Septoria Canker on Nursery Stock of Populus Deltoides

Septoria musiva Peck is capable of establishing itself on unwounded first-ye ar stems of eastern cottonwood. Natural infections have been observed since 1969 in three forest nurse ries in Mississippi, and inoculations have confirmed that both conidia and ascospores are capable of causing stem infections . This paper offers evide n ce that the fungus Septoria musiva Peck is capable of causing cankers o n unwounde d stems of eas te rn cottonwood, Populus deltoides . ~. deltoides is the mos t widely distribute d Populus species in the Southern United States; it occurs in na tural stands along major rivers and its use in plantations is increasing. Throughout the United States range it is attacked by a l eaf spot disease caused by ~. musiva (2), butit has been regarded as resistant to stem infection by this fungus. Bier (1) reported tha t the fungus cau sed canke rs on P . ba lsamifera (= P . d e ltoides) when inoculations were made in s te m wounds, but he found nO-infection when un;ounded stems were inoculated . The present authors observed naturally occurring stem cankers during 1969 in a n experimental cottonwood nursery at Stoneville, Mississippi. Later in the year, c ankers were found at Winona and Mount Olive, in State nurseries growing cottonwood cuttings for tree planters. The information reported here includes observations on the severity of the disease a t the Stoneville a nd Winona nurseries, and on the manner in which stem infection occurs in s pring . In addition , inoculations were made to identify the fungus and to test its pathoge nicity. MATERIALS AND METHODS Nursery Obs e rvations: Cottonwood nurseries a re usually started by planting cuttings in l ate winter or early spring . The cuttings take root rapidly and send up shoots that m ay reach heights of 10 to 14 fee t by the end of the first nursery season . Cuttings for field planting are then made fro m the aboveground parts, and the rootstocks are left for a nothe r year's production . Stoneville: The Stoneville nursery, which is maintained by the Southern Hardwoods Laboratory, had been in production for about a decade and contained rootstocks of various ages and provenances . During 1969, 1000 leaves were examined for the presence of pycnidia . The leaves were from the current season's shoots on rootstocks i n the first, second, or third year of growth . In addition, stem cankers were studied on 20 clones planted in 1968 . These clones had been collected within a 50-mile radius of Stoneville and represented both a natural population and selections with above average growth potential. All stem s on these rootstocks were examined in July 1969 and June 1970 -a total of 10,296 stem s in 2 years. Pycnidia or pustules on the cankers were taken as evidence that the fungu s was present. Cankers were randomly selected for microscopic examination and culture isolations. Winona: Cuttings taken at random from the natural population had been planted at Winona in 1963, 1968, a nd 1969. All stems (12,232 ) and 750 randomly selected leaves were examined in July 1969 to determine the relation between age of rootstock and incidence of infection. Random sampl es of cankers were collected for isolations and spore measurements. Inocul ations : In May 1970, 190 leaves on 19 potted c uttings were inoculated with a single spore Septoria isolate obtained from a canker. A spore-mycelium suspension was applied to leaves with an air brush . After 48 hours of incubation in a mist chamber at 90980/0 relative humidity (RH), plants were transferred to a greenhouse maintained at 23 3 0°C , 65-850/0 RH. 1 Plant Pathologists at the Southern Hardwoods Laboratory, Stoneville, Mississippi. The Labo ratory is maintained by the Southern Forest Experime nt Station, USDA Forest Service, in cooperation wi th the Mississippi Agricultural Experiment Sta tion and the Southern Hardwood Forest Research Group. 2Formerly Plant Geneticist a t the Southern Hardwoods Laboratory; presently Associate Profes sor of Forestry, University of Minnesota , St. P aul. 3PlantGeneticist at the Southern Hardwoods Laboratory, Stoneville, Mississippi. Vol. 55, No. 5--PLANT DISEASE REPORTER--May 1971 FIGURE 1. Symptoms and signs caused on Populus deltoides by Septoria musiva and its ascogenous form Mycosphaerella populorum. Upper left: Septoria canker on 6-month-old cottonwood stem. Upper right: Septoria spots on green cottonwood leaf. Center left: Conidia isolated from canker of inoculated cottonwood stem. Center right: Cross section of (a) pycnidium and (b) conidia in leaf tissue. Lower left: Ascospore of M. populorum isolated May 1970 from cottonwood leaf collected in October 1969. Lower right: Perithecium, asci, and ascospores of M. populorum produced in cottonwood leaf collected October 1969 and stored in a lath house until March 1970. 461 462 Vol. 55, No. 5--PLANT DISEASE REPORTER--May 1971 Additional leaf inoculations on 24 cuttings were made with 12 singlespore isolates obtained from cankers. Each isolate was tested on one dorsal and one ventral leaf surface of each cutting. The inoculum was in the form of 2 mm agar disks containing spores and mycelium and affixed with masking tape. Infection counts were made after 60 days in the greenhouse. A pathogenicity test on stems was conducted by making 200 inoculations on 47 potted cuttings. One hundred inoculations were made by placing a 2-mm agar disc containing spores and mycelium beneath the bark in a stem wound 0.25 x 0.5 inch in diameter, and 100 inoculations were made on bark surfaces of unwounded stems. All inoculation points were covered with masking tape. The inoculum was from singlespore isolates of cottonwood leaf spots in the Stoneville nursery. Percentage of infection was tabulated after 30 days . Natural spore discharge and infection was used in May 1970 to test the pathogenicity of the ascogenous stage of~. musiva (Mycosphaerella populorum G. E . Thompson) (2) . Leaves containing Septoria infections were collected in October 1969 at the Stoneville nursery and stored overwinter in a lath house. In March 1970 they were placed to completely cover the soil below cuttings growing in a flat measuring 24 x 32 inches . The flat was kept in the mist chamber at 90-98% RH for 7 days, then transferred to a greenhouse maintained at 23-27°C, 65-85% RH, and 50% shade. Infection was computed from the number of exposed stems that had cankers after 90 days. A total of 25 stems on 18 rootstocks were exposed .