Posts Tagged ‘soybean charcoal rot’

Charcoal Rot of Soybeans

Friday, August 17th, 2012

Nancy Gregory, Plant Diagnostician; ngregory@udel.edu

Charcoal rot was diagnosed on soybean plants in early August, much earlier than we normally see this disease. We usually see charcoal rot near maturity, and these plants had small pods about ¾ inch in length. Charcoal rot is a stem rot caused by Macrophomina phaseolina, a soil-borne fungus with a wide host range, infecting many bean species and corn. Charcoal rot often shows up in Delaware fields in the late summer season, as soybeans mature, especially in drought stressed beans. Plants may show a loss of vigor or smaller than normal leaves. Leaflets may yellow, but stay attached at the petioles. Dark lesions may extend up the stem from the soil line. Eventually, the fungus colonizes the water-conducting tissues of the bean stems, and plants die. The fungus produces small survival structures called microsclerotia; I think of them as small rubber-band-balls of fungal strands. The numerous small dark microsclerotia look like charcoal dust, giving the disease its name. These microsclerotia can survive in soil and debris for several years; however, they do not survive well in wet soils. The fungus can survive on seeds in small cracks. Often, infections occur early, when soil moisture is good, and symptoms become obvious late in the season as plants become stressed from drought. To control, rotate away from fields that have been heavily infested, for at least two years. Irrigation and cultural practices such as good fertility and average seeding rates should help avoid the stress that brings on symptoms of dieback. There are no good resistant varieties, but breeding work is ongoing. Foliar fungicides are not effective for controlling charcoal rot.

Spraying Soybeans with Fungicides After Hurricane Irene

Friday, September 2nd, 2011

Arvydas (Arv) Grybauskas, Extension Plant Pathologist, University of Maryland; arvydas@umd.edu

The question has come up after the tremendous rainfall and damage caused by hurricane Irene whether or not one should spray their soybeans with fungicides. There was also a question just before the hurricane as to whether or not it was advisable to throw in a fungicide since many fields were to be sprayed for insects like podworm and stinkbugs. The answer to both those questions is highly dependent on each individual situation and a blanket recommendation cannot be made, especially because soybean rust is not going to be an issue this season.

Before we discuss the factors that play a part in determining fungicide need let me briefly explain why we rely on statistics and statistically valid research to answer these questions. Most of you are familiar with yield monitors and their output either because you have equipped your combine with one, or the custom combining operation that does the harvesting has one, or you have seen sample maps in a trade publication. If you haven’t then I’m sure you are still aware of the issue of variability in a field because of all the hoopla associated with any precision Ag equipment article or ad. These have been invaluable tools as they have really shown that an apparently uniform 40 acres is really quite variable. The obvious reasons for the variability are variations in soil type, depth, moisture and fertility just to name a few. But there are also many other less obvious reasons associated with pests since they are not uniformly distributed across any field as well. Some fields are more variable than others but every field is to some degree not uniform. The fact is if you took any production field and arbitrarily split it in half the average yield in each half would not be identical. In fact I would suspect that the vast majority in our area would easily have about 3-5 bu/A yield difference between each half. Again some might have greater uniformity and a smaller difference and some might have more. It is this variability that illustrates the need for statistics. Think of it this way. If the same field that we split arbitrarily in half produces yields that are different by 3-5 bu/A without a treatment then how do we know if we put a fungicide (or anything else) on half that it did anything? We use statistical tools and design experiments following certain statistical rules to answer that question. Two of the most important rules are replication and randomization. We will replicate so that we test a treatment more than once and get a measure of the variability in the response. We will randomize where each replicate of each treatment goes in the field to make sure they all have an equal chance of being in a high yield spot or low yield spot in a field. By using these and other tools we can then use the variability that is there to say whether or not a treatment has a certain probability of having an effect. The point is a bigger number doesn’t always mean that the treatment has a real or significant effect for those conditions. Sometimes a 3-5 bu yield difference could just be chalked up to being part of the background variation.

