Posts Tagged ‘field corn diseases’

Dealing With Corn Ear Rot Infected Grain

Friday, September 23rd, 2011

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

We continue to get samples of stalk rot, and now ear rots in field corn. More samples of Fusarium ear rot caused by Fusarium moniliforme also known as Fusarium verticillioides have been received in the diagnostic lab this week. 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% of ears have a significant amount of mold (25% 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.

Fusarium ear rot caused by Fusarium moniliforme

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.)

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.

 

Corn and Soybean Disease Update – August 12, 2011

Friday, August 12th, 2011

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

Corn
Gray leaf spot is increasing in irrigated corn but it is too late to affect yield. This late in the season we often see an increase in gray leaf spot as well as Northern corn leaf blight.

We have seen several samples of ears with the milk line half-way up the kernels with random discolored kernels. This is Fusarium ear rot caused by Fusarium moniliforme. The fungus travels down the silks and infects the individual kernels. Often white fungal growth can be seen as well. Hybrids vary in their susceptibility and infection is favored by hot, dry weather. When severe the whole ear can be whitish. Often most of the fungus growth is limited to the tips of the ears. If grain is to be stored it is important to dry it sufficiently to prevent growth of the fungus to prohibit growth of the mycotoxin, fumonisin.

Individual random kernels and some tip infection by Fusarium moniliforme.

Soybeans
Despite the hot, dry weather downy mildew is showing up in full season irrigated soybeans now. Varieties vary in their resistance, but this fungus disease has never been yield limiting or damaging here. The disease produces numerous small yellow spots on the upper leaf surface and a tuft of grayish fungal growth on the corresponding lower leaf surface.

Downy mildew on the upper leaf surface of soybean caused by Peronospora manshurica.

Grayish tufts of the downy mildew fungus on the lower leaf surface.

Agronomic Crop Disease Update – July 8, 2011

Friday, July 8th, 2011

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

Soybeans
Now is the time to start checking soybeans for soybean cyst nematode. Once soybeans have reached the 3rd trifoliate leave stage (roughly about 28-32 days from planting) the white or yellow female cysts can be seen on the roots. If you see irregular patches of stunted soybeans don’t presume the stunting is from drought. Digging the plants carefully may reveal SCN is present and could be the cause of the stunting. If you are seeing many cysts and stunting on resistant soybeans it is time to rotate out of that field to reduce SCN egg numbers.

White and yellow female soybean cyst nematodes on roots, 34 days after planting

Corn
Three corn samples arrived in the plant clinic this week with bacterial stalk rot. If you are irrigating from surface water sources, such as ponds or ditches, there is a risk of bacterial stalk rot. The bacterial can be in the irrigation water and get trapped in the whorl, the ear leaf sheath, and the ear shank. These places provide a place for water to sit and the bacteria can enter the stalks and cause a soft decay of leaf sheath, stalk, and ear shanks. It is foul smelling as well. It appears as random infected plants in the field and as a result it does not cause major losses. Corn is thought to be susceptible for a short period of time and the older the corn the less likely infection will occur. There is no chemical control for bacterial stalk rot. Treating irrigation water in the system with hypochlorite is an alternative solution.

Bacterial stalk rot

 

Bacterial Stalk Rot in Corn

Friday, July 9th, 2010

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

If you are irrigating from surface water sources such as ponds or ditches, there is a risk of bacterial stalk rot. The bacteria are in the irrigation water and if the whorl or the ear leaf sheath and ear shank provide a place for water to sit, bacteria can enter the stalks and cause a soft decay of leaf sheath stalk and ear shanks. It is foul smelling as well. It appears as random infected plants in the field. Corn is thought to be susceptible for a short period of time and the older the corn the less likely infection will occur. There is no chemical control for bacterial stalk rot. Treating irrigation water in the system with hypochlorite is an alternative.


Bacterial stalk rot

Corn Disease Update

Friday, September 4th, 2009

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

Northern Corn Leaf Blight and Gray Leaf Spot
Northern corn leaf blight and gray leaf spot have been pretty common this season on many hybrids. The cooler season and increased rainfall in many areas has resulted in more of both diseases this year. Fungicide applications to corn should pay dividends this season.

rectangular lesions of gray leafspotNote the rectangular lesions of gray leaf spot.

Northern corn leafblightNorthern corn leaf blight lesions are wide with bluntly rounded ends. Note the gray leaf spot lesion above it for comparison.

Stalk Rot and Ear Rot in Corn
Be on the lookout for stalk rots and ear rots as harvest approaches. Diplodia ear rot has been diagnosed in two sweet corn fields last week and Fusarium and Gibberella ear rot will probably be seen as well. The following article from Kentucky is on the Kent County Ag Blog and is a good treatment of the rots were are likely to see this fall.

