3.7.51
STEM CANKER ON SEA BUCKTHORN (HIPPOPHAE RHAMNOIDES L.) IN FINLAND

P PARIKKA 1 and S KARHU2

1Agric. Res. Centre of Finland, Plant Production Research, Plant Protection; 2Agric. Res. Centre of Finland, Plant Production Research, Horticulture, Finland

Background
Sea buckthorn (Hippophae rhamnoides L.) is a plant species native to the Baltic Sea region. Cultivation of sea buckthorn has recently aroused interest, and Russian varieties have been introduced for berry production. Breeding programmes in the70s and 80s at the Agricultural Research Centre of Finland produced hybrids of Russian varieties and wild strains originating from the Baltic Sea region. Two varieties, the female Raisa and the male Rudolf have already been introduced from this material. Plant diseases became a problem in growing sea buckthorn only in the early 90s. Stem cankers were detected for the first time in 1992.

Materials and methods
The breeding material of sea buckthorn was planted for selection in 1989 at four sites, Piikki6 and KokemAki in Southern Finland and Ruukki and Sotkamo in Northern Finland. Stem cankers were observed in late summer or autumn during 1993 - 1997. The injuries to the plants were graded from 0 (none) to 5 (very severe). Growth and spore production of the pathogen isolated from sea buckthorn were tested on agar plates at different temperatures (OoC - +30oC) on three agar media. Infection and disease development was tested on young plants with mycelium placed on buds or small wounds.

Results and discussion
A dematiaceous fungus species, Stigmina Sacc. (Thyrostroma Hohnel.) was isolated from stem cankers. Fungi pathogenic to sea buckthorn have been investigated in Russia [1] but this species has not been reported. In Estonia Verticillium dahliae is suspected of being the fungus which causes wilting [2]. The optimum of growth and spore production of Stigmina sp. are at + 15o - +20oC and maximum at +25oC. Mycelial growth and spore production occur at at a temperature as low as 0 - +50C. The fungus placed on buds of healthy but susceptible sea buckthorn in autumn caused small cankers around the buds during autumn and spring. Stigmina sp. causes stem cankers on stems and twigs of sea buckthorn. The lesions formed around buds are first reddish and later turn dark brown and sunken. The cankers are elliptical, measuring from 0. 5 to 7 cm in length and girdle the stems which wilt or produce abnormal growth. Stigmina sp. produces spores on dead buds and cankers. The mycelium grows under the bark and the spose mass protru- de the cracking surface . The dark olivaceous spore mass contains conidia measuring 40 - 66 x 20 Ám.

The stem cankers were widespread in all the experimental fields of sea buckthorn and the severity of the disease increased during the observation period. Most heavily infected were the Russian varieties originating in the Altai region and their progenies. The variety Vitaminnaya had severe cankers at all four experimental sites, the injuries being most severe near the coast (Piikki6 and Ruukki). The average infection level of the variety increased in four years (1993-97) from 0.8 to 3.5 at Sotkamo. At Piikki6 the increase was from 2.5 to 5 while at Ruukki the infection level of Vitaminnaya was constanly 4.5 to 4. The Finnish native strain Siipyy had very few small cankers. The variety Raisa, a hybrid of Finnish and Caucasian wild strain, was infected during the observation period and had mild to rather severe infection depending on the site. At Sotkamo the average disease level of Raisa plants increased from 0 in 1993 to 1.5 in 1997. Stem canker was affected by climatic conditions, the injuries being most severe where autumn temperatures were high. Stigmina sp. has not previously been re-ported in native Finnish sea buckthorn stands. In 1997 we did, however, detect it in plants growing in the Aland islands in South-Western Finland.

References
1. Zhukov A. M. 1972. Mikol. i Fitopatol. 6:322-327.
2. Siimisker, T. 1996. J. Agric. Sci. (Estonia) 7: 177-181.