Ok, getting back to should we spray fungicides on soybeans after this heavy rainfall and wind damage. Let’s start with the damage. There are have been no independent scientifically sound experiments that show a consistent and reliable yield advantage to spraying fungicides in the absence of diseases. Physical damage by itself is not going to be corrected by a fungicide. Some fungicides reportedly have a positive effect on plants by counteracting components of plant stress responses. However, there is no proof from independently conducted field trials that this affect is large enough to affect yields in the field in non-diseased plants.

Stands that are lodged, leaning or matted down because of high winds tend to favor the development of certain diseases because the canopy remains closed and therefore remains wet for longer periods. Also, the distance a fungus needs to move either from leaf to leaf or soil to leaf in lodged or leaning stands is generally less. Heavy rains play a more important role than the stand damage in driving disease development. The rain from Irene provided two main avenues for greater disease development: spread of disease-causing agents if they were present; and a wet or humid environment that favors pathogen growth and infection of plants. A key that cannot be ignored is there has to be a pathogen around and in a form capable of being moved.

There are soybean diseases that will not respond to foliar fungicides. Bacterial blight and Bacterial pustule are diseases that are favored by hard driving rains. They look very similar to several fungal diseases, but because bacteria cause these fungicides cannot control them. Don’t get talked into spraying just because there are some leaf spots. There are also some fungal diseases that will not respond to foliar applied fungicides. The stem and root diseases in particular, Charcoal rot, Fusarium wilt, SDS and Phytopthora all are fungal diseases that cannot be controlled either preventatively or curatively at this time of year. One on-line source of brief disease descriptions with pictures is http://cipm.ncsu.edu/ent/ssdw/soyatlas.htm .

The soybean diseases that can occur in Maryland that can have an impact on yield, and can be managed with foliar applied fungicides are primarily: soybean rust, frogeye leaf spot and brown spot. Soybean rust due to the earlier drought in the South is not likely to be a problem this year. Its reestablishment and increase on kudzu has been severely delayed and is at the lowest level of infestation in August since monitoring for this disease began in 2005. Hurricane Irene therefore was highly unlikely to pick up enough spores from Florida and deposit them in Maryland to be an issue. Of the remaining two diseases Frogeye appears to have more potential for damage but is fortunately less common than Brown spot. Brown spot is the most common foliar disease but tends to be a lower canopy problem. Both are much more likely to develop if the previous crop was soybeans. I cannot find any data that would indicate there is much of a chance of getting a yield response from a fungicide by controlling frogeye or brown spot when the fungicide is applied after the R3 growth stage. The reproductive growth stages are: R1 initial flowering, R2 full flower, R3 small pods are formed at upper nodes, R4 full pod, R5 beginning seed, and R6 full seed. One on-line source for soybean growth stages can be found at: http://extension.agron.iastate.edu/soybean/production_growthstages.html . There are other common diseases that tend to form late in the season and tend to be considered seed quality issues. Another way to understand these late season diseases is that fungicides reduce them but without a significant change in yield. Only seed quality is improved. Although seed size is often improved, only rarely does that translate to significant yield increases. The most common of these diseases are purple seed stain, pod and stem blight, and anthracnose. There is a third category of generally late appearing diseases that are reduced by fungicides but we have little evidence that they respond to the fungicide with a yield increase, for example Phyllosticta leaf spot. In short as a general rule it takes a fair amount of foliar disease that starts early in the reproductive period, if not before, to cause enough damage to reduce yields that can be countered with a fungicide.

With regard then to our current post-Irene situation, here are my thoughts on fungicides in soybeans.

● Scout first. If you know there is a specific problem that is starting then you can much more easily decide on the need for a fungicide.

● If you are a seed producer and the soybeans are between R1-R3 then consider spraying. You will protect some seed quality and have the potential for managing brown spot or frogeye if present and in that case may see a yield benefit. If the beans are between R4 and R5 then consider spraying if you haven’t sprayed earlier. However, expect only a seed quality improvement.

