Swotting up on main threats to your crop
Swotting up on main threats to your crop
In this weeks technical
insight article we follow-up
on our detailed coverage of
Septoria two weeks ago by
taking a closer look at the
other key foliar diseases
of cereals
WHEAT DISEASES
Mildew
WITH more later drilled crops in the ground this year, the risk of mildew is higher than in previous years, says UAP technical director Chris Bean.
"These crops will grow faster, putting on leaves at the expense of laying down wax defence mechanisms. Some spring wheat varieties are also more susceptible to the disease."
Cool, humid conditions suit the fungus. "A dry, cool start to the season will bring mildew into crops. It doesnt like very wet weather, as the fungus tends to get washed off the leaf."
Growers will see white, fluffy growths on leaves and stems where the disease is present. Yield loss is believed to be as much as 20% in a bad year, although there is some debate about the figure.
"Crops which receive a lot of nitrogen are more at risk, as well as those grown on manganese deficient soils," adds Mr Bean. "And this year rooting may be poor in certain situations, so crops will be vulnerable."
He stresses that although there hasnt been much mildew around for the past two or three seasons, the inoculum is always present. "It is climate and variety which will affect whether it comes in, not the previous season."
Control
If mildew is developing on new growth across a field, spraying is advised. "If it gets a hold, it will be there throughout the season and will get onto the ears. But if you spray in time, it can be controlled easily." Fungicide choice is important as there have been shifts in sensitivity to the morpholines and the triazoles.
"And you must be careful with the strobilurins, which need to be partnered with another product."
Winter varieties at risk include Claire, Consort, Equinox, Genghis, Madrigal, Option, Reaper and Deben. Chablis is the worst of the spring wheats.
Whether you target the disease before or at T1 will depend on disease levels, advises Mr Bean. "Choose Fortress (quinoxyfen) for long-term protectant activity. If disease knock-down is required, Neon (spiroxamine) or Tern (fenpropidin) are the most active of the morpholines.
"Caramba (metconazole) is a good triazole choice. And if a strobilurin is to be used, then Landmark (kresoxim-methyl + epoxiconazole) and Twist (trifloxystrobin) should be used ahead of Amistar (azoxystrobin)."
Yellow rust
In the wrong season and with a susceptible variety, yellow rust is potentially the most serious foliar disease, believes Mr Bean. Yield loss can be higher than 30%.
"Its already present in some crops this year, as the green bridge effect has been around. Growers should be on guard from now on. It will depend on the weather."
First signs of the disease are that crops lose their lustre. Orange, raised pustules running in lines up and down the leaf will then be noticed.
"The other thing about yellow rust is that there are different strains which affect different varieties," he adds. Equinox, Genghis, Madrigal, Napier, Reaper, Rialto and Savannah are all susceptible.
Control
Product choice is important as the disease recycles within the plant tissue for seven days. "Thats why you can get symptom expression after spraying. It means the wrong fungicide was used."
Landmark and Amistar are the best strobilurins, while Caramba, Opus (epoxiconazole) and Folicur (tebuconazole) are the top triazoles. Neon and Corbel are his choice of the morpholines.
"For fast burn-out, go for a triazole/morpholine mix," says Mr Bean. "For longer term protection, use a strobilurin." His preferred mixes are Landmark/Neon or Amistar/Opus/Neon.
Brown rust
A more regional disease, brown rust likes warm, humid conditions and needs moisture on the leaf to germinate. Coastal Kent and East Anglia are the main areas affected.
It often comes into a crop later in the season, from flag leaf onwards. "It recycles very quickly, so left untreated it will strip flag leaves in 2-3 weeks."
In a bad year, yield reduction will be 15-25%. Brown, raised pustules will be dotted all over the leaf where it is present. Conditions in May and June will determine if the disease takes hold. Soissons is particularly bad for brown rust, while Buster is also very susceptible.
Control
Fungicide choice is more limited as the fungus recycles so quickly. "Amistar is the best product by far," he says. "Folicur and Caramba are the preferred triazoles and fenpropimorph is better the fenpropidin." Normal control timings are flag leaf and ear emergence.
BARLEY DISEASES
Rhynchosporium
Cool, wet weather favours the development of Rhynchosporium, which is why it is more of a problem in high rainfall areas. As well as damaging the leaves and reducing yields by up to 40%, it also affects quality by causing pinched grains.
"Like mildew, it is declining in its susceptibility to fungicides," says Mr Bean. "So mixed strategies with at least two active ingredients are needed."
It can come into crops very early and may be apparent from mid-February, he adds. "Growers will see a pale centred lesion in the centre of the leaf. It can be confused with magnesium deficiency, where the lesion is on the edge of the leaf."
Control
There are four major fungicides for Rhynchosporium control. "Twist and Landmark are very effective, while a morpholine/triazole combination such as Colstar (flusilazole+fenpropimorph) will also do a good job."
Surprisingly good results have come from a cyprodinil/cyproconazole mix, such as Radius. "It combines protectant and eradicant activity and has performed very well."
He says the disease is easy to control where it is dry. "But in wetter areas, it can be a real battle. Dont cut dose rates and choose varieties which minimise the impact of this disease. But even varieties with an 8 rating will still get infections in bad years."
As it is developing a tolerance to mbcs and triazoles, Mr Bean recommends using one of the newer triazoles in a mixed strategy. "They definitely have the edge."
Net blotch
Net blotch is already present in crops and may threaten more than Rhynchosporium this year, Mr Bean warns.
"Its not quite as damaging, but the cool and damp weather will keep it going. The disease shows itself in a discrete, rectangular lesions containing a net or webbing pattern."
Siberia, Regina and Pearl are all susceptible and there are sizeable acreages of these varieties in the ground. Angela and Gleam are also at risk.
Control
If net blotch is there at the T1 timing, it should be treated. "Some growers will consider control before T1. Amistar is the best strobilurin, while Radius is another good choice. The morpholines give useful, additional control when partnered with another product."
Mildew
Feed varieties which receive plenty of nitrogen will be at risk from mildew because the disease favours lush, soft growth. Mildew on barley restricts root development and growers can help by not encouraging a very thick plant stand.
In cool, humid conditions it will develop rapidly and has the potential to reduce yields by 10-15%.
Control
As with wheat, the aim is to get on top of the disease early on. The control options are similar. Mr Bean prefers Landmark and Twist out of the strobilurins, but Radius, Colstar, Neon, Corbel and Fortress are all good alternatives.
Brown rust
Brown rust in barley should be tackled early with a susceptible variety, advises Mr Bean. "Its a result of previous infection and the variety being grown. So if you know you are running a risk, be prepared."
The disease prefers warm, humid conditions and can strike at different times in the season. "It might not come in until flag leaf, or we may start to see it in two weeks time."
Fanfare, Gleam, Heligan, Pearl, Siberia and Vertige are all prone, as is the spring variety Prisma.
Control
Again Amistar is the best strobilurin, while Neon, Corbel, Radius and Colstar are also good choices. "Mix Amistar with one of these for best results. The disease also affects quality as it damages the awns." *
What goes in the spray tank depends on what you see in the field.With crop-walking severely hit by foot-and-mouth restrictions reviewing the main foliar disease threats outlined here, plus our Septoria focus two weeks ago, could prove to be time well spent for many cereal growers.
Keeping crops free from disease until harvest will be a challenge this year. Know your enemy and ensure fungicides are used to best effect, advises UAP expert Chris Bean.