Vol. 17, No. 11 June 1, 2007 ********************************************************************** Crazy Top of Corn By Laura Sweets Conditions have been favorable for the development of crazy top in corn, so a brief discussion of this disease seems appropriate. Crazy top of corn is caused by the downy mildew fungus, Sclerophthora macrospora. The causal fungus is a soilborne fungus, which causes infection when young plants are subjected to saturated soil conditions or water accumulating in whorls or leaf sheaths. In corn, crazy top is likely to occur when young corn plants are subjected to saturated soil conditions for 24- 48 hours from planting to about the five-leaf stage of growth. Accumulation of soil and water in the whorl of small plants may also result in infection. The disease causes a deformation of plant tissues including excessive tillering, rolling of leaves, proliferation of the tassel until it resembles a mass of leafy structures and stunting of corn plants. Leaves of infected plants may be narrow and strap-like in shape, leathery in texture and yellow or yellow striped in color. In seasons with wet springs or rains after corn has emerged, young corn plants subjected to saturated soil conditions may show symptoms of crazy top. Occasionally a band of affected plants may encircle a drowned out spot in a field. Some hybrids may be more susceptible to crazy top. This disease is seldom severe enough to cause significant losses. The downy mildew fungus which causes crazy top of corn (Sclerophthora macrospora), has been reported on more than 140 species of perennial and annual grasses. In addition to corn, downy mildew occurs on wheat, barley, rice, oats, sorghum, crabgrass, green foxtail, barnyard grass and numerous other grasses. In addition to surviving in various grass hosts, the fungus produces survival structures called oospores which can persist for months in infested crop residues and in the soil. Losses from crazy top are seldom severe enough in corn to warrant control. Furthermore practical management options for crazy top are very limited. Improving soil drainage or water management may be beneficial. Rotation to nongrass crops may help may also be of some benefit. Laura Sweets SweetsL@missouri.edu ********************************************************************** Early Season Soybean Diseases By Laura Sweets This could be an interesting year for early season soybean diseases in Missouri. In spite of the erratic weather patterns, soybean planting has made good progress and is only a few days behind average. However, the unusual fluctuations in both soil moisture and soil temperatures could increase the potential for Pythium seed decay and seedling blight as well as Phytophthora in beans that were planted early. The early season soybean diseases include those that cause seed decay, seedling blights and root rots of soybean. Most of these early season soybean diseases are caused by fungi in the soil that are found wherever soybeans are grown. Pythium, Phytophthora, Rhizoctonia and Fusarium are the most common of these early season pathogens, although Macrophomina (charcoal rot fungus) may also cause early season seedling problems. Soybean seedling blights have the potential to cause losses in Missouri soybean fields every year. The specific seedling blights that occur and their severity vary with the environmental conditions each season. With the changes in weather patterns this spring and soybean planting delayed in much of the state because of wet soil conditions, it is difficult to predict which, if any, seedling blights may occur or may cause significant problems this season. Pythium and Phytophthora are favored by wet conditions and are more likely to be serious problems when wet conditions exist at or just after planting. Rhizoctonia and Fusarium are not as restricted by soil moistures and soil temperatures but still need some moisture to initiate infection. Macrophomina phaseolina grows best at temperatures between 82-95 degrees. Infection of seedlings with Macrophomina is most likely to occur if conditions of high soil temperatures and low soil moisture exist during the first two to three weeks after planting. Symptoms of Pythium damping-off range from seed rot or preemergence damping-