● If you are growing soybeans for general production and are between R1-R3. Scout first! If you have symptoms of frogeye leaf spot or of brown spot in the middle of the canopy then a fungicide application may be warranted. This is much more likely to occur if the soybeans were planted no-till into soybean stubble. If the field is at R4 and symptoms of frogeye or brown spot are evident then spraying is much less likely to improve yields. If the soybeans are at R5 then there is little evidence that you will get your money back from a fungicide application especially if it is just brown spot. If there is frogeye, it’s R5, and it continues to be wet the disease will get worse but there is no evidence that you will get a return on the fungicide. If you are not comfortable with not spraying then consider spraying in strips. In essence run your own test. It will cost you less and you will have a better basis for making the decision next time.

● Knowing the field history and rotation history are helpful. If you have soybean planted no-till into soybean stubble (it could be wheat-bean or barley-bean stubble) then you have the best chance of having diseases that can be reduced with fungicides and therefore get a yield response with a fungicide. The next level of risk based on crop rotation is when soybeans are just a year apart in the rotation, e.g., full-season beans followed by corn followed by wheat or barley and then double-crop beans. The lowest level of risk is there is a two-year or more rotation to soybeans. The most likely thing to happen with a fungicide application at R3 to R4 is improvement of seed quality (for example reduced purple seed stain or improved test weight) but not necessarily improve yields. The higher the risk based on rotation then the higher the chance a yield improvement will be attained. In my opinion, you have a 30-50% chance of getting a yield boost (3-5 bu/A) in soybeans with the higher chance coming from the higher risk situation and spraying at R3.

● Last point. Spraying for yield protection without knowledge of what is out there has already led to the development of a strain of frogeye that is resistant to strobilurins (Headline, Quadris, etc.). This has been confirmed in IL, KY and TN. It’s not a very good insurance program to spray against diseases when you just spray for yield boost because you are destroying one of the best tools you might need later on.

Soybean Disease Update – August 26, 2011

Friday, August 26th, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Sudden death syndrome was diagnosed on soybean from Sussex County. This was an unexpected find since I associate SDS with cool, wet seasons. The only control for SDS is to plant resistant varieties, or, if susceptible soybeans are to be planted, double crop soybeans are rarely infected. I would not expect this to be widespread but we did see it so be on the lookout for symptoms.

Foliar symptoms of SDS

 Infected root on the left with diseased leaf compared to healthy stem and leaf.

Charcoal rot was also confirmed on soybean as well as corn. Soybeans are killed by the fungus resulting in dead, stunted plants in irregular patterns in the field. Symptoms are worse on full season early maturing varieties (Maturity Group III and possibly early Group IV’s). The diagnostic symptoms are the ashy gray stems and lack of roots. Splitting the lower taproot and stem longitudinally with a knife reveal the best diagnostic feature which is the presence of the small black microsclerotia in the pith and often the cortex. Charcoal rot is favored by hot, dry weather and is common in Delaware soils that have been heavily cropped to soybeans over the years. The tiny sclerotia overwinter in the soil and it just takes the right environmental conditions for the disease to occur. There is no usable resistance in soybeans to charcoal rot. The stress during flowering and early pod set triggers development of symptoms if that coincides with hot, dry, stressful weather. Most initial infections occur during the seedling stage and remain latent until flowering and later. Later maturing soybeans typically flower and set pods most years when conditions are less stressful and escape the yield reducing effects of the infection. Double crop soybeans are rarely affected by charcoal rot here in DE. Two year rotation will help reduce microsclerotia levels in the soil but if corn is used at least three years is suggested. Longer rotations would be better if they could be implemented.

 Split soybean stem showing the charcoal gray discoloration in the pith with microsclerotia.

 Close up of gray pith from the presence of the tiny microsclerotia of the fungus that causes charcoal rot.

