Factors affecting dieback in the rare plant Hypericum edisonianum (Edison's St. John's-Wort).

摘要

Hypericum edisonianum (Small) Adams & Robson (Ascyrum edisonianum Small), a state-endangered plant found in only four counties in central Florida, has been experiencing sporadic diebacks with no known etiology. This study was initiated to determine the causes of diebacks and whether narrow genetic variation combined with unique environmental factors place the plant at risk from disease.; Two newly recorded fungal pathogens attack H. edisonianum. Collelotrichum gloeosporioides causes foliar and stem lesions in H. edisonianum that can ultimately lead to stem death. However, no C. gloeosporioides infestations were found in any wild populations. The originally infected stem appears to have been the result of contaminated greenhouse conditions. Sphaeropsis tumefaciens also attacks H. edisonianum, causing the formation of woody galls and witches' brooms. Disease incidence ranged from 0 to 83% within seven field populations of H. edisonianum however S. tumefaciens infection was only significant in one population from 1998 to 2000.; All study populations of the plant grew in seasonal ponds that were unique in their soil nutrient characters and hydrology. Taller stems of H. edisonianum were found centermost in most ponds and also contained the greatest number of galls. Plant stem heights were correlated with gradients in soil moisture, soil nutrients, organic matter content and soil saturation.; Amplified fragment length polymorphisms (AFLP) indicated considerable genetic diversity in 10 populations of H. edisonianum sampled from 3 counties.; A new, undescribed, leafmining microlepidopteran (Coleotechnites sp.) (Gelechiidae) inflicts extensive foliar damage on H. edisonianum but does not play a significant role in H. edisonianum stem mortality.; In summary, neither fungal pathogens nor insect infestations are solely responsible for the observed decline in H. edisonianum populations. Rather, environmental factors, particularly drought conditions, play a pivotal role in the synergistic effects of both disease and insect damage.