Integrated Pest & Crop Management Newsletter University of Missouri-Columbia Vol. 16, No. 17 September 15, 2006 Discolored Soybean Seed By Laura Sweets This may be a year when there are numerous questions concerning various types of soybean seed discoloration. Soybean diseases are one of several factors which can cause discoloration and deterioration of soybean seed. But soybean diseases are only one of the factors which may be involved in this year’s problem. Much of the state was extremely dry during July and August. Drought stress may be contributing to small seed size, wrinkles or cracks in seed coats and even some of the seed discoloration. If the soybean plants died prematurely because of drought stress and/or late season diseases such as charcoal rot, there may be immature, green seed in the harvested grain. Discoloration of soybean seed this year appears to be the result of environmental stress compounded by late season stem and pod diseases. When the late season pod and stem diseases occur, maturing plants have a blackish cast and black to gray spots, blotches and streaks may cover stems, branches and pods. The late season diseases led to increased problems with discolored and damaged soybean seed. Purple seed stain; a general blotchy brown discoloration that might be the result of the Cercospora or Colletotrichum species which cause anthracnose and tipblight; bleeding hilum which can be caused by virus diseases such as soybean mosaic and bean pod mottle; a white mold growth which could be Phomopsis seed decay or secondary fungi entering through pods damaged by insects are all showing up in beans. The diseases which contribute to discolored soybean seed are usually favored by wet conditions late in the season. Weather conditions from now through harvest will have a major influence on how severe discoloration and deterioration of soybean seed is this season. Symptoms of the seed damage which may result from Phomopsis seed decay, purple seed stain, frogeye leaf spot, virus diseases and Colletotrichum anthracnose and tipblight are described below. Phomopsis seed decay Phomopsis seed decay results when the fungi which cause pod and stem blight move from the stems and pods onto the seed. Plants infected with pod and stem blight may be stunted and have discolored stems. Black pycnidia or fruiting bodies of the fungi Phomopsis sojae or Phomopsis longicolla develop on the lower portion of the main stem, branches and pods as plants reach maturity. The pycnidia may be limited to small patches usually near the nodes or may cover dead stems and pods. On stems, the pycnidia are usually arranged in linear rows while on pods they are randomly scattered. Prolonged periods of warm, wet weather during flowering and pod fill favor the development of pod and stem blight. If conditions remain warm and wet, the fungus may grow through the pods and infect the seed. Infected seed is oblong or misshapen and may have a white moldy appearance. Purple seed stain Cercospora kikuchii can infect soybean seeds, pods, stems and leaves but is most commonly found on the seed. However, during the last several years leaf spot and leaf blight caused by this fungus have been prevalent in parts of the state. Foliage infection occurs on the uppermost leaves and begins as reddish purple to reddish brown angular to somewhat circular lesions on the soybean leaves. These lesions may coalesce to kill larger areas of leaf tissue. The entire uppermost trifoliolate leaf and petiole may be blighted and brown. One striking symptom of this disease can be the premature yellowing and then blighting of the youngest, upper leaves over large areas of affected fields. In most fields symptoms do not progress down the plants more than one to two nodes. Pods at the uppermost nodes may develop round, reddish purple to reddish brown lesions. Infected seed show a conspicuous discoloration varying in color from pink to pale purple to dark purple. The discoloration may range from small specks to large blotches which cover the entire surface of the seed coat. Warm, humid weather favors disease development. Yields are usually not reduced but a high percent of seed stain may be evident at harvest. Frogeye leaf spot Cercospora sojina causes frogeye leaf spot on soybean. Symptoms occur primarily on leaves although the causal fungus may also infect stems, pods and seeds. Lesions are small, circular to somewhat angular spots that develop on the upper leaf surfaces. Initially the spots are dark and water soaked in appearance. As the lesions age they develop a dark reddishbrown border. The center of the lesion becomes light brown to light gray in color. Lesions may merge to kill larger areas of the leaf. Heavily spotted leaves may wither and drop prematurely. Stem lesions usually develop later in the season. Young stem lesions are deep red with a narrow, dark brown to black margin. As the stem lesions age, the centers become brown to smoky gray in color. Lesions on pods are circular to elongate, slightly sunken and reddish brown. The fungus can grow through the pod wall to infect maturing seed. Infected seeds may show discoloration of the seed coat that ranges from small specks to large blotches of light gray to dark gray or brown. Virus diseases There are several virus diseases which