Today's Date: 2016-07-23
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Daily Growing Degree Days for 2016-07-23
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Plant Pathologist Commentary: Small Grains Disease Update 06/02/2016

So far this year the small grains have been looking pretty good throughout the state. However, with recent heavy rainfall and some important spraying decisions to be made, it’s time to start thinking about diseases!

What’s out there?

Current scouting reports from the last two weeks have revealed some BYDV showing up, particularly in the winter grains in the southern part of the state. This is not surprising as we had an early immigration of cereal aphids back in early May. Some of these aphids tested positive for BYDV. Watch for aphids in the crop. The main management tool for BYDV is to limit secondary spread of viruliferous aphids by spraying to control aphid populations. There are no thresholds set for aphid control taking in to account BYDV but there are for aphid feeding pressure which is when 80% of stems have one aphid or more.

Stripe rust has made an appearance in the southern part of the state but has been somewhat limited in development by dry warmer conditions. However, this may change rapidly with the advent of significant rainfall in some areas. Stripe rust requires cooler night time temperatures and free moisture on leaf surfaces to infect and develop well. Stripe rust has been reported in Kimball, Le Center adn St. Paul.

Root and Crown Rots seem to be making their presence felt this year. I have had several reports of stand loss both in commercial fields and in some of our research plots. Conditions have been conducive in drier warmer areas for development of Fusarium root and crown rot. Look for seedling death, poor root growth and brown or back discolorations of roots, sub crown internodes and stem bases in established plants. Pink discoloration automatically indicates Fusarium R and CR but the pink color is not always associated with Fusarium infection and it may just look brown. If you have these symptoms in your fields it will be important to think about using a seed treatment the next time it is planted to small grains, especially if you have a shorter rotation which involves corn.

Tan Spot we often talk about scouting for tan spot on Memorial Day which has just passed. Samples for the mawg scouting efforts are coming in this week that look to have early stage tan spot infections. Early infections, especially on younger plants, can present as leaf tip yellowing before the discreet brown lesions with yellow halos develop. BEWARE contact herbicide injury. With the cooler temperatures there has been a lot of contact herbicide injury from products such as Bronate (Bromoxynil). This can really look like a leaf spot disease with white/tan looking lesions or even grey. Usually the distinction can be made because these lesions look pitted, as if the leaf surface has flaked off. This is not the case with tan spot. In addition, contact herbicide injury also does not have the yellow halos associated with tan spot. Although the herbicide injury can look devastating, the new growth which did not come in to contact with the product will be free of spots and the plants will do just fine.

Spraying Decisions to Be Made With winter wheat close to or at flowering the decisions need to be made about spraying for Fusarium head blight (scab). All winter wheat varieties grown in Minnesota are highly susceptible to FHB except for AC Emerson which has some resistance. So it is time to turn to the FHB forecasting tool at http://www.wheatscab.psu.edu and select Minnesota in the drop down state menu. Remember that this model takes in to account the weather from the last 7 days in its prediction. So, although at present the risk is trending low (denoted by green areas). The recent rains will come in to play shortly and the risk will likely trend higher in the next few days. Also remember that FHB only really needs humid, warm conditions to thrive and not necessarily prolonged periods of rainfall. Early flowering (50% of heads on main stems with yellow anthers showing in the middle of the head) is the critical window for spraying. Flowering only lasts 3-4 days in wheat so the window is short. Recent data from the USWBSI shows that reasonable efficacy can also be achieved up to five days past flowering. Spraying early, prior to flowering, is not optimal for good disease control.

Spray Timing for Rusts and leaf disease verses FHB

I got asked questions today about the need to spray now for stripe rust in situations where the crop roughly ten days away from needing its FHB application. If there is no disease present, then there is no reason to spray ‘just in case’. Even with the wetter conditions favorable for fungal growth, there will be limited damage cause in this time period. The products routinely used for FHB will also provide pretty good control of most leaf diseases. In the case of stripe rust, it takes roughly 7-10 days if conditions are favorable for the disease for it to go through a round of sporulation. If you are only seeing a few pustules low in the canopy, then again, there is probably no need to spray. However, disease high in the canopy at the time of flag leaf emergence can be a problem. Products which contain only a strobilurin fungicide (e.g. Aproch or Headline) must be sprayed before stripe rust infection occurs. This is due to the mechanism by which the strobilurin chemistries move in the plant. If there is stripe rust infection which has already occurred and needs to be controlled, then use products which contain an azole (e.g. Folicur) or a combination of azole and strobilurin (e.g. Quilt). Bear in mind that if temperatures get too warm (above 80° F) and dry, stripe rust will stall. Optimal temperatures for disease development are 50-65° F with moisture (dew) present. However, the whole story changes if you are more than ten days away from your FHB application. Then the decision becomes based on where the disease is in the canopy relative to the growth stage, bearing in mind the aim is to keep the flag leaf free of disease. Rusts (leaf and stripe) and tan spot are all polycyclic diseases and therefore their progression must be watched closely as they can go through multiple infection cycles in a season. As with all chemistries, check current labels for use restrictions and pre-harvest intervals. Consult the NCERA 184 fungicide table for relative efficacy of different AI’s for different diseases http://bulletin.ipm.illinois.edu/wp-content/uploads/2015/04/NCERA-184-Wheat-fungicide-table-2015_V3.pdf

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