UK: Improving knowledge and control of Downy Mildew

Downy mildew in basil is a devastating disease caused by the plant pathogen Peronospora belbahrii. Work on the disease had already determined the conditions required for sporulation, however little work has been done to address the sources of infection, conditions required for infection and spore survival, or the most effective ways to control the disease. The current project aimed to address these knowledge gaps through a series of field, glasshouse and laboratory experiments as follows, (i) PCR analysis and seed washing to establish whether the main source of infection is through seed contamination; (ii) epidemiological studies to determine how light, temperature and leaf wetness affect infection and whether oospores are produced and spore survival (iii) determine whether alternate hosts for P. belbahrii exist within the Lamiaceae family; (iv) establish strategies for disease control (cultural (lighting) and chemical).

Data from the project has established
1) That most of the seed samples tested contained P. belbahrii DNA. As PCR testing cannot distinguish between viable and non-viable DNA it was not clear whether the DNA detected in the seed was infective and so could act as a source of infection. No oospores were detected in any of the seed washings which suggested that P. belbahrii was not present in basil seed lots as surface contaminant but as an internal contaminant of the seed. To try and establish whether infections could be established from seed containing P. belbahrii DNA a number of small scale trials were set up however, these did not lead to any downy mildew infections developing in the crop. This lack of infection may be due to low levels of infective seed within any seed lot, as a result much more extensive set of trials are required to try and confirm that P. belbahrii infections are seed borne.
 
2) Epidemiological studies indicated that infection of basil by P. belbahrii occurred in the dark but not in the light, suggesting that basil downy mildew infections were likely to occur overnight rather than during the day.

Downy mildew infection was highly dependent on both temperature and leaf wetness. Infection occurred when plants were incubated at temperatures between 5 and 25ºC, with limited or no infection at 30ºC irrespective of leaf wetness duration. No infection occurred when leaves were wet for 2 hours or less. Based on these data and data on the effect of humidity on leaf wetness a risk grid has been produced to help growers determine when basil plants were most at risk of infection by downy mildew.

Oospores of P. belbahrii were not found in any infected plant material examined or soil samples taken from fields containing basil downy mildew infected crops. The presence of P. belbahrii DNA and the absence of oospores in soil samples suggested that the DNA was from contaminated crop debris and not infective propagules.

3) Fourteen plant species from across the Lamiaceae genus were tested for susceptibility to P. belbahrii. Of the plant species tested agastache, lavender, common sage and catnip were the only ones which showed symptoms associated with P. belbahrii. Profuse sporulation was observed following infection of agastache and lavender, sporulation was sparse following infection of common sage and no sporulation was observed on catnip. This is the first report of lavender and catnip being possible alternate hosts for P. belbahrii. All the alternate hosts identified were herb crops so growers should take care if growing the alternate host crops at the same time as basil. The lack of weed crops in the list of alternate hosts should make disease management easier as there appears to be no route for overwintering/spread of P. belbahrii via these plants.

4) An outdoor trial was established during the summer of 2016 using 10 individual treatments and three programmes. Disease control was more effective following the application of programmes than the individual product. The most effective programme consisted of treatments with Revus (mandipropamid) and a coded product HDC F226, however programmes using Fubol gold (metalaxyl-M + mancozeb) and Revus, and Fenomenal (fenamidone + fosetyl-aluminium) and Revus also proved effective.

Five products identified as having potential for the control of basil downy mildew were tested to establish the most appropriate time between fungicide applications. Paraat (dimethomorph), Fenomenal (fenamidone + fosetyl-aluminium) and Revus (mandipropimid), provided good protective activity when applied up to ten days prior to infection and as a result have the potential to be used as part of a weekly fungicide programme for the prevention of downy mildew infection of basil. As part of this testing it became apparent that Fubol Gold (metalaxyl-M + mancozeb) was ineffective against the isolate being used. This, and subsequent testing, indicated that the isolate used was resistant to metalaxyl-M isolate was used in this trial. Resistance to metalaxyl-M could lead to disease control problems as this active is used in both seed and foliar treatments as a result it is vital to establish the extent of metalaxyl-M resistance within the P. belbahrii population.
 
Aim: The project aims to generate data that will improve the control of basil downy mildew on crops grown under protection and outdoors

Objectives
  1. Determine whether P. belbahrii infections can be transmitted to plants from P. belbahrii contaminated seed
  2. Determine the level of Metalaxyl-M resistance in P. belbahrii
  3. Dissemination of outputs from project through best practice guidelines, an article in the AHDB Grower Magazine and presentation at appropriate grower meetings
Benefits to industry:
The additional work will befit the industry in two main ways,

i) Establishing whether the occurrence of the pathogen in UK seed stocks leads to crop infection will allow the industry to set in place measures to remove the source of infection. The measures taken will depend on whether the disease is truly seed-borne (systemic) or exists as a contaminant on the seed surface.

ii) Understanding the prevalence of metalaxyl-M resistance within the P. belbahrii population will allow levy payers to evaluate the threat and to adopt or adapt growing practices which will help them best mitigate losses from downy mildew.
 
Source: AHDB Horticulture

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