DIVERSITY OF FUNGI COLONIZING LEAVES OF RHODODENDRON ( Rhododendron L . ) CUTTINGS

Rhododendrons (Rhododendron L.) are shrubs whose attractiveness is determined by their multi-coloured flowers and evergreen leaves. Necroses visible on the leaves of rhododendron cuttings diminish the suitability of nursery material for marketing. These symptoms are most frequently caused by fungi. The investigations were conducted in 2010–2011 in an ornamental shrub nursery to identify fungi colonizing the phyllosphere of rhododendron cuttings and causing leaf necroses. The material for analysis consisted of leaves of 11 rhododendron cultivars. 550 leaves were collected from 110 half-yearold cuttings for mycological analysis. Over 350 fungal colonies belonging to 15 species were isolated from the leaves of rhododendron cuttings. The dominants included: Pestalotiopsis sydowiana, Trichoderma koningii and Alternaria alternata. The influents included: Aspergillus brasiliensis, Mucor hiemalis f. hiemalis, Epicoccum nigrum, Sordaria fimicola and Umbelopsis isabellina. A large majority of the fungi preferred the phyllosphere environment of Yakushima rhododendron (R. yakushimanum) cultivars ‘Sneezy’ and ‘Golden Torch’ as well as of the large-flowered cultivars ‘Flautando’, ‘Dominik’, and ‘Simona’. The phyllosphere of the large-flowered cultivars ‘Bernstein’, ‘Nova Zembla’, and ‘Goldbuckett’ was a reservoir for many fungal colonies and fungi species. The cultivars less susceptible to colonization by fungi and the most promising for planting in green areas and home gardens are the large-flowered cultivars ‘Bernstein’, ‘Nova Zembla’, ‘Goldbuckett’, ‘Rasputin’, and ‘Roseum Elegans’.


INTRODUCTION
Evergreen rhododendron (Rhododendron L.) shrubs owe their attractiveness not only to flowers enchanting with their colours and smell, but also to the leaves forming the canopy.Necroses greatly decreasing the suitability of nursery material for marketing have been diagnosed on leaves of rhododendron cuttings in production nurseries.Symptoms of discolouration, leaf spot and necroses resulting in dieback and leaf fall in long-standing shrubs were presented in the papers by K o w a l i k and M u r a s [1] as well as by K o w a l i k et al. [2][3][4], whereas the agents of leaf infection (fungi and fungus-like organisms) were identified in the works of K o w a l i k [5,6] and K o w a l i k et al. [7,8].
Determining the agents causing necroses on the leaves of rhododendron cuttings provides the possibility to select cultivars less susceptible to colonization by pathogens causing dieback and falling of leaves to be planted in green areas and home gardens.
The aim of the study was to identify fungi living in the phyllosphere of rhododendron cuttings and causing leaf necrotic spots.

MATERIALS AND METHODS
The research was conducted in 2010-2011 on cuttings of evergreen rhododendron (Rhododendron L.) in an ornamental shrub nursery in the region of Silesia.
The analyzed material consisted of 11 rhododendron cultivars: R. brachycarpum 'Flautando', R. calophytum 'Dominik', R. catawbiense 'Nova Zembla', 'Rasputin', 'Roseum Elegans', 'Simona', R. dichroantum 'Bernstein', R. forrestii 'Baden Baden', R. wardii 'Goldbuckett', R. yakushimanum 'Golden Torch' and 'Sneezy'.550 leaves showing necrosis were collected from 110 half-year-old cuttings and used for mycological analysis.The analysis was conducted using standard methods applied in mycology [5].Fungi were isolated from the border of the living and necrotic tissue.Isolated fungal colonies were classified using the following keys: D o m s c h et al. [9], E l l i s and E l l i s [10], G u b a [11] and S u t t o n [12].The system of K i r k et al. [13] was accepted as the basis of the classification and the authors' epithets were quoted by the fungi names after I n d e x F u n g o r u m [14].
In terms of the proportional share of species in the whole community of isolated fungi, division was made after K o w a l i k [15] into dominants, influents and accessory fungi.

