Decision guide for foliar disease controlin irrigated wheat

STEP 1: ESTIMATE THE YIELD POTENTIAL OF THE FIELD (Do this for each field.)

Estimated Yield In Bushels Per Acre ___________________.

If the estimated yield is greater than 44 bushels per acre for hard red spring (HRS) wheat or 31 bushels per acre for soft white spring (SWS) wheat, then determine disease incidence in the field. If yield is less than the thresholds, it is not worthwhile to spray; do not continue.

Adjust with up-to-date figures for a more accurate calculation of when the cost of spraying equals the gain in yield. The above estimations are based on the cost of $15 per acre for spraying ($12 for chemical and $3 for application). The price used for HRS was $4.50 per bushel and for SWS it was $4.00 per bushel, and the loss in yield due to foliar disease was 7.5% in HRS and 12.0% in SWS. The formula is:

Cost of spraying ($15.00)
____________________________

Grain price($4.50 HRS)($4.00 SWS) x Loss(.075 HRS)(.120 SWS)

= 44 bushels per acre HRS or 31 bushels per acre SWS where cost equals gain

STEP 2. DETERMINE DISEASE INCIDENCE FOR THE FIELD

Randomly collect 100 tillers (a single stem with all its leaves) along a zig zag pattern when plants are between the flag leaf emergence (booting) (G.S. 40) to the flowering complete (G.S. 69) growth stages. Collect about five tillers every five to ten paces. With a mid-May seeding date, this usually occurs between July 1 to 15. Examine the third leaf down from the top (flag) leaf of each tiller. Divide these leaves into two categories (piles): diseased, not diseased. Spots greater than 2 mm in diameter indicate the presence of disease. DO NOT include other damage such as yellow or brown discoloration or physical damage, both of which tend to occur on the leaf edges and tip.

STEP 3. DETERMINE THE COST EFFECTIVENESS OF SPRAYING

If more than 50% of the third leaves are diseased, then spraying may be cost effective as there will likely be a yield loss in the order of 7.5% for hard red spring wheat and 12.0% for soft white spring wheat. If less than 50% of the third leaves are diseased, then it is not worthwhile to spray.

If greater than 50% of the third leaves are diseased, then calculate:

Yield loss = ________ x _________ x $_________ = $_________

(.075 for HRS) x (Estimated Yield from Step 1) x (*HRS Grain Price/bu)

or
(.120 for SWS) x (Estimated Yield from Step 1) x (*SWS Grain Price/bu)

* Note: Use up-to-date figures for the calculation.

The estimated cost of spraying per acre (chemical + application) is $15.00. If you have a more accurate figure use it in the calculations. If the yield loss is less than the cost of spraying, then the costs are greater than the return; do not spray. If the yield loss is greater than the cost of spraying, determine the potential cost benefit. An additional benefit from spraying is that kernel weight and seed protein may be increased.

Potential Cost Benefit =__________ x Number of Acres =$________

Consult the most recent issue of Saskatchewan agriculture and food publication " Guide to Crop Protection" for registered fungicides and their use.