Horticulture notes April 2009
Downy Mildew Alert
David Kerr, the CAFRE Ornamental Crops Adviser, is warning of the need to be vigilant for downy mildew fungus on various bedding plants and pot crops this season.
David commented: “Growers of Impatiens (Busy Lizzie) should certainly watch for symptoms of downy mildew this year.” David continued: “Impatiens downy mildew (Plasmopara obducans) was first identified in the UK in 2003. Last season it caused production and garden problems in European markets.”
Generally, downy mildew symptoms are stunted, distorted plant growth and pale foliage. These symptoms result from systemic infection i.e. there is fungal growth within the plant tissues. Secondary spread can result in discoloured fungal patches, often angular in shape, on the upper leaf surface. 'Downy’ growth may be found on the leaf underside.
However, downy mildew may be present but symptomless in early infection, so infected plugs that arrive at nurseries may not show signs of disease initially. Downy Mildew fungi cause disease on a wide range of ornamental plants including:
- Antirrhinum (snapdragon)
- Pansy, Viola
- Matthiola (Stock)
- Impatiens (Busy Lizzie) especially Impatiens wallerana
- Hebe
David also summarised some characteristics of this disease, namely;
- Symptoms vary between species and in Impatiens the foliage may turn yellow and drop prematurely, leaving bare stems.
- Downy mildew fungi tend to be host specific, so they don’t cross-infect other species of plants.
- Development of downy mildew is favoured by conditions of high humidity and periods of leaf wetness.
- Disease spores may spread by air movement, water splash or physical movement, via people or equipment.
There are a number of approaches toprevention and disease control:
- Reduce humidity through ventilation, or heating plus ventilation. Use fans to improve air flow, especially in the early morning to avoid condensation.
- Avoid overhead irrigation unless there is enough time for leaves to dry before dark.
- Clean plant debris and compost residues from growing areas before the arrival of new plants.
- Where possible, use new plug trays, pots and packs.
- Check plants/plugs bought in and keep them isolated for a period.
- If a small number of plants are infected, remove them and dispose of carefully.
The above measures should reduce the need for fungicides but where a fungicide programme is required, there are
some guidelines:
- Use fungicides from chemical groups which have different modes of action to reduce resistance.
- Always test whether a spray is crop safe by trying on a small number of plants.
- It is important to achieve good leaf cover.
- Do not mix fosetyl-aluminium with anything else, as phytotoxicity can occur.
- Reduce spray intervals to 7 days if downy mildew is present.
- Broad spectrum protectants e.g. chlorothalonil (Bravo) or mancozeb can be used followed by a more specific product for downy mildew such as mancozeb + Metalaxyl (Fubol Gold).
- ‘Aliette’ (fosetyl-aluminium) stimulates the plant to produce substances to help it defend itself.
- Strobilurins (azoxystrobin or kresoximmethyl) are broad spectrum and effective against downy mildew.
- Always check and follow instructions on product labels before use.
Please contact me if you need further information.
Top Fruit - Sprayer Checks
You’ve started your spray programme against apple scab, but are you sure your sprayer is at its best performance? A routine, weekly, sprayer check before operating can prevent failures in output and potentially damaging leaks. Some of the main faults that arise are:
- Nozzles wearing out
- Hoses cracked/hardening
- Dirty and perished filters
- Worn ‘O’ rings and seals
- Pressure gauge inaccurate
Correcting these common faults will maintain accurate spray dosages every time and save you money in the long term.
New Guinea Impatiens
