ISSUE  10   July 12, 2007

SOYBEAN PLANT STARTING THE REPRODUCTIVE STAGES

At this time of the year the soybean plant is starting the reproductive phase of the production cycle. The reproductive or R stages in soybean are divided in four parts: R1-R2 describe flowering, R3-R4 are pod development, R5-R6 are seed development and R7-R8 indicate the maturation phase. Although the soybean plants are starting to bloom the vegetative (V) growth will continue during the reproductive part of the plantís development.

Table: Reproductive stages of Soybean

Stage

General Description

Specific 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 fully developed leaf.

R3

Beginning pod

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

R4

Full pod

Pod is ĺ inch long at one of the four uppermost nodes on the main stem with a fully developed leaf.

R5

Beginning seed

Seed is 1/8 inch long in the 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.

R7

Beginning maturity

One normal pod on the main stem that has reached its mature pod color, normally brown or tan, depending on variety.

R8

Full maturity

Ninety-five percent of the pods have reached their mature pod color.1

1 There are 5 to 10 days of drying days needed to bring the moisture percent down to less than 15%.

Flowering of the soybean plant is initiated on the third to sixth main stem node and continues upward and downward from there. Pollination and fertilization are usually accomplished before the soybean flower opens. Hot and dry conditions can cause stress for the soybean plant and flowers may be aborted. If conditions are favorable and adequate moisture is available a normal number of seeds per pod may develop. A healthy and stress free soybean plant will average about 2.5 seeds per pod. Drought conditions will reduce the number of seeds per pod and the seed size.

Soybean plant dry matter is increasing rapidly through the V stages and R1, as leaves develop and the canopy is closing. At R2 the accumulation of dry weight in the plant is constant through late seed fill. Most of the dry weight increase is initially going to the vegetative plant parts. At about R4, the reproductive parts start to expand and the developing pod and seed accumulate dry matter.

At R2 the rate of Nitrogen fixation by the soybean plant increases quickly and peaks at R5 and drops thereafter. For more details and pictures about the reproductive phase see Soybean Growth Quick Guide at: http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/a1174/a1174-2.htm#Reproductive.

Source: How a soybean plant develops, special report no. 53 Iowa State University

Hans Kandel
NDSU Extension Agronomist

 

FUSARIUM WILT IN ALFALFA

Fusarium wilt, caused by Fusarium oxysporum f. sp. medicaginis, has infected the 2005 alfalfa variety performance trial in 2007. Fusarium wilt is a vascular disease that occasionally infects susceptible alfalfa under moist, warm conditions. The most characteristic symptom of Fusarium wilt is a reddish-brown discoloration that appears as partial or complete rings in cross section of the woody cylinder of the tap root. Above-ground symptoms of Fusarium wilt include bleached leaves and stems on plants scattered throughout the field that wilt and tend to recover overnight. As the disease progresses, dead plants occur, which leads to thinning of the stands. This disease occurs only occasionally in North Dakota having occurred at Fargo about once every 10 years or so.

Most improved commercial alfalfa varieties have good resistance to Fusarium wilt. Only older varieties like Vernal, which is still commonly grown in North Dakota and used as a check variety in performance trials, are susceptible. Vernal is very competitive yield wise in the absents of disease. However, this year Fusarium wilt has caused major damage on three of four replicates of the 2005 seeding. One plot has less than 40% stand with other plots having 50 to 70% stands. Forage yields of Vernal in the second harvest will be 30 to 50% less than the 11 other commercial varieties in the experiment. Interestingly, there are no symptoms on Vernal in the 2004 and 2006 performance trial seedings. The reason for this is unclear. Since good varietal resistance is available, Vernal, Ladak, and several other older varieties should not be used in eastern North Dakota, especially in the Red River Valley; on high water table soils; or under irrigation due to the potential loss from Fusarium wilt.

Dwain W. Meyer
Extension Forage Specialist
dwain.meyer@ndsu.edu

 

CAN APPLYING FUNGICIDES IMPROVE YIELDS IN CORN IN NORTH DAKOTA?

This past week we have received inquiries about the profitability of applying fungicides to corn at tasseling. Applying fungicides to corn has recently become the focus of marketing and as well as discussions within the research community. Foliar fungicides are recommended and will likely be profitable when leaf diseases in corn reach damaging levels. In North Dakota, however, leaf diseases, such as leaf rust, Northern corn leaf blight, or eyespot, on most of the hybrids commonly grown, rarely develop to the levels that can be considered damaging in most years. To date, there have been no reports that diseases are threatening the corn crop this year. Nevertheless, the question then remains; can there be a profitable yield response in corn even when diseases are limited? This question arises because some data suggest that strobilurin fungicides boost yields in several crops (including corn) even in the absence of foliar diseases. Unfortunately, we do not have any data from research conducted by NDSU in North Dakota to help us in answering this question. Data from other states indicate that the response of corn to strobilurin fungicides when there is little disease pressure can be variable. As an example, the recent article by Emerson Nafziger summarizes the results of three years of research that included trials conducted on land with various rotations and a range of yield levels in Illinois. This research found significant improvements in yield with the application of fungicides in seven of ten experiments. However, in only three of the ten experiments were the responses large enough to be profitable at todayís corn prices. The complete article can be found at: http://www.ipm.uiuc.edu/bulletin/article.php?id=793. These data indicate that in the absence of significant disease, obtaining a profitable yield increase with fungicides can be risky. We have no data to know how well this information might extrapolate to conditions in North Dakota.

Joel Ransom
Extension Agronomist

Marcia McMullen
Extension Plant Pathologist


NDSU Crop and Pest Report Home buttonTop of Page buttonTable of Contents buttonPrevious buttonNext button