Charcoal Rot Identified in Corn

Friday, August 26th, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Charcoal rot was identified in corn this week. Charcoal rot caused by the fungus Macrophomina is a common disease in soybean during hot, dry seasons. Occasionally it is seen in corn causing a stalk rot that in the early stages can look like the common stalk rots that we see here such as Diplodia, Fusarium and Giberella stalk rots. However when the stalk is split the characteristic sign of charcoal rot is the abundant small reproductive structures (sclerotia) inside the rind, especially on the vascular bundles. The inside of the stalk especially the lower 3-4 nodes are gray black giving it the name charcoal rot. Hybrids that have good resistance to other stalk rots often have some resistance to charcoal rot but the hot, dry conditions and early senescing of stalks can lead to infection and symptom development. Harvest in a timely manner to avoid lodging problems.

Charcoal rot on corn. Note the black sclerotia attached to the vascular bundles in the pith. That is the best diagnostic feature for identifying charcoal rot.

 

Soybean Disease Update – August 5, 2011

Thursday, August 4th, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Soybeans
Phyllosticta leaf spot was diagnosed on soybean this week. We have observed it in years past during periods of hot, dry weather on full season beans with a good canopy. It is a weak, pathogenic fungus and is probably infecting stressed plants. There should be no cause for concern since the disease is considered of minor importance. It will produce irregular spots often with small brown to black fruiting bodies of the fungus in the dead spots.

Phyllosticta leaf spot

Close up of Phyllosticta leaf spot

Alternaria leafspot on mite damaged soybean leaves.

With the dry weather look for the other drought related diseases in soybeans such as charcoal rot, and Alternaria leafspot. Alternaria is often observed when the leaves have been damaged from spider mites and this weak pathogenic fungus colonizes the leaves following the mite damage.

More Charcoal Rot in Soybeans

Friday, August 27th, 2010

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Two more Charcoal rot samples were identified this week. Charcoal rot is favored by dry weather that follows wet weather soon after planting. Usually irregularly sized patches of infected stunted, dying plants are seen in the field, but sometimes you can see individual plants in the row infected, and dead or dying. When this happens you see scattered plants dying either singly or several in a row flanked by healthy ones. Carefully dig up plants and look at the roots and lower stem. They will often be gray and if you scrape the gray lower stem or roots you can see many small black flecks that look like the tissue was covered with powered charcoal. Often if the plants are dead and you split the stems the pith is full of these tiny microsclerotia as well. Rotation will help to some degree, as well as planting later maturing varieties. This disease will always be a threat to Group II and III soybeans planted full season in fields previously cropped to soybeans when a season is wet early and then the crop is drought stressed during pod fill like this year. Double-crop soybeans are not usually infected with charcoal rot. As predicted, we will likely be seeing more of this as dead spots appear.

 

Charcoal rot on split soybean stem. Note the powdered charcoal appearance of the microsclerotia imbedded in the stem tissue and scattered in the pith.

Charcoal Rot is Showing Up in Soybeans

Friday, August 20th, 2010

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Charcoal rot was identified this week on a late Group III soybean variety. Charcoal rot is favored by dry weather that follows wet weather soon after planting. Usually irregularly sized patches of infected stunted, dying plants are seen in the field, but sometimes you can see individual plants in the row infected, and dead or dying. When this happens you see scattered plants dying either singly or several in a row flanked by healthy ones. Carefully dig up plants and look at the roots and lower stem. They will often be gray and if you scrape the gray lower stem or roots you can see many small black flecks that look like the tissue was covered with powered charcoal. Often if the plants are dead and you split the stems the pith is full of these tiny microsclerotia as well. Rotation will help to some degree as well as planting later maturing varieties. This disease will always be a threat to Group II and III soybeans planted full season, if planting in fields previously cropped to soybeans when a season is wet early and then the crop is drought stressed during pod fill like this year. Double-crop soybeans are not usually infected with charcoal rot. We will likely be seeing more of this disease as the month progresses.