may occur on soybean in Missouri including bean pod mottle, soybean mosaic and tobacco ringspot or budblight. Of these, soybean mosaic virus and bean pod mottle virus are most likely to cause symptoms on the seed. Seed infected with soybean mosaic or bean pod mottle virus may have a symptom called bleeding hilum. This is a discoloration, usually black or dark in color, that bleeds from the hilum down the sides of the seed. The affected area may be quite small and near the hilum or may be quite extensive and cover most of the seed. It is important to keep in mind that bleeding hilum is also a genetic characteristic of certain soybean varieties. The intensity of the discoloration can be influenced by environmental conditions during the growing season. Colletotrichum anthracnose and tipblight Colletotrichum truncatum and several other Colletotrichum species cause anthracnose of soybean. Typically, anthracnose is a late season stem and pod disease of soybean. Symptoms occur on stems, pods and petioles as irregularly shaped, light to dark brown spots, streaks or lesions. Eventually black fungal structures may be evident in these lesions. Anthracnose may also cause a tipblight. The tipblight phase of anthracnose causes a yellowing or browning of the uppermost leaves and pods. The blighted tips may dry up and die prematurely. This fungus may also infect seed. Seed may be smaller than normal and severely infected seed may be a moldy, dark brown in color and shriveled. Anthracnose is favored by warm, wet weather, and the tipblight phase of anthracnose is most likely to occur after a rainy period. The incidence and severity of the soybean diseases which cause seed discoloration and deterioration are greatly increased by warm, wet conditions late in the season. For grain crops there are no potential rescue treatments. Fields should be harvested as soon as possible to prevent further seed damage. Many of the pathogens causing seed discoloration and deterioration can survive on soybean seed. Heavily infected seed, if planted, could produce diseased seedlings resulting in stand problems. Therefore, seed from infected fields should not be saved for planting. If infected seed must be used for planting, it should be thoroughly cleaned, a sample submitted for a germination test (preferably a stress test) and a fungicide seed treatment applied. Many of the pathogens that cause these diseases may also survive in infested residues left on the soil surface. Thus, crop rotation is an important means of preventing or reducing disease outbreaks. At least one year between soybean crops in recommended. Varieties may differ in their reaction to these various diseases and, if possible, good quality seed of resistant varieties should be planted. Laura Sweets 573-884-7307 ******** Time to Consider Fall Herbicide Applications Nick Monnig and Kevin Bradley As summer turns to fall most producers across the state have their minds focused on the upcoming harvest but it’s not too early to be thinking about fall herbicide applications. Some producers like fall herbicide applications because it enables them to reduce spring workloads by controlling winter annual weeds in the fall rather than using a burndown prior to planting in the spring. Many have also speculated that fall herbicide applications have a beneficial impact on the insect populations and soil conditions experienced at planting. In an effort to learn more about the impact of fall and spring herbicide applications on winter and summer annual weed populations, soil conditions, and insect populations, we have conducted a number of experiments in no-till corn and soybean fields over the past several years. Here we provide a brief summary of the results. First, we have found in both our corn and soybean experiments that fall applications of non-residual herbicides like glyphosate plus 2,4-D are often ineffective because additional germination of winter annual weeds usually occurs in the spring. This usually translates into poor winter annual weed control at planting, thereby negating the whole purpose of the fall herbicide application. For this reason, for fall applications we generally recommend a residual herbicide like Canopy EX or Valor in soybean or Princep, Basis or Valor in corn for winter annual weed control. Our results also indicate that applications of residual herbicides made in early March can provide the same level of winter annual weed control as applications of these herbicides in the fall. Furthermore, our data indicate that early spring applications of residual herbicides provide better control of emerging summer annual weed seedlings than fall herbicide applications. This is especially the case with the Canopy EX treatment in our soybean experiments. Although we realize that this timing--early March--defeats some of the purposes of the fall application timing, our results indicate that with our current herbicide arsenal, the early spring timing is better if your objective is to achieve excellent winter annual and some summer annual weed control. If your primarily goal is just to eliminate your winter annual weed populations, then our experiments show that fall and early spring applications of residual herbicides will provide similar levels of control. In our evaluations of the