RESULTS
Over 350 fungal colonies related to 15 species were isolated from the leaves of evergreen rhododendron (Rhododendron L.) cuttings showing necrosis.
Between 12 and 51 colonies belonging to between 3 and 7 species were isolated from the leaves of cuttings of the individual rhododendron cultivars.The highest number of fungal colonies were identified on the leaves of R. yakushimanum 'Sneezy', a slightly lower number was isolated from the large-flowered cultivars R. brachycarpum 'Flautando', R. calophytum 'Dominik', and R. catawbiense 'Simona'.A comparable number of fungal colonies were isolated from Forrest's 'Baden-Baden' and Yakushima 'Golden Torch' rhododendrons.The least number of colonies and fungal species were identified on the leaves of the largeflowered varieties R. dichroantum 'Bernstein' and R. catawbiense 'Nova Zembla'.The greatest diversity of fungi was observed in the phyllosphere of largeflowered rhododendrons R. catawbiense 'Simona' and R. yakushimanum 'Sneezy' (Table 2).

DISCUSSION
A comparison of the fungal communities isolated from diseased cuttings and leaves of long-standing rhododendron shrubs revealed that A. alternata and Pestalopsis sydowiana, syn.Pestalotia sydowiana [2,3,6,8] dominated in the populations.Frequently occurring Botrytis cinerea, Septoria azalea, Gloeosporium rhododendri and fungi of the genera Mortierella and Mucor were also observed in these communities.These fungi can be considered to play the main role in the leaf necrotizing process and in infection of rhododendron inflorescence buds [1,4,8,16].
Botrytis cinerea, Septoria azalea, Gloeosporium rhododendri and fungi of the following genera: Aspergillus, Chaetomium, Cladosporium, Cylindrocarpon, Fusarium, Humicola, Penicilium, Phialophora, Phoma, Rhizoctonia and other [5,6,8], were also found on the leaves with necrotic spots of rhododendrons and azalea of many years' growth, whereas the same fungi did not occur in the phyllosphere of rhododendron cuttings.Numerous occurrences of Trichoderma koningii were recorded on the leaves of rhododendron cuttings, whereas the fungus was observed only sporadically on azalea leaves [7].
Individual cases of the fungus Harzia velata, isolated from the leaves of rhododendron cuttings, have not been found so far in the phyllosphere of evergreen rhododendrons or leaf dropping azaleas.
In the literature of the subject, Phytophthora spp.fungus-like organisms are reported among the most dangerous pathogens that worsen the health of rhododendrons in nurseries [19].The conducted mycological analysis did not find these pathogens either on the leaves or on the root system of cuttings, which is evidence that the tested material was acquired for nursery production.
A vast majority of the fungi preferred the phyllosphere environment of the Yakushima rhododendron cultivars ‹Sneezy› and ‹Golden Torch› and of the largeflowered cultivars R. brachycarpum 'Flautando', R. calophytum 'Dominik', and R. catawbiense 'Simona'.The phyllosphere environment of the large-flowered varieties R. dichroantum 'Bernstein', R. catawbiense 'Nova Zembla', and R. wardii 'Goldbuckett' was the reservoir for few colonies and species of fungi.The cultivars that were less susceptible to fungal colonization and best suited for planting in green areas and home gardens are the following varieties of largeflowered rhododendrons: R. dichroantum 'Bernstein', R. catawbiense 'Nova Zembla', 'Rasputin', 'Roseum Elegans', and R. wardii 'Goldbuckett'.

CONCLUSIONS
Handling Editor: Elżbieta Weryszko-Chmielewska This is an Open Access digital version of the article distributed under the terms of the Creative Commons Attribution 3.0 License (creativecommons.org/licenses/by/3.0/),which permits redistribution, commercial and non-commercial, provided that the article is properly cited.©The Author(s) 2014 Published by Polish Botanical Society

Table 2
Number of fungal colonies and species on the leaves of cuttings of particular rhododendron cultivars