Wheat Disease Update: March 29

Wheat leaf rust, notice the non-uniform pattern and larger pustules size as well as deeper orange color.

Leaf rust

Several new reports of leaf rust have been made over the past few days.  At present, leaf rust has been detected in Adams, Bolivar, Carroll, Coahoma, Covington, DeSoto, Grenada, Issaquena, Jefferson, Noxubee, Oktibbeha, Panola, Quitman, Rankin, Sharkey, Smith, Stone, Sunflower, Tallahatchie, Tunica, Warren, Washington, and Winston counties.  I am fairly certain that more leaf rust is present throughout the MS production area but I have not received additional reports nor have I seen any additional rust in my travels over the past two weeks.  Scout fields for the presence of leaf rust and keep in mind there are some major distinguishing features to help you tell it apart from stripe rust.  Ideally, leaf rust pustules are larger in size, typically random in their presence on the leaf (no real “pattern”), and the pustules rupture with spores that are more of an orange color when compared with the spores present in the ruptured pustules of stripe rust.  However, making some of the comparisons can be difficult if you only have one type of rust in a given field situation.  To this point in the season, leaf rust levels have been low in almost all cases; however, one field had some heavy leaf rust on plants along the edge and another field in eastern MS was reported to have moderate leaf rust.  However, with that said, generally speaking our current daytime high temperatures are outside of the typical range of optimum temperatures.  Generally, 68-77F is considered the optimum temperature range.  But, our nighttime temperatures are likely contributing to some continued development especially with the heavy dews we are experiencing.

One important thing to note regarding field scouting, if you hold a wheat leaf up to the sun and see some yellow “flecking” present on the leaves this is not indicative of a rust epidemic occurring in the future.  Many factors can contribute to the presence of the “flecking” and in the past I have not encountered a situation where the flecking became heavy rust.  Numerous scenarios can result in the leaf flecking.

Stripe rust

Stripe rust has been positively identified in Bolivar, Claiborne, Coahoma, DeSoto, Grenada, Issaquena, Leflore, Lowndes, Marshall, Newton, Noxubee, Oktibbeha, Panola, Quitman, Sharkey, Sunflower, Tallahatchie, Tate, Tunica, Warren, and Washington counties.  On Monday (3/26/12) I was able to find leaves in DeSoto and Tunica counties with low levels of the disease.  Over the past few weeks the amount of stripe rust in MS has seemed to decrease.  Leaves on Monday with stripe rust were generally lower in the canopy and it appeared that the disease was no longer able to move up to the flag leaves present throughout most of the wheat fields I surveyed.  However, on Wednesday (3/28/12), a field was identified in Panola County with heavy stripe rust infection on the flag leaves.  With that in mind, I would suggest that in some cases environmental conditions are still conducive for a limited development of stripe rust.  While I am not sure of the variety present in the field in Panola County it is likely this was a fairly susceptible variety due to the level of infection.  In the majority of the other fields I have scouted, stripe rust has not been present on the flag leaf.

Several fields were sprayed with a fungicide that ranged from triazoles (propiconazole, tebuconazole) to a strobilurin + triazole pre-mix (trifloxystrobin + propiconazole).  I have had some reports from at least two consultants where propiconazole was applied to manage stripe rust and the fungicide did not appear to reduce the level of the disease.  Even though propiconazole is labeled for stripe rust it is not one of the preferred fungicides (refer to blog update: https://www.mississippi-crops.com/2012/03/02/wheat-leaf-rust-and-stripe-rust-update-march-2-2012/).  Stripe rust can still be detected at low levels in some of the fields that received a fungicide.  However, it is possible that with the increased daytime temperatures the disease will have a harder time reproducing.  But, in the past we have identified a strain of the fungus that has the ability to reproduce at higher temperatures.  In general, temperatures in the 80s are not considered to be conducive for stripe rust development.

Powdery mildew of wheat.

Powdery mildew

Several reports of powdery mildew have been made from throughout the state over the past week.  I have seen more powdery mildew this season than any of the previous seasons since I arrived in MS.  Typically, powdery mildew is more of an issue in the lower canopy and tends to favor the environment since moisture and humidity are plentiful.  However, powdery mildew can favor situations where the wheat stand was planted thick due to a high seeding rate or in situations where either excessive nitrogen fertilizer was applied or in situations where wheat may be grown in the bottom of an old catfish pond.  Over the past week I’ve received at least two calls claiming the disease has moved up onto the flag leaf.  I suspect we are seeing a little more powdery mildew this season since we’re experiencing quite heavy dews.  One thing to remember, downy mildew of wheat is a completely different disease and will typically only be observed along water furrows or in other areas of the field where water was standing for an extended period of time.  The presence of the white tufts of fungus on the leaves will be present when powdery mildew is observed.

Barley yellow dwarf virus (BYDV)

In some isolated cases on Wednesday I was able to detect what I would call BYDV.  Typically, plants are detected in an isolated, small case, with yellow to purplish leaves.  However, unless the infection occurred shortly after planting (in the fall) the plants will likely not be stunted and a yield loss will not result.  In the few fields I observed BYDV I don’t believe a yield loss will occur since only a few plants were infected and it appears the infection occurred sometime during the spring since plants are not stunted (dwarfed).

Pre-harvest fungicide interval

One of the most important things to be aware of has to do with the interval of time between harvest and fungicide application timing.  If you are to make a fungicide application, be aware that the cut off (pre-harvest interval) for most fungicides is heading complete (10.5).  Once the field reaches flowering (Feekes 10.5.1) almost all fungicides should not be applied due to the potential of residue issues in the harvested crop.  The only fungicide labeled past 10.5 is Prosaro; however, the only labeled disease in this situation is Fusarium head blight (FHB).  Presently, based on environmental conditions I’m not concerned about the appearance of FHB since the disease requires an ultra-specific set of conditions to occur.  Included is a table that lists the specific pre-harvest interval for the majority of the fungicides available in our production system.  If you have specific questions please feel free to call and we can discuss the issues.

pre-harvest interval fungicide table