Wheat Disease

Overview:
Wheat disease in our region is generally less damaging that it is east of us, but yield losses can be significant in some years. KSU estimates that fully 20% of wheat production in Kansas is lost to disease, and in some cases, 50% of yield is lost. These are significant numbers, and it is possible that many farmers underestimate the risk of disease to wheat production.

Disease can overwinter in our area in mild winters, but it often arrives on strong south winds. Warm nights and humid weather--even for only a few days--can cause a disease outbreak. Treatment is mostly preventative, and should be applied at flag leaf emergence. Farmers often cannot wait until disease is fully developed to make a treatment decision: wheat price and yield potential often dictate strategy in the real world--see below for more detail.

Economic levels are more likely to be reached when wheat prices exceed $3.50-$4.00 per bushel, and in fields with a yield potential of 45-50 bushels per acre. Treatment is more likely to pay dividends in susceptible varieties, in irrigated wheat, and in minimum till or no-till wheat. 

Here is a flyer from BASF, makers of Headline fungicide, which shows some profit figures for $5 per bushel wheat, and also shows some photos of diseased wheat leaves compared to treated wheat. (PDF, 94 Kb)  

There are many diseases which can occur in wheat: these include wheat soilborne mosaic, wheat spindle streak mosaic, barley yellow dwarf, leaf rust, stem rust, Septoria leaf blotch, glume blotch, tan spot, and powdery mildew, In this region, rusts are more common, and are discussed in some detail below.

Cultural practices which minimize disease yield loss include crop rotation, planting resistant varieties, and controlling volunteer wheat in the fall. Seed treatment and delayed planting, as well as crop residue reduction can also help with disease control.

More reading:
The KSU Wheat Production Handbook discusses disease in some detail, and has much information of value. KSU also publishes a document entitled Wheat Foliar Fungicides. It is somewhat dated, but does have some worksheets and fungicide control data which are interesting.

Finally, KSU rates wheat varieties for both insect and disease resistance. All three of these documents can be downloaded from this page. 

Rust in wheat:
There are three types of rust which infect wheat in our area: striped rust, stem rust, and leaf rust. To jump to a specific type of rust, click on it in the line above. Otherwise, just scroll down to learn more.

Rust is caused by a parasitic fungus called Puccinia recondita f. sp. tritici. This fungus blows up from Texas each spring on strong southerly winds. In addition, with warm winters and sufficient volunteer wheat, rust can and does overwinter in our area.

Most wheat varieties have good resistance to stem and leaf rust, but stripe rust can cause significant yield losses on many varieties. According to Denis Gaudet, a Canadian plant pathologist, stripe rust has recently has undergone a "shift in biology" and now it can tolerate the warmer temperatures of the Great Plains. Prior to the year 2000, this had not been seen.

Additionally, new rust threats are always emerging: Ug99 is a stem rust from Uganda which is being watched carefully by the ARS--read more about Ug99 here.

Yield losses from rust in wheat can be very severe: more than 50% of the yield is possible. Irrigated wheat is at the greatest risk, but rust can form in relatively dry weather if the temperatures are in the critical range and dew is present for 6-8 hours. Resistant wheat varieties help minimize the damage from the rust, and the amount of plant resistance should be considered when deciding when to treat. 

Best economic return occurs in high yield wheat and when prices are high.  To read an Oklahoma State newsletter on wheat disease, click here. (PDF, 761 Kb)

Several fungicides are available to control wheat rust and these make good economic sense, especially on fields with yield potential above 40 bushels per acre. Texas A&M says that a $2.50 return for every dollar invested is likely. 

Fungicides may increase seed size and decrease seed clean out, and are sometimes used on seed production fields for these benefits.

Timing for fungicides is normally at about 2/3 of the flag leaves emerged. Proper timing is critical. A good memory aid is to "protect the flag leaf."

In wheat, the contribution to yield from the flag leaf is 43%, the leaf below the flag contributes 23%, and the head 22%. This is why control is timed to 65-75% flag leaf emergence: Feeks 8. Late applications help, but yield reduction will be much greater than with properly timed fungicide applications.

Leaf Rust: This rust can be a major problem in western Kansas and eastern Colorado. It forms a very small pustule (1/32 inch long by 1/64 inch wide) on the leaf. Each pustule can produce 1,000 spores of the rust fungus daily. It is orange in color and forms a random pattern on the leaves. Kernel weight is affected more than the number of kernels. Control with fungicides can be good, if properly timed.

To the right is a photo of leaf rust (photo credit: Bill Willis).

KSU says this: "Although leaf rust may initiate tiny orange spots on culms and heads, it does not form large, open pustules on these organs. This helps distinguish leaf rust from stem rust."

Striped Rust: This rust was a major problem just south of our area in 2005. Striped rust is the most important wheat disease world-wide, according to Kurt Jones, of Syngenta Crop Protection. This rust can be on the leaf and head, is yellowish in color, and appears in straight lines, hence the name. To see a photo of striped rust, click here.

Optimal temperatures for striped rust are 55-75 degrees F. When you have overnight temperatures above 65 degrees and highs in the mid-80s, you have a chance for rapid rust development. Leaf rust and striped rust can coexist on the same leaf. White wheat is more susceptible than red wheat, and in both plants the epidermal tissue is damaged by the rust. Rust reduces photosynthetic leaf area, and reduces kernel weight and number. Yield loss of over 50% is possible on susceptible varieties, and 10-15% on moderately susceptible varieties.

Crop rotation and delayed planting date does not seem to reduce striped rust disease probability: resistant varieties are the only cultural practice available.

Stem Rust: This rust appears on the stem and leaf, is dark red, and needs temperatures in the range of 65-85 degrees Fahrenheit. It has not been a major problem in our area. A resistant variety of wheat is the best protection.

To see a listing of wheat varieties which are resistant to disease and insects, go to this page.