SPHAEROPSIS SAPINEA AND WATER-STRESS IN A RED PINE PLANTATION IN CENTRAL WISCONSIN

A study was conducted to determine the effects of water stress resulti ng from competing vegetation on disease development of Sphaeropsis sap inea in red pine plantations. A 9-year-old plantation was selected in 1992 and experiments were conducted for three consecutive years. Four treatments were assigned at random to individual trees: no treatment, herbicide to kill surrounding weeds, supplemental water, and both herb icide and supplemental water. Two isolates of each S. sapinea morphoty pe (A and B) were used to inoculate wounded lateral shoots. Disease de velopment was measured as the maximum distance below the inoculation s ite at which necrotic needles were observed. Nonwatered trees with com peting vegetation (nontreated condition) had significantly lower preda wn needle water potentials (more water stress) and more severe disease development than trees that received the herbicide, water, or combine d herbicide and water treatments. The most severe disease occurred in the driest year and the least in the wettest year. Competing vegetatio n indirectly affected disease development by inducing water stress, ev en in relatively moist years, on trees previously considered well esta blished. Isolates of morphotype A were more aggressive than isolates o f morphotype B. Conclusions from this research have implications for s ustainable management of the region's conifer forests.