10⁴ conidia per ml, and incubated in the light at 20^oC.  Periodically after inoculation, we acquired and saved digital color images of disks and later analyzed these images to quantify symptom development.  After 30 and 48 days, inoculated disks from each clone were more necrotic than control disks (p < 0.01).  however, both inoculated and control disks from upper leaves were less necrotic than corresponding disks from lower leaves (p < 0.01).  Thus, leaf maturity can influence the response of plant material used in this bioassay.  Morphological, biochemical, and physiological characteristics of the leaf tissue, developed while on the intact plant or after excision and incubation, as well as responses of leaf-associated microorganisms, may be responsible for the observed effects.