The common late-season stalk rots are caused by fungi and include: Gibberella stalk rot (Gibberella zeae = Fusarium graminearum), anthracnose (Colletotrichum graminicola), Fusarium stalk rot (Fusarium moniliforme), charcoal rot (Macrophomina phaseolina), and Diplodia stalk rot (Diplodia maydis). It is common for more than one stalk rot organism to attack a plant at the same time.

Symptoms
Stalk rots caused by Gibberella, Fusarium and Diplodia fungi are not usually apparent until several weeks after pollination. Diseased plants may die suddenly in various areas within the field, with leaves first turning a dull, grayish-green similar to the color caused by frost or drought damage. Death of the entire plant follows within 7 to 10 days in susceptible hybrids. The lower internodes turn from green to tan, straw-colored, or dark brown and are spongy and easily crushed. When the stalks are split lengthwise, only the vascular strands are intact and the pith tissue is decayed.

Stalks infected with the Gibberella fungus have a characteristic pink to reddish discoloration of the pith and vascular strands. The breakdown of pith tissues starts at the nodes soon after pollination and becomes more severe as the plant matures. Rotting also commonly affects the roots and crown as well as the lower internodes. An additional identifying feature is the presence of small, round, bluish-black perithecia (fungal-fruiting bodies) which form on the surface of Gibberella-infected stalks in the fall or the following spring. These fruiting bodies are easily scraped off with a thumbnail. Fusarium stalk rot looks similar to Gibberella, except that the discoloration of infected tissues commonly varies from whitish-pink to salmon.

Diplodia stalk rot can be distinguished from other stalk rot diseases by the numerous, small, black dots (pycnidia) which the fungus produces at or near the lower nodes of infected stalks. Unlike the perithecia formed by the Gibberella fungus (which may also be clustered near the lower nodes), the pycnidia of Diplodia are embedded in the rind and cannot be scraped off with a fingernail. However, individual stalks may have fruiting bodies of both fungi if a double infection has occurred.

Corn anthracnose has become much more prevalent in Kentucky since the early 1970s. In addition to rotting the lower stalk, the anthracnose fungus is capable of attacking the stalk above the ears, causing dieback and breakage of the plant tops (borer injury in the top of the plant may cause similar symptoms). The fungus also commonly causes a leaf blight. Although the lower stalk rot phase of anthracnose may cause very susceptible hybrids to be killed before pollination, most hybrids are killed only a week or two before normal maturity. A shiny black or dark brown discoloration of the rind late in the season is a typical symptom of anthracnose on the stalk. This black discoloration usually extends up the stalk for several internodes and may uniformly discolor the rind or give it a blotchy or speckled appearance. The pith tissue beneath these lesions becomes brown or black, especially around the nodes. When lodging occurs, it is usually higher on the plant than with other stalk rot diseases.

Here are two pictures of Diplodia ear rot on sweet corn which would also be applicable to field corn:

Diolodia enters the ear at the leaf sheathDiplodia enters the ear at the leaf sheath and progresses up the ear shank causing the rot. It usually is not found on the tips of the ears initially.

Diplodia ear rot -- look for white fungal growth and small black pycnidia 

Look for the white fungal growth and the small black reproductive structures of the fungus on the husks.

Corn and Corn Fungicides

Thursday, July 2nd, 2009

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

The following was written by Dr. Arv Graubaskas, Extension Field Crops Plant Pathologist at the University of Maryland in his Maryland Field Crop Disease Notes. The issue of applying fungicides to corn when part of the field is replanted is addressed. This question has been asked and this is the best answer we know.

In some parts of the state the earliest planted corn is close to developing a tassel. There has also been a lot of corn that had to be replanted due to drowning. In many other cases corn was only recently planted because the soil finally dried out enough to get equipment onto the field. There is thus a tremendous range of growth stages of corn out there and in many cases in the same field. Considering the weather pattern is still pretty wet the legitimate question of whether or not a fungicide program is warranted is raised.

First don’t get sold on the plant health or plant physiology response to strobilurin fungicides (Headline, Quadris, and to a lesser extent Quilt and Stratego) alone. Key word here is alone. The usual sales pitch involves stating that millions of acres have been treated and the average response has been 8, 10 or even 15 bu/A. These averages include cases that had significant foliar diseases as well as cases with little or no disease. These fungicides are great disease management tools and when diseases like gray leaf spot are significant will outperform other classes of fungicides. The performance of these fungicides when diseases are clearly yield limiting is so outstanding that it skews the average response number.

