The fungus Bremia lactucae, responsible for downy mildew in lettuce, belongs to a fungal group including Pythium and Phytophthora. Like the group's other members, downy mildew is causing growers unrelenting and increasingly complex problems.
As in any crop protection strategy, the use by growers of a range of actives from different chemical groups within the control strategy is something that I would strongly advocate.
Within lettuce crops there can be up to three weeks between downy mildew spores landing in the crop and the appearance of symptoms. An integrated control strategy should therefore aim to use complementary active ingredients to achieve long-term control as well as helping avoid an over-reliance on certain actives.
The use of products such as Aliette 80WG (fosetyl aluminium) is one such fungicide option that should be incorporated within the spray programme as an effective treatment against downy mildew.
Importantly, there is no known resistance of downy mildew to Aliette 80 WG in the UK and as a result Aliette has come to form the cornerstone fungicide treatment in downy mildew control for many lettuce growers to give them the best chance against the disease.
Aliette 80WG is a systemic fungicide that can provide protection within the plant - protecting new growth so new roots, new shoots and new leaves are all protected. There is also definite activity on the soil pathogens Pythium and Phytophthora.
Growers have the opportunity to incorporate Aliette into the control strategy under a specific off-label approval that permits application at either propagation or following planting out in lettuce crops. It allows application as either a spray or drench treatment, subject to a 14-day harvest interval. As with any off-label approval, growers are reminded that use of the product is at their own risk.
Growers planting lettuce that has been treated in the propagation stage have also noticed that the young plants "get away" more quickly with improved root growth L
- Alan Horgan is a technical officer at Certis
RESISTANCE & HYGIENE
Rijk Zwaan salad crops account manager Gerard van der Hut says: "Although Rijk Zwaan is constantly developing varieties with resistance to evolving mildew strains, the problem lies in that new races and strains of the disease are constantly appearing. "There are currently 27 known races and new ones are appearing at an alarming rate."
Combining protective chemistry with the use of tolerant varieties is therefore vital. "It would not be sensible to rely entirely on varietal resistance. An effective supporting fungicide programme is very important," he explains.
Hygiene is also critical for growers looking to further reduce the risk, says Ian Gillott, author of the assured protocol on outdoor lettuce. "By leaving infected crop residues in the field for any length of time after harvest, you encourage wind-blown spores to spread very quickly into adjoining fields, so get rid of the infected trash as soon as possible post harvest," he says.
Due to the disease's ability to survive as oospores (resting spores), rotations are another significant factor. "With some growers constantly growing lettuce crop after lettuce crop, without a suitable rotation, sources of inoculum in the soil will build up that are capable of infecting new crops. The disease will develop rapidly when conditions are favourable," says Gillott.
These conditions include leaf wetness and warm temperatures that will encourage germination of the spores and the spread of the disease.
"When you see heavy dews that have developed overnight on your car on a July morning combined with warm days, then it's time to be aware that conditions are ripe for the development of downy mildew," he adds.
"Although there's no simple answer, resistant varieties and good hygiene combined with good rotations and a diverse fungicide programme will give you a fighting chance."