Hedera: Biology and epidemiology of Xanthomonas leaf spot

• All bacterial isolates from diseased ivy, identified as Xanthomonas, were pathogenic on Hedera helix cv. Green Ripple and can therefore be considered as Xanthomonas hortorum pv. hederae (Xhh); none of the non-Xanthomonas isolates were pathogenic. 
• Contrary to reports from the USA, isolates of Xhh did not infect Brassaia (syn. Schefflera) actinophylla.
• Carbon utilisation tests and DNA-fingerprinting indicated that all isolates of Xhh are very similar. 
• Thirty-five ivy cultivars were tested for resistance to ten isolates of Xhh. Most were fully susceptible, and none could be considered resistant, although some (H. rhombea, H. rhombea cv Variegata and H. helix cv. Tanja) were ‘less susceptible’. 
• There was no evidence for the existence of pathogen races, i.e. all isolates gave a similar responses on all cultivars. 
• Two new semi-selective media, Brilliant Cresol Blue Cellobiose (BCBC) and modified Tween, were developed for use in epidemiology studies using data from carbon utilisation and antibiotic sensitivity tests.
• An antiserum to Xhh was produced for detection and rapid confirmation of the identity of isolates from selective media plates.
• Nursery-studies showed that Xhh could be present on stock plants with no visible symptoms of disease.
• The numbers of Xhh recovered from the cuttings decreased during rooting.
• Cross-infection between trays of cuttings was only detected after they had rooted, polythene covers had been removed and overhead watering started.
• Results from one nursery indicated that plants with visible symptoms provide a more important source of inoculum than plants without.

Bacterial leaf spot of ivy (Hedera spp.), caused by Xanthomonas hortorum pv. hederae was identified as the second most prevalent bacterial disease during a survey of hardy nursery stock (HNS 71: Roberts, 1997) carried out on behalf of HDC during 1996/97. The pathogen was found to be present at seven of the eight nurseries visited and on a range of Hedera spp. growing both under protection and in the open and at all stages of production – rooted cuttings, liners and finals. Primary symptoms of the disease are irregular, dark, water-soaked spots/areas on the leaves. Severe symptoms can result in defoliation or even plant death. Plants become unsaleable due to a poor appearance resulting from the leaf spots or due to lack of foliage and dieback. The problems experienced by some growers have been so severe that they have ceased production of ivies. 

Copper sprays have been used by some growers in an attempt to control the disease, but apparently with little success. In the absence of chemical control agents, control measures should be based on disease avoidance and/or disease resistance. However, there was almost no information on this disease available in the scientific literature, in particular, there was no information on pathogenic variation (i.e. races), host resistance, epidemiology (i.e. sources of infection, survival of the pathogen, mechanism of spread) and diagnosis. Hence it was impossible to target control measures. There was therefore a clear need to gain some understanding of the basic biology and mechanisms of infection for this disease, to determine the existence of any race/cultivar specific interactions, to develop diagnostic reagents and typing methods and thence determine the primary sources of the pathogen. 

The commercial objective of this project was to use bacterial leaf spot of ivy as a model patho-system to provide basic information on the biology and epidemiology of bacterial diseases of HNS which can be used in the development of an effective integrated control strategy. It was anticipated that the principles developed could also be applied to a wider range of bacterial diseases of HNS. The choice of this particular crop/pathogen system was based not only of its high degree of importance to the industry but also because it represents an ideal model system for carrying out research in terms of availability of plant material, range and variation of species/varieties, ease and convenience of inoculation and symptom production. 

The main objectives of this project were to:  

• Establish a collection of isolates of the pathogen. 
• Characterise isolates using physiological tests and DNA fingerprinting to assess phenotypic and genetic variability. 
• Develop a routine method for pathogenicity testing. 
• Determine if there is any pathogenic variability amongst different strains of the pathogen. 

• Screen a collection of ivy cultivars for resistance to a range of isolates of the pathogen. 
• Develop a routine detection method for use in epidemiological studies. 
• Conduct epidemiological studies to identify the primary source(s) of the pathogen and conditions for spread and infection.