Asian Soybean Rust

Keith Fontenot  |  7/16/2009 12:30:09 AM

The first find of Asian soybean rust, in soybeans in the state for 2009 was discovered in a sentinel plot of Maturity Group 5 beans in St. Martin Parish. Previous findings earlier this year were in kudzu located in Iberia, St Mary, and East Baton Rouge parishes.

The question arises at this point: what do we do now with our soybeans in the field? This question almost has to be answered on a field by field basis taking into consideration several factors. In some parts of the state earlier planted beans have reached growth stages where an ASR infection would not cut down production. In Evangeline Parish and most parts of southwest Louisiana, just the opposite is true of our later planted soybean crop. Let’s take a look at the factors we need to consider.

The first factor is that of plant stage of growth. To better understand stages of growth the following information may be helpful:

From the time the soybean seed is planted and it begins growth, the soybean is in the vegetative or (V) growth stages.There may be several of these, determined by the number of nodes on the main stem of the plant. Vegetative stages of growth end once the first flower is seen or noted on the plant. At this point the plant enters the reproductive stages or (R) growth stages.

The early stages of soybean reproductive development involve flowering and pod development. Determining these stages is the same, regardless of the maturity group planted. The main stem is used for determining these stages. Begin by finding the newest fully developed leaf near the terminal of the plant. This will be considered node number one and is key in properly identifying the reproductive stage of the plant. These different stages are given certain designations that begin with the letter “R” and an abbreviated title. The table below describes the early stages of reproductive development (R1 - R6).


Stage

Number

Stage Title

Description

R1

Beginning Bloom

One open flower at any node on the main stem

R2

Full Bloom

Open flower at one of the two uppermost nodes on the main stem with a fully developed leaf

R3

Beginning Pod

Pod 3/16 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf

R4

Full pod

Pod 3/4 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf

R5

Beginning Seed

Seed 1/8 inch long in a pod at one of the four uppermost nodes on the main stem with a fully developed leaf

R6

Full Seed

Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf


As with vegetative development, reproductive development is environmentally sensitive. Temperature and day length plan an important role in the rate of reproductive development. Shorter days and high temperatures have been documented to increase the rate of soybean reproductive development. The adjoining table provides a good estimate for determining the number of days between reproductive growth stages. When attempting to determine the growth stage of a particular field, be sure to evaluate numerous plants in different sections of the field to account for field variability.

Stages

Average Number of Days

Range in Number of Days

R1 to R2

0*, 3

0-7

R2 to R3

10

5-15

R3 to R4

9

5-15

R4 to R5

9

4-26

R5 to R6

15

11-20


* Stages R1 and R2 generally occur simultaneously in determinate varieties.

The time interval between R1 and R2 in indeterminate varieties is about 3 days

In preventing or controlling ASR, in the economic situation our growers are in today, we need to look at trying to time fungicide applications for the best effect for our dollar. In some cases R2 or full bloom on a field may be the earliest application if rust has been detected nearby. R3 is the earliest stage of application we would like to see, if no rust has been detected in the field.

We have a sticky situation in that you don’t want infection in the field, but you also want to apply fungicides for the maximum length of duration of protection in the field. In many cases an early application will almost have to be followed by a second, later application.

On the flip side of the coin, late R6-R7 is the growth stage where the beans have filled, are beginning to dry down and separate in the pod, and plants are beginning to turn color from green to the yellowish tan of maturity. This is the stage when the disease can no longer affect yield of plants.

Yield potential as always is a major factor in deciding whether or not to apply the fungicide. In many cases we have been catching spotty showers following a prolonged dry spell, which has helped growth of the bean plants. With the rain chances for disease increase including, ASR, Aerial Blight, Frogeye leaf spot, lesser diseases, and the late season problem Cercospora. We need to keep these other disease problems in mind as well as ASR.

Single vs. multiple applications: In the 70’s and 80’s, two fungicide applications were the standard practice for many years. When the price of beans went down & fungicide prices kept going up, the single application at a higher rate became more economical. Now with the added threat of ASR, with the other diseases, we try to time one application at the R3-4 stage to control problems. Weather conditions, the spreading of ASR, or the lack of it will determine if one application will hold up or not.

Many producers get concerned with Green Bean Syndrome, when fungicides are used. Fungicides do not cause green bean syndrome. Fungicides prevent diseases, allowing the plants to live longer and produce larger seed which usually result in higher yields.

Concerning Green Bean, another consideration is whether or not an insecticide should be added to the fungicide tank mix. If insect problems are present this is a no brainer. If insect pressure is light a decision has to be made. Most insecticides will not persist & be effective for an extended period in field conditions. So, is the insect pressure enough to spray now for the cost of the insecticide, or do you wait and possibly spray later, with an additional airplane cost?

After speaking with State Plant Pathologist Dr. Clayton Hollier, the following fungicides are recommended for ASR and other fungal disease control. This list does not include all products on the market, but does include most products available in our area.


Fungicide

Active Ingredient (ai)

Rate/Acre

Quadris

Azoxystrobin

6.2-15.4 oz

Headline

Pyraclostrobin

6-12 oz

Tilt 3.6EC

Propiconazole

4-8 fl oz

PropiMax 3.6EC

Propiconazole

4-8 fl oz

Bumper 41.8EC

Propiconazole

4-8 fl oz

Folicur 3.6F

Tebuconazole

3-4 fl oz

Laredo 25EC

Myclobutanil

4-8 fl oz

Laredo 25EW

Myclobutanil

4.8-9.6 fl oz

Domark 230ME

Tetraconazole

5-6 fl oz

Stratego 2.08F

Propiconazole + Trifloxystrobin

5.5-10 fl oz

Quilt 1.66EC

Propiconazole + Azoxystrobin

14-20.5 fl oz

Alto

Cyproconazole

2.75-5.5 oz.

Punch

Flusilazole

See label

Topguard

Flutriafol

See label

Proline

Prothioconazole

2.5-3.0 oz.


As information is gathered and updated on the spread or lack of it of ASR, and also on the other soybean diseases, we will keep you as informed and updated as possible.

Questions may be directed to ASR Hotline at 1-866-641-1847 or contact Dr. Levy at the Dean Lee Research Station in Alexandria at (318)-290-8747, or Dr. Clayton Hollier, Plant Pathologist with the LSU AgCenter in Baton Rouge at (225) 578-4487. You may also wish to contact our office at (337) 363-5646 with this or other concerns on the crop.

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