effects of fall herbicide applications on soil conditions at planting, the results from our trials indicate that winter annual weeds can "wick" significant amounts of moisture from the soil. At several different periods throughout the spring, the untreated plots that contained winter annual weeds in our experiments had a lower soil moisture content than plots that were treated with herbicides and contained no winter annual weeds. This difference was amplified in the spring of 2006 when the soil was already deficient of moisture. This tells us that in years when we experience a wet spring, we are less likely to see any impact of winter annual weeds on overall soil moisture content, but in years with a dry spring, the impacts of winter annual weeds on soil moisture will be readily apparent. In our evaluations of the effects of fall herbicide applications on soil temperature, we found that the removal of winter annual weeds with fall herbicide applications did increase soil temperatures compared to untreated plots, but only when air temperatures reached 70 degrees. Therefore, based on our research, the notion that fall herbicide applications will always increase soil temperatures is somewhat misleading. Insect populations in our experiments were highly dependent on the presence of winter annual weeds and it was difficult to see any specific effects of individual herbicides or application timings. In our soybean studies, several weeks after planting insect pest populations in the untreated plots that contained winter annual weeds were 10-fold higher than those in plots treated with a herbicide to remove these weeds. The predominant insect that we encountered in these studies was the negro bug, which appears to be an emerging pest of no-till soybean in Missouri. Although we saw little to no differences in the incidence and severity of insects in our corn experiments, the impact of fall herbicide applications on black cutworm populations should not be overlooked. It is known that black cutworm moths are attracted to fields with dense populations of winter annual weeds, and that black cutworm larvae can cause severe injury in corn. It is clear from the results of our experiments that there are many other factors, other than weed control, that you should consider when deciding whether or not to make a fall herbicide application. Probably the most important one for a grower that hasn’t been mentioned thus far is cost. Make sure that the fall herbicide program matches your needs and won’t just be an added cost to your operation. As mentioned previously it is of little value to apply a herbicide in the fall to control winter annual weeds and have to come back with a burndown in the spring anyway. Due to the wet weather usually experienced in Missouri in the early spring, we realize that this timing may not be as convenient or feasible as the fall application timing. However, our results clearly show that with our current herbicide options, you will achieve similar levels of winter annual weed control and even better levels of summer annual weed control with residual herbicide applications in early March compared to the fall. This does not mean that you won’t need to apply another herbicide in-crop for summer annual weed control--you will--but the need for your burndown should be eliminated. Kevin Bradley Nick Monnig 573-882-4039 ******** Late Season Soybean Diseases By Laura Sweets This has been an unusual year for soybean diseases. Although some of the different late season diseases are showing up, no one disease appears to be prevalent across the state. Symptoms of environmental stress are more pronounced than disease symptoms in many fields. And in some cases, environmental stress has led to colonization of these weakened plants by root and stem fungi. Stunted, poor vigor plants may have cankers or lesions close to the soil line. Macrophomina phaseolina (the charcoal rot pathogen), Rhizoctonia solani, Phomopsis species and Fusarium species can be recovered from these damaged stem areas. Sudden death syndrome has developed in some fields, particularly fields planted early prior to the extended period of cool, wet weather in late spring. It has also become more prevalent in the northern part of the state where cooler temperatures and rain came several weeks ago. Septoria brown spot has been causing yellowing or browning of leaves and premature leaf drop in some fields. Cercospora kikuchii and Colletotrichum spp. have been causing leaf spot, yellowing and shoot dieback. Charcoal rot is evident in fields that suffered moisture stress earlier in the season or in areas suffering from drought conditions this fall. Losses from soybean cyst nematode may be greater when plants are stressed. This would be a good year to sample fields for SCN. Yield losses from these various late season diseases will vary depending on when symptoms began to occur, number of plants infected, severity of disease in infected plants and weather conditions from now to harvest. In some cases although yellowing of the upper nodes may be quite widespread and spectacular in a field, damage is limited to the uppermost leaves and pods so yield loss should be minimal. In other cases, especially with sudden death syndrome, the entire plant may have been killed prematurely. If