Charcoal rot on split soybean stem. Note the powdered charcoal appearance of the microsclerotia imbedded in the stem tissue and scattered in the pith.

Agronomic Crop Diseases

Friday, September 19th, 2008

 Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Small Grains
Be sure that you plant wheat varieties with high levels of disease resistance. Select varieties with high levels of resistance to powdery mildew, leaf rust and stripe rust. Seed should be treated with Baytan, Raxil or Dividend to protect plants from loose smut and common bunt. Varieties that are susceptible to powdery mildew should be treated with Baytan, Dividend or other seed treatment that will protect them from early infection.

Soybeans
Do not ignore soybean cyst nematode. Soil sampling after harvest before any fall tillage is recommended for fields to be planted next season to soybeans following this year’s crop. Do not plant SCN susceptible varieties without soil testing first. Soil sample bags are available from the county Extension offices for $10/sample bag.

Septoria brown spot and downy mildew are still evident on some late beans. Most levels that I have seen should not limit yield in any way.

Charcoal rot was diagnosed this week on a Group IV soybean in Sussex County. The irregular spots of dead and dying soybeans at first observation looked as if they were maturing due to the drought but upon closer observation of the roots and stems that it was charcoal rot. This soilborn fungus infects soybeans early in the season and if stress conditions persist, infection results in lower stem decay and prematurely dead plants. The spots were not that large and the plants did set some pods but it would have been easy to just think it was the drought. Rotation and planting later maturing varieties in areas where charcoal rot has been a problem in the past is the best control program.

 

Charcoal rot on split soybean stem. Note the powdered charcoal appearance of the microsclerotia imbedded in the stem tissue and scattered in the pith.

Corn
Corn harvest is underway so be sure to check corn fields for lodging potential by squeezing the lower nodes or pushing on the stalks. A simple way to do this is to walk through the field and, keeping your hands at chest height, push stalks 8-10 inches from vertical. If 10-15% of the stalks lodge, schedule the field for early harvest before a strong wind results in severe lodging.

Fusarium ear rot caused by Fusarium moniliforme also know as Fusarium verticillioides has been seen this week in several locations. Hybrids that have been holding their ears vertical and have poor ear cover can be more susceptible to ear rots that benefit from moisture trapped in the ears. Ears that have been damaged by insects, particularly corn earworm, can also have more ear rot fungal infections. Fusarium moniliforme can produce mycotoxins called fumonisins (see below), but not all isolates of the fungus produce fumonisins. Infected grain should be dried to 15% or below to prevent mold growth in storage.

When evaluating an ear rot problem, remember that certain ear rots are a warning sign to suspect toxins, but ear rots do not always lead to toxin problems. When potentially toxigenic ear rots are noticed in the field, grain can be managed to minimize toxin development. If more than 10 percent of ears have a significant amount of mold (25 percent of the ear or more), these fields should be harvested and the corn dried as soon as possible. The combine will remove some of the moldiest kernels.

The best option for moldy grain is to feed it or sell it instead of storing it. However, it should be tested for toxins before feeding. Testing for mycotoxins can be done before putting the grain in storage. The best sampling method is to take a composite sample of at least 10 pounds from a moving grain stream, or to take multiple probes in a grain cart or truck for a composite 10-pound sample. If toxins are present, it is possible that it can be fed to a less sensitive livestock species, such as beef cattle (depending on the specific toxin and its concentration). A veterinarian or extension specialist can help with these decisions. If the grain is sold, there may be a reduced price due to mold damage.

Cleaning the grain removes fine particles that are usually the moldiest and most susceptible to further mold development. “Good storage conditions (proper temperature and moisture content, aeration, insect control, clean bins) and regular inspection are essential in preventing mold and toxin development in any stored corn.” G. Munkvold (Iowa State Univ. Ext.)

 

Fusarium ear rot caused by Fusarium moniliforme