Let me illustrate with my data from last season. I had 16 replicated “trials” across the state across a range of crop histories, planting dates and hybrids that compared untreated corn vs. a strobilurin fungicide. The average response across these trials was 7.3 bu/A, but in only five of these trials was the positive (yield beneficial) response statistically significant. More importantly in those five trials gray leaf spot was a serious problem and the actual responses to the fungicide in those five cases ranged from 24 to 38 bu/A. In other words, where I did not have enough gray leaf spot to cause losses and thus the fungicide could only provide a yield advantage through alteration of plant physiology the average response was -0.7 bu/A. The overall average looks good but it is skewed by the cases that really benefited which were those cases where the fungicide primarily worked as a disease control agent. The plant physiology or plant health type effect occurs in concert with disease control to often outperform other classes of fungicides when diseases are a problem. Where there is no disease the plant physiology side benefits have little or no effect on yield or stand on a consistent basis. The bottom line is an insurance program where there is no need for insurance will only cost you money. Use a fungicide when you have a real risk of a foliar disease. The highest risk of getting gray leaf spot, the primary foliar disease of corn, involves three factors: 1) a susceptible corn hybrid, 2) no-tilling corn into corn stubble, and 3) a relatively wet season.

There is one additional factor that needs to be considered regarding fungicides in corn, especially this season. It gets us back to the comment that there is quite a range of growth stages out there and in many cases in the same field. Fungicides applied by air in a fullgrown corn crop generally perform better if surfactants are used. The surfactants help the fungicide to penetrate the canopy and be better distributed throughout the canopy. Arrested ear syndrome, where the development to corn ears is damaged resulting in a percentage of small malformed ears, has been associated with fungicides applied with a non-ionic surfactant (NIS) or sometimes other products (certain formulations of tankmixed products) especially if the corn is in the late vegetative stages of development just before tassel. Most of the damage seems to be associated with NIS surfactants but the fungicide formulation and tank-mixed products are not completely exonerated. It is therefore important that if one decides on using a fungicide that it is applied at or after tassel formation is completed. If it is close or there are parts of the stand that are not yet in tassel then NIS surfactants should not be used and be wary of any tank-mix products. One final comment on fungicides in corn, these products have not been shown to directly reduce stalk rots. Fungicides affect stalk rots and therefore improve stand, when there is a foliar disease. There is no direct action of the fungicides applied at or near tassel on the stalk rot pathogens that develop much later in the season. However, significant loss of effective leaf area from foliar diseases predisposes plants to stalk rots. Therefore, when there is a foliar disease problem and you manage it with a fungicide you get the additional benefit of reducing stalk rots.

The bottom line is fungicides have been proven to be excellent disease management tools for foliar diseases in corn and have an indirect effect on lodging due to stalk rots of the crop. The additional benefits of strobilurin fungicides are the reason they often outperform other fungicide classes with regard to yield when diseases are an issue. They should be considered when the risk of a foliar disease is high. Otherwise they are expensive insurance with a relatively low chance of a return if significant foliar diseases do not develop.

Corn and Wheat Disease Update – June 5, 2009

Friday, June 5th, 2009

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

Corn
No need for me to tell you how bad the weather has been so far for corn. Stands are still being reduced by excessively wet soils and the Pythium and Fusarium damping-off that is occurring as a result of the wet soils. Fungicide treated seed, good drainage and some warm temperatures would help considerably in getting the plants out of the ground and growing.

Wheat
Fusarium head blight or scab is being seen in some fields in Kent and Sussex counties. The occurrence and severity so far has been variable but, in general, I think we dodged a bullet this time. Our wheat for the most part was already in flower before the most favorable weather came for scab (Figure 1).

fusarium head blight 

Figure 1. Fusarium head blight or scab.

Take-all was diagnosed this week as well from two fields. Take-all is characterized by patches in the field that can vary in size but the wheat is generally stunted and the heads bleach out prematurely. Infected plants can be easily pulled out of the ground due to the extensive root rot that occurs. The other symptom is the dark streaking at the base of the stem (lowest node under the leaf sheaths), see Figure 2. Take-all can be controlled by rotating out of wheat for a year. However planting wheat followed by double crop soybeans followed by wheat is not an effective rotation for take-all control. Manganese levels also interact with take-all. Be sure that soil levels of manganese are adequate for the crop and check pH so that the managanese is available. High pH makes manganese unavailable.

takeall

Figure 2. Take-all symptoms on the lower nodes. Note lack of roots as well.

Tan spot (Figure 3) has been present for almost three weeks in wheat. This foliar disease can look like Septoria (Stagnospora) leaf and glume blotch. It is caused by the fungus Pyrenophora tritici-repentis.

 tanspot

Figure 3. Tan spot symptoms on wheat.

It has been widespread on Delmarva this season because of the amount of rainfall that we have had. It is too late for any control, but this disease is favored by wet, warm weather. Most of the spots are in the lower canopy and may reach the flag leaf before the plants begin to dry down. Applications of foliar fungicides at heading or earlier have been providing good control of this disease. At present most of the infection is in the lower canopy and the effect on yield should be minimal if the disease does not move up to the flag leaf or the leaf below the flag leaf.