DISEASE GRADIENT OF CRONARTIUM-FLACCIDUM ON ITS INTERMEDIATE HOST VINCETOXICUM-HIRUNDINARIA

The disease gradient of Cronartium flaccidum, a bark blister rust on t wo-needled-pines, was examined on the intermediate host Vincetoxicum h irundinaria. A 500 x 50 m transect containing 100 pine trees with spor ulating aeciospores was laid out. Vincetoxicum plants were stationed 1 0 - 100 - 500 and 1,000 m away from the edge of the transect. In each V. hirundinaria growth area, two spore traps were set up to determine the amount of inoculum transfer to that area. The traps were operated at the time believed to be that of maximum sporulation: 12 noon - 2 p. m. Spore counts were taken 15 - 30 - 45 and 60 days after the first ae cia appeared on the pine trees. The disease gradient was calculated on the basis of the number of uredia on three pairs of Vincetoxicum leav es, from the lower, middle and upper part of the plant, respectively, according to the exponential model. The mumber of aeciospores captured varied with the distance from the inoculum source and with the inspec tion times. Differences between sites and between inspection times wer e highly significant. The disease gradient decreased with increasing d istance from the inoculum source. Increasing the distance from 10 to 1 ,000 m brought a 90.8% decrease in the number of uredia. It is recomme nded that new pine plantations should be laid out at a safe distance f rom the intermediate host, the distribution of which in Italy is alrea dy known.