Soybeans: Other Crop Problems

 

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Pub 811: Agronomy Guide > Soybeans > Other Crop Problems

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

 

Insects and Diseases

Figure 2-4, Soybean Scouting Calendar, shows insects and diseases that could be causing the symptoms in the field. Individual descriptions of insects and diseases, scouting and management strategies can be found in Chapter 13, Insects and Pests of Field Crops, or Chapter 14, Diseases of Field Crops.

Recommended treatments to control insects, pests and diseases can be found in OMAFRA Publication 812, Field Crop Protection Guide.

Frost and Hail Damage

Early Season

Plants damaged below the cotyledons by early-season frost or hail will not recover. If frost or hail damages the growing point of the seedling, but not the stem portion below, the plant will send out new shoots from the base of the leaves or cotyledons (see Plate 16). Wait 3 or 4 days and watch for new growth to emerge from the point where leaves attach to the stem (leaf axils). Research trials show that leaf loss at early growth stages has little impact on final yield or maturity. Table 2-19, Percent Yield Loss of Indeterminate Soybean at Various Levels of Leaf Area Loss and Growth Stages, summarizes the expected yield loss from leaf loss at various life stages.

Plate 16. Frost injury. If frost damages the growing point, but not the stem, the plant may recover by new growth emerging at the leaf axils.

Photo showing frost injury. If frost damages the growing point, but not the stem, the plant may recover by new growth emerging at the leaf axils.

Figure 2-4 Soybean Scouting Calendar

Table 2-19. Percent Yield Loss of Indeterminate Soybean at Various Levels of Leaf Area Loss and Growth Stages
Growth
Stage
Percent Leaf Area Destroyed
10 20 30 40 50
VC-Vn
-
-
-
-
-
R1
-
1
2
3
3
R2
-
2
3
5
6
R2.5
1
2
3
5
7
R3
2
3
4
6
8
R3.5
3
4
5
7
10
R4
3
5
7
9
12
R4.5
4
6
9
11
15
R5
4
7
10
13
17
R5.5
4
7
10
13
17
R6
1
6
9
11
14
R6.5
0
1
1
3
4

Printed with permission from National Crop Insurance Services (NCIS) 2008. Not to be reproduced without permission.

Growth
Stage
Percent Leaf Area Destroyed
60 70 80 90 100
VC-Vn
-
-
-
-
-
R1
4
5
6
8
12
R2
7
9
12
16
23
R2.5
9
11
15
20
28
R3
11
14
18
24
33
R3.5
13
18
24
31
45
R4
16
22
30
39
56
R4.5
20
27
37
49
65
R5
23
31
43
58
75
R5.5
23
31
43
58
75
R6
18
23
31
41
53
R6.5
5
7
13
18
23

Printed with permission from National Crop Insurance Services (NCIS) 2008. Not to be reproduced without permission.

Stem Damage

Broken or cut-off stems have greater impact than leaf loss on yield and maturity. If stem loss is under 50% prior to flowering, yield loss will be less than 10%. When evaluating hail damage, check for bruising on the plant stem. Severe damage to the stem will make it more difficult for the plant to recover ( See Plate 17). It can also make the plant more susceptible to disease. Bruising, which does not cause stem breakage, causes minimal loss in yield.

In terms of yield reduction, soybeans are most vulnerable during the flowering and seed fill period. This is particularly true if stems are broken, resulting in a reduction in the number of pods. Delays in maturity and seed size also occur.

Plate 17. Hail damage. Soybeans are most vulnerable to hail damage during flowering and pod fill.

Photo showing hail damage. Soybeans are most vulnerable to hail damage during flowering and pod fill.

Late Season Cold Temperature and
Frost Injury

Soybeans are regarded as a warm-season crop and are therefore more susceptible to cold temperatures, especially during flowering. It is believed that sustained cold temperatures (less than 10°C) during flowering affect proper formation of pollen in the flower. This results in poorly developed pods called parthencarpic pods (also called "monkey pods"). There is some variety difference in tolerance to cold temperatures.

Varieties that have tawny pubescence (i.e., yellowish-brown hair) are often more cold tolerant than those that have grey pubescence.

Frost during flowering and pod fill can drastically reduce yield and quality. A severe frost during these stages can reduce yield by up to 80%. Freezing during pod fill will result in severely damaged beans with a greenish, "candied" appearance. Even moderately frosted beans with a greenish colour and slightly wrinkled seed coat are considered damaged and can be discounted if present in excess of limits. The seed will eventually dry down with a wrinkled seed coat. Frost-injured plants may reach maturity earlier but will have seed moisture equal to non-frosted plants. Germination will also be severely reduced. The Canadian Food Inspection Agency classifies frost-damaged soybeans as those "soybeans whose cotyledons, when cut, are green or greenish-brown in colour with a glassy, wax-like appearance."


Yield reductions from late season frost injury are smaller as the crop matures. Frost during the R5 stage reduces yield by 50%-70%. Frost at the R6 stage will cause losses of 20%-30%. Once the crop reaches the R7 stage only a 5% yield loss is expected. No yield reductions occur once the plants have reached full maturity.


Lightning Damage

Lightning damage is confined to small circular or oval regions with a diameter of 5-10 m. Plants are usually killed but can sometimes survive on the edges of the affected area. The affected area has a clearly defined margin, making diagnosis relatively straight forward (Plate 18). The affected area does not grow over time. Stems are often darkened with dead leaves remaining attached to the plant.

Plate 18. Lightning damage occurs in small circular areas that have a clearly defined margin.

Photo showing how lightning damage occurs in small circular areas that have a clearly defined margin.

Mature Green Seed


An extremely dry growing season can result in green soybean seed at harvest even if seed moisture is below 13% (Plate 19 ). The problem is generally the most severe in those regions that are extremely dry during July and August in soils with poor water holding capacity. Since the beans are dry, the "activity" inside the seed is minimal. The enzyme that normally breaks down the chlorophyll cannot function at such low moistures, therefore the green colour will not disappear over time. There may be some improvement to the green tinge on the outside of the bean over time, but the green discolouration inside the bean will remain if left in the field or in storage. There is little that can be done to avoid having green beans since this problem is weather related. A good crop rotation along with choosing the best varieties suited for the area is the best defence.

Plate 19. Mature green seed occurs when chlorophyll is not broken down during pod fill in drought-stressed plants. The right side shows mature green seed damage.

Photo showing how mature green seed occurs when chlorophyll is not broken down during pod fill in drought-stressed plants. The right side shows mature green seed damage.



For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 29 April 2009
Last Reviewed: 29 April 2009