Rust resistance of some varieties and recently bred genotypes of biomass willows

Abstract

Rust resistance of 23 commonly grown biomass willow varieties and 71 genotypes produced from a UK-based willow-breeding programme was tested in laboratory inoculation experiments. Leaf discs were inoculated with six pathotypes of Melampsora larici-epitea and disease was scored based on uredinial pustule area and inoculum density. In the field, rust severity of the same varieties and genotypes was scored using an assessment key. Of the varieties and commonly grown willows tested, several North American willows and Salix schwerinii×Salix viminalis ‘Bjorn’ and ‘Tora’ were highly resistant. Overall, the great majority of the willows produced through breeding showed high levels of resistance to rust. There was a significant correlation between infection-type scores obtained from the laboratory testing and field rust severity (Spearman's rank correlation coefficient=0.624, P<1×10⁻⁶). There was also a good agreement in field disease severity scores between the years (Kendall's coefficient of concordance=0.669, P<0.001).

Introduction

Rust caused by Melampsora is the most serious disease of willows grown in short-rotation coppice (SRC) plantations for renewable energy. Rust defoliates susceptible plantings prematurely and, when severe, reduces yields by as much as 40% [1]. The most widespread and destructive Melampsora species on SRC willow is M. larici-epitea Kleb. In the UK, a number of rust pathotypes (a pathotype is defined as one or a group of pathogen genotypes showing the same virulence/avirulence patterns on a set of host differentials) were identified under formae speciales larici-epitea typica (LET), larici-retusae (LR) and larici-daphnoides of M. larici-epitea [2], [3]. Studies using a clone/rust interaction trial network [3] showed that most of the 24 clones grown in the trial were infected by LET or LR types. Among the LET types, LET1 was the most prevalent, causing severe infections on several Salix viminalis clones each year. The host range of the pathotypes belonging to LR was restricted to Salix burjatica/Salix dasyclados clones.

Breeding for disease resistance is a priority in many crop systems. Deployment of resistance genes against pathogens can be particularly important in low-input crop systems such as SRC willow. Studies have shown that there are abundant sources of resistance in Salix against rust [2], [4]. Willows are dioecious (occurring as male or female), hybridise with relative ease, and those used for biomass production usually reach sexual maturity within 1–2 years. Furthermore, new genotypes can be multiplied in a relatively short period of time as biomass willows are propagated by cuttings. These attributes make breeding for resistance a favourable option to combat rust disease.

Biomass willow breeding in Europe first started at Svalöf Weibull AB, Sweden, in 1987 [5]. Through breeding, the average yield has been increased by 50% and levels of leaf rust reduced considerably. A UK-based willow-breeding programme (the European Willow Breeding Partnership (EWBP)) was initiated in 1996 under the partnership of IACR-Long Ashton, Svalöf Weibull AB and Murray Carter Ltd., with the aim of producing high-yielding, disease- and pest-resistant varieties for the biomass energy industry. Over the years, the EWBP produced a wide range of crosses. Many new genotypes were selected for their yield potential and disease and pest resistance and grown in field trials for further evaluation. Although the EWBP was dissolved in 2002, due to the closure of IACR-Long Ashton, UK-based breeding efforts have been continuing at Rothamsted Research, Hertfordshire, UK, funded by the Department for Environment, Food and Rural Affairs, UK.

This study was carried out to characterise rust resistance in selected willow genotypes produced by the EWBP. A number of existing biomass willow varieties and commonly grown genotypes were included in the study to provide a comparison between the newly bred genotypes and existing varieties.