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The epidemiology and control of heuchera rust
Puccinia heucherae, rust, epidemiology, detection
[Above] Symptoms of heuchera rust on a leaf; details of rust pustules and teliospores
Heuchera rust, caused by the fungus Puccinia heucherae, was first detected in UK gardens in 2004 on a leaf sent to RHS Gardening Advice (Henricot et al., 2007). It was subsequently detected in nurseries in 2005, and by 2010 it was reported from across the UK in both gardens and nurseries.
P. heucherae is widespread in the USA and East Asia but there are no previous records of rust disease recorded on Heuchera in the UK. The rust has proven to be widespread and difficult to eliminate as growers are unsure how and why heuchera plants are now becoming infected and how to control the spread of the fungus.
Heuchera rust results in small brown spots, sunken depressions or raised bumps on the upper leaf surface corresponding to raised, orange to brown spore-producing pustules on the underside of the leaves. The pustules may turn a greyish-white colour under humid conditions. Young leaves can become puckered and distorted and heavily-infected older leaves may turn brown and shrivel. These symptoms are unsightly and disfiguring to the plant. As a result AHDB Horticulture funded the RHS and ADAS in 2014 to undertake a project to improve the understanding of heuchera rust and its origin and occurrence in the UK.
Objective 1: Survey and review for further information relevant to heuchera rust incidence
1.1 A survey will be conducted of UK heuchera growers to understand the pathway of disease introduction and conditions in which the disease is most likely to occur.
1.2 A review of global knowledge of heuchera rust and relevant epidemiological information from rusts on other hosts will guide investigations into conditions favourable to heuchera rust.
Objective 2: Determine if latent rust infection can be detected in heuchera from nurseries
2.1 In the UK the closely related rust, P. saxifragae, has been found on the genus Saxifraga and in the USA this species has been recorded on Heuchera. The two species, P. heucherae and P. saxifragae are difficult to separate by morphological characteristics but can be differentiated by DNA analysis (Henricot et al., 2007). A molecular test will be developed to confirm if the infection is due to P. heucherae, or another rust species, and to detect the fungus on infected plants before rust pustules develop.
2.2 The molecular diagnostic test will used in combination with observation of quarantined symptomless plants to determine whether heuchera rust exists as a latent pathogen within the supply chain of heuchera in the UK. Points in the supply chain where rust is entering and/or causing disease will be established. Two growth conditions will be studied to determine whether growth conditions may be influencing the expression of latent infection.
[Above] Heuchera plants growing at the RHS Field Research Facility
Objective 3: Study the effects of leaf wetness period and timing in the day on plant infection by rust
Inoculation experiments will investigate how differing periods of leaf wetness, high humidity and light may affect rust infection incidence on heuchera plants. Knowledge of infection requirements will guide best practice procedures for growth of heuchera plants in nurseries.
This project will provide industry growers and home gardeners with a better understanding of the source and spread of heuchera rust on Heuchera plants. Communication of the results to industry partners will ensure that there continues to be enthusiasm to provide the wide range of Heuchera cultivars of various colours and forms to home gardeners.
The project will seek to share best practice on cultural control measures such as watering timing and hygiene measures. Information from a grower survey and experimental results will indicate the time of year or plant ages at which rust has a greater chance of infecting or being seen. This may guide fungicide control measures, whether to implement more stringent segregation or quarantine of plants and whether to alter current nursery grower practices such as overwintering heuchera plants.
RHS advice on heuchera rust
Henricot, B; Denton, G; and Lane, C (2007). First report of Puccinia heucherae on Heuchera spp. in the UK. Plant Pathology 56: 352.
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