Aerial oomycetes: A review of management and control options available for the UK horticulture industry

Summary: The Oomycetes are a group of fungus-like organisms related to algae that cause diseases on a range of plant species. Some cause symptoms on aerial plant parts, includingPhytophthora species such as Phytophthora ramorum, Albugo species and downy mildews from genera including Peronospora, Plasmopara, Hyaloperonspora, Pseudoperonospora andBremia. Recently, foliar infection by some Pythium species has been recognised (Denton, 2014). Other Phytophthora and Pythium species cause emergence failures, damping-off, root rots and stem base diseases. Economically significant losses due to Oomycetes occur across the majority of horticultural production systems. They are often considered to be fairly ubiquitous in the environment, with some aerial oomycetes surviving in soil and growing media, and are associated with high leaf wetness duration. There had been recent industry concern over downy mildews not previously a problem in the UK including on basil, impatiens and aquilegia and aerial Phytophthora species including P. ramorum.

A large number of projects on oomycetes have been commissioned by the AHDB in recent years across the crop sectors. Work specifically targeted against aerial oomycetes has included downy mildews on rose (HNS 173), hardy nursery stock and herbaceous plants (HNS 186), onions (CP 099c) and peas (FV 436) plus Albugo sp. on brassicas (FV 053e) and Phytophthora white tip on leeks (FV 172). There have been recent investigations monitoring metalaxyl resistance in impatiens downy mildew (PO 011 and PO 11 a and b) and fungicide control (PO 012). Basil downy mildew research is to be funded. HNS 185 on understanding and managing crop protection through Integrated Crop Management includes grower responses on crops requiring treatment for downy mildews and the level of satisfaction with the level of control achieved.

Research on root and stem rotting oomycetes (as listed in the 1^st October 2012 workshop for growers) can also be applicable to oomycetes that show leaf and shoot infection. AHDB has funded a recent review of worldwide research on root-rotting oomycetes (CP 126) and provided knowledge transfer of the outcomes (CP 128) with information on diagnostics and monitoring, and chemical and biological control particularly applicable to aerial oomycetes. Diagnostic techniques are being developed to identify, detect or quantify pathogens (CP 136). Molecular methods have been used for roots, substrates and soils (PC 281), conifers (HNS/PO 181) raspberries (SF 130) and carrot (FV 353) that should apply to aerial oomycetes. Strategies for disease control for organically grown field vegetables were examined in the DOVE project (Gladders, 2002).

Techniques which can be used by growers (baiting and lateral flow devices) for detection of the pathogens in water (HNS/PO 188) have relevance to the dispersal phases of aerial Phytophthora and Pythium species. Work on monitoring P. ramorum in the field has led to the development and use by UK plant health inspectors of portable molecular diagnostic equipment. Chemical and biological oomycete control products have been tested for strawberries (SF 121), raspberries (SF 123) and carrots (FV 391). There is a vast body of oomycete research globally. For example, chemical and biological control products have been investigated in the USA IR4 programmes and also in the UK Sustainable Crop and Environment Protection – Targeted Research for Edibles project (SCEPTRE). European reviews have been conducted on the use of biocontrol products and integrated crop management (ENDURE, 2009).

Resting spores of aerial oomycetes are formed in plant tissue and will remain in soil and green waste that can be used in mulches. In the UK, composted waste materials, mulching and pathogen control has been the subject of projects in various crops funded by The Waste and Resources Action Programme (WRAP) (Noble and Roberts, 2003).

The detachable (caducous) sporangia of aerial oomycetes means that they can be carried in wind-blown rain and rain-splash and either land on hosts directly or enter irrigation water. The spread of Phytophthora ramorum in this way to certain trees and shrubs has been of national concern (Defra, 2005.). Water monitoring and treatment, including the use of slow sand filters and reed beds have been studied at various centres (Pettitt & Hutchinson, 2005).

The EU COST programme has involved work on the diagnostics and control of Phytophthora spp.. In the UK and USA there has been much recent research on Phytophthora ramorum epidemiology, diagnostics and chemical and cultural control measures.

The proposed study follows on from work by Tim Pettitt (CP 126 and CP 128) on root and stem infecting oomycetes and will focus instead on aerial oomycetes. There will be some cross-over between these desk-studies, in particular for Phytophthora and Pythium species with water-borne zoospores and when considering environmental, chemical and cultural control methods. Information from CP 126 and 128 that is relevant to aerial oomycetes will be incorporated within the reviews of the current study emphasising the key aspects relating to above ground infection.

It has been directed that as other AHDB work is planned on oomycete pathogens belonging to the Genus Bremia and those causing downy mildews of spinach and grapes these will not be reviewed. Potatoes fall outside the remit of AHDB Horticulture and so information on Phytophthora infestans will only be reviewed where information such as diagnostics and control methods could be applied to other known UK hosts (e.g. tomato). Crops within all of the AHDB Horticulture crop sectors will be examined for aerial oomycete problems: Field Vegetables (including brassicas, legumes, alliums, salads, asparagus, outdoor cucurbits, herbs and carrots), Protected Edibles (vine salad, leaf salad), protected and outdoor hardy nursery stock and protected ornamentals.

Aims: The primary objective is to undertake a literature review and collect data and grower comment across a broad range of aerial oomycete pathogens in order to identify where there are gaps in our knowledge, and so direct future research and knowledge exchange work programs.

The scope of this project has been defined by the AHDB to include crops within all of the AHDB sectors: Field Vegetables, Protected Edibles, Ornamentals (including protected and field grown nursery stock, bulbs and cut flowers), Mushrooms, Soft Fruit and Top Fruit.

Reviews will be carried out on aerial infections by downy mildews (except those of spinach, grapes and Bremia spp.) and species of Phytophthora (except P. infestans of potato) and Albugo.

Objectives:

1.    To survey stakeholders across crop sectors to determine which aerial oomycetes are having the greatest financial impact and specific factors contributing to this (AHDB Objective 4)

2.    To compile a list of AHDB projects and factsheets on the main crop-pathogen interactions and determine key outcomes (AHDB Objectives 1,2 & 3)

3.    To compile a list of UK and overseas research publications covering the main UK aerial oomycete diseases and to record the outcomes of those currently most relevant and gaps in scientific knowledge (AHDB Objectives 1,2 &3)

4.    To review future threats including pesticide resistance and emerging disease problems (AHDB Objectives 5)

5.    To produce a final report by 30 March 2016

6.    To produce two factsheets

7.    To produce an article for AHDB Horticulture magazine

8.    To deliver four presentations to grower groups