The Response of Boxwood Accesstions from Hyrcanian Forest Areas of Northern Iran to Blight Disease (Agent: Calonectria pseudonaviculata)

The Caspian boxwood (Buxus sempervirens subsp. hyrcana) trees are endangered, evergreen and endemic species in the Hyrcanian forests areas of northern Iran growing in Guilan, Mazandaran and Golestan provinces. This protected evergreen trees species is considered a unique and genetically valuable forest resource for Iran. The habitats of this valuable species, which is located in the lowlands, like other similar trees, always faced with overharvesting problems. But in addition to excessive harvesting, box tree moth Cydalima perspectalis and boxwood blight disease caused by the pathogen Calonectria pseudonaviculata (also known as box blight or buxus blight), as growing problems, are additional causes of boxwood habitats degradation. the outbreak of caspian buxus die-back due to boxwood blight disease was first reported in the summer of 2012 in the buxus forests located in the north of Iran and quickly infected more than 70% of the boxwood forests in the northern part of the country. Since then, there was intense concern that this disease might be the end of boxwood plants, and due to the importance of the subject, various research projects were implemented in research centers and universities in Iran, like other countries in the world. Currently, processes related to biodiversity protection of Hyrcanian boxwood are performed by Natural Resources and Watershed Management Organization as the best protection strategies, in which there are two protection strategies (in situ/ex situ) with different techniques. As the current approach to disease management in the world largely depends on the use of tolerant cultivars, research into boxwood blight resistance continues, so new resistant cultivars continue to be introduced worldwide. However, despite the importance of Caspian boxwood trees, there are no such studies in Iran. The objective of this study was to fill this knowledge gap by evaluating possible hyrcanian buxus accessions resistant to boxwood blight as a further step in improving the effectiveness and efficiency of this species protection management in the conditions of re-facing with the disease. Results of this study will enable growers and landscapers to better manage box blight disease.

In order to investigate the resistance of boxwood accessions from hyrcanian Forest areas of northern Iran to blight disease, infected boxwood habitats were visited in Golestan, Mazandaran and Gilan provinces, and to find boxwood plants with natural resistance, healthy buxux seedlings were collected from these habitats. Because in these types of studies, the process of resistance assessment begins by searching in natural forests for trees that remain healthy relative to their counterparts after exposure to an interest pathogen. In order to ensure buxus seedlings health, the seedlings that were collected from 12 boxwood forest habitats were kept for six months in the Alborz Research Complex (affiliated to the Research Institute of Forests and Rangelands of Iran) and their health was monitored. Finally, from the seedlings of each habitat, the freshest and most equal-sized seedlings were selected and used for greenhouse evaluations. In order to prepare the inoculum from the disease-causing fungus, the combination of four isolates of C. pseudonaviculata species, which had appropriate genetic diversity based on previous studies, was used. Seedlings were inoculated through foliar spraying of fungal spores (1× 105 spores/ml). Three weeks after inoculation, the average number of spots created on the stem and leaves of each boxwood plant as well as the average diameter of the largest leaf spot for each boxwood accession were recorded and analyzed.

The results of inoculation of blight pathogen on boxwood accessions showed that there were similar results in measuring the diameter of the leaf spot with the number of blight spots in each accession, so that, in plants with a higher number of blight spots, there was also a larger diameter of the leaf spot. Also, the results showed that although no accession is completely resistant to this disease and the disease appeared with different degrees in all accession; but a number of accessions are less sensitive to boxwood blight and the disease occurred in them with a lower severity. These results suggest to breeders to find natural resistance or create resistance in boxwood plants against blight disease through breeding programs.

In this study, for the first time, the resistance of different accessions of Buxus sempervirens subsp. hyrcana from boxwood habitats in the north of the country (as the only native and endangered boxwood species of Iran) to blight disease was evaluated. Based on the obtained results, it was determined that some degrees of disease tolerance can be found among different Hyrkani boxwood accessions. These results show that there is potential for plant breeders to select and recommend less susceptible boxwoods or to develop cultivars with improved resistance to boxwood blight.