large areas of a field are thus affected, yield losses will be greater. Although it is too late in the season to do much to control these diseases this year, management strategies to prevent or minimize these diseases next season are also given below. Sudden death syndrome Symptoms of sudden death syndrome (SDS), caused by a strain of Fusarium solani, may appear several weeks before flowering but are more pronounced after flowering. Foliage symptoms begin as scattered yellow blotches in the interveinal leaf tissue. These yellow blotches may increase in size and merger to affect larger areas of leaf tissue. Yellow areas may turn brown but veins remain green giving the leaves a striking appearance. Infected plants may wilt and die prematurely. Severely affected leaflets may drop off the plant leaving the petiole attached or may curl upward and remain attached to the plant. Root systems may show deterioration and discoloration of lateral roots and taproot. When split open, internal tissues of the taproot and stem may show a light gray to light brown discoloration. Management options for SDS are somewhat limited but should include planting varieties which have performed well where SDS has been a problem, improving drainage in poorly drained fields, avoiding compaction, staggering planting dates, delaying planting until soils are warm and dry, avoiding continuous crop soybean, maintaining good crop vigor, avoiding crop stress including stress from soybean cyst nematode and harvesting fields with SDS in a timely fashion. Septoria brown spot Septoria brown spot causes small brown spots on the unifoliolate and lower trifoliolate leaves. The individual spots may run together forming irregularly shaped brown blotches on the leaves. Infected leaves may yellow and drop prematurely. Brown spot usually starts on the lower portion of the plant. Under favorable weather conditions (warm, wet weather), the disease may move up through the plant. Brown spot was evident in many Missouri soybean fields earlier this season. Hot, drier weather during July slowed down disease development. But late season rains can trigger a reoccurrence of Septoria brown spot. Symptoms move up through the canopy of soybean plants. Lower leaves may show heavy spotting, yellowing and dropping prematurely. Upper leaves may also show spotting and yellowing. Some fields which have a yellow cast from the road may be showing symptoms of Septoria brown spot rather than SDS. The fungus which causes this disease, Septoria glycines, survives in infested residues left on the soil surface. Fields with continuous soybean production are more likely to show damage. Planting disease-free, good quality seed of resistant varieties, rotating crops with at least one year between soybean crops and maintaining good plant vigor should reduce losses from Septoria brown spot. Frogeye leaf spot Frogeye leaf spot begins as small, reddish brown circular lesions on soybean leaves. The individual lesions do not increase greatly in size but as they mature the center of the lesion turns light tan to gray. The mature lesions have a light center with a dark purple or brown border. On severely infected leaves the lesions may merge killing larger areas of leaf tissue. These areas may drop out giving the leaves a very tattered or lacy appearance. From a distance fields with high levels of frogeye leaf spot may have a dry, brown cast. The fungus which causes this disease, Cercospora sojina, can survive in infested residues left on the soil surface. Fields with continuous soybean production are more likely to show damage. The disease is favored by warm, wet weather, high humidity and conditions which increase moisture levels in the canopy. Shaded fields and fields with heavy tree growth along the edges and river bottoms have been more likely to have severe frogeye leaf spot this season. Planting disease-free, good quality seed of resistant varieties, rotating crops with at least one year between soybean crops and maintaining good plant vigor should reduce losses from frogeye leaf spot. Cercospora leaf spot and purple seed stain Cercospora kikuchii can infect soybean seeds, pods, stems and leaves but is most commonly found on the seed. However, this year we are seeing some cases of leaf spot or leaf blight caused by this fungus. Infection is primarily occurring on the uppermost leaves and begins as reddish purple to reddish brown, angular to somewhat circular lesions on the soybean leaves. These lesions may coalesce to kill larger areas of leaf tissue. The uppermost trifoliolate leaf and petiole may be blighted and brown. One striking symptom of this disease may be the premature yellowing and then blighting of the youngest, upper leaves over large areas of affected fields. In most fields, the symptoms have not progressed down the plants more than one to two nodes. Pods at the uppermost node may develop round, reddish purple to reddish brown lesions. This pathogen may also infect seed causing purple seed stain. Infected seed show a conspicuous discoloration ranging in color from pink to pale purple to dark purple. The discoloration may range from small specks to large blotches which cover the entire surface of the seed coat. Temperatures of 82-86 degrees with extended periods of high humidity favor disease development. At this point in the season control of Cercospora leaf spot and purple seed stain is not feasible. It is important to remember that since this fungus can infect the seed, seed from heavily infected fields should not be used for seed. If infected seed must be planted, seed lots should be thoroughly cleaned and an appropriate seed treatment fungicide used. Rotating soybean with crops other than legumes will also help reduce Cercospora leaf spot and blight in future soybean crops. Colletotrichum species Colletotrichum truncatum and several other Colletotrichum species cause anthracnose of soybean. Typically, anthracnose is a late season stem and pod disease of soybean. Symptoms occur on stems, pods and petioles as irregularly shaped, light to dark brown spots, streaks or lesions. Eventually black fungal structures may be evident in these lesions. Anthracnose may also cause a tipblight. The tipblight phase of anthracnose causes a yellowing or browning of the uppermost leaves and pods. The blighted tips may dry up and die prematurely. Anthracnose is favored by warm, wet weather, and the tipblight phase of anthracnose is most likely to occur after a rainy period. Again, at this point in the season control of anthracnose is not feasible. This fungus may also infect seed so seed from heavily infected fields should not be used for seed. If infected seed must be planted, seed lots should be thoroughly cleaned and an appropriate seed treatment fungicide used. Rotating crops with at least one year out of soybean will also help reduce anthracnose. Charcoal rot Charcoal rot may cause a seedling infection in soybean but is more common later in the season. Infected plants are less vigorous and may have smaller leaves. Leaves may turn yellow and wilt. The taproot and lower stem may develop a silvery gray to light gray discoloration of the epidermis. Fine black specks or microsclerotia may be evident in tissues below the epidermis and eventually in epidermal tissues. If the lower stem and taproot are split open, a reddish brown to blackish discoloration may be seen in vascular tissues of the taproot and stem. The fungus which causes charcoal rot, Macrophomina phaseolina, is a common soil fungus in Missouri. Corn and grain sorghum may also be hosts of the charcoal rot fungus. Charcoal rot is favored by hot, dry weather so symptoms usually appear at temperatures between 82-95 degrees. Management of charcoal rot of soybeans should include rotating to cereals, cotton or other nonhosts for one or two years. Maintaining good crop vigor through the season will help reduce losses from charcoal rot. Laura Sweets 573-884-7307 ##################################################################### Weather data for the Week Ending September 11, 2006 By Pat Guinan -------------------------------------------------------------------------------- | Monthly | Growing Weekly Temperature (deg. F) |Precip (in.)|Degree Days^ -----------------------------|------------|------------ Ext- Ext- Depart| Depart|Accum Depart Avg.Avg. reme reme from |Sep 1 from |since from Station County Max.Min. High Low Mean avg. |Sep 11 avg |Apr 1 avg. ------------------------------------------------------|------------|------------ Corning Atchison 78 58 85 53 67 -4 | 1.52 +0.13 | 3449 +489 St. Joseph Buchanan 78 59 83 56 68 -3 | 0.44 -0.92 | 3394 +380 Brunswick Chariton 82 57 88 52 68 -3 | 1.57 +0.68 | 3395 +337 Albany Gentry 77 56 85 49 66 -5 | 2.33 +1.27 | 3219 +200 Auxvasse Audrain 83 58 87 52 69 -2 | 1.22 +0.23 | 3360 +288 Columbia Boone 82 59 85 55 70 -2 | 0.14 -0.79 | 3474 +285 Sanborn Field Boone 84 61 87 57 71 -2 | 0.20 -0.73 | 3688 +429 Williamsburg Callaway 84 58 88 54 69 -2 | 1.59 +0.60 | * * Novelty Knox 80 58 86 53 68 -3 | 0.38 -0.45 | 3134 +122 Linneus Linn 80 56 86 51 67 -3 | 0.48 -0.46 | 3192 +256 Monroe City Monroe 81 57 85 52 68 -3 | 1.96 +0.99 | 3256 +187 Versailles Morgon 86 60 89 55 71 -1 | 0.75 -0.20 | 3722 +486 Green Ridge Pettis 82 59 87 52 69 -2 | 0.01 -0.68 | 3609 +578 Lamar Barton 85 58 87 51 70 -4 | 0.05 -1.26 | 3713 +335 Cook Station Crawford 84 53 88 48 67 -4 | 0.49 -0.48 | 3317 +45 Alley Spring Shannon 85 50 92 46 66 -5 | 0.38 -0.62 | 3199 +86 Round Spring Shannon 83 52 88 48 65 -6 | 0.34 -0.66 | 3227 +113 Delta Cape | | Girardeau 83 57 87 54 69 -4 | 0.38 -0.69 | 3494 -90 Cardwell Dunklin 87 57 92 53 71 -4 | 0.44 -0.26 | 3926 +74 Clarkton Dunklin 87 58 91 56 71 -4 | 0.00 -0.72 | 3850 +44 Glennonville Dunklin 85 58 90 55 70 -4 | 0.00 -0.68 | 3832 +43 Charleston Mississippi 84 58 89 54 70 -4 | 0.28 -0.37 | 3714 +175 Portageville- | | Delta Center Pemiscot 86 61 90 56 72 -2 | 0.38 -0.48 | 3987 +213 Portageville- | | Lee Farm Pemiscot 86 60 90 56 72 -2 | 0.24 -0.62 | 3994 +248 Steele Pemiscot 88 60 91 56 72 -3 | 0.82 -0.03 | 4084 +315 -------------------------------------------------------------------------------- * Complete data not available for report ^Growing degree days are calculated by subtracting a 50 degree (Fahrenheit) base temperature from the average daily temperature. Thus, if the average temperature for the day is 75 degrees, then 25 growing degree days will have been accumulated. -------------------------------------------------------------------------------- Pat Guinan, Commercial Agriculture Program (573) 882-5908