A foliar disease such as leaf rust causes the most damage when it is severe at heading, flowering or milk, and not as damaging at soft dough or later, as explained by Oklahoma State University Extension.

More than 20 years of university fungicide trials including years with little or no disease and several years with high disease pressure have documented an average yield increase of approximately 10 percent from using fungicides.

Such an increase usually justifies fungicide use if the yield potential and price of wheat are high. OSU Extension in its fact sheet explains that a foliar fungicide will not help a grower regain yield. (An ag retailer should never get caught making a promise about a fungicide recouping yield.) Foliar fungicides can only help protect the yield potential present at application.

In general, the yield potential of the wheat should be 30 to 40 bushels per acre minimum, but can go up or down depending on the price of wheat, for a foliar fungicide to show a return on investment.

Foliar diseases do the most harm when infection is severe at stages such as heading, flowering and milk. So, growers should apply a fungicide before disease is severe. The Extension chart below provides a good guide as to expectations of yield loss at different stages of growth and severity of rust.

Approximate percent loss of yield caused by leaf rust at combinations of leaf rust severity and growth stage of wheat:

Severity (%) of leaf rust on the flag leaf

 

10

25

40

65

100

Growth Stage

------------------------% yield loss------------------------

Flowering

10

15

20

30

35

Milk

2

5

8

14

20

Soft dough

1

3

4

7

10

Hard dough

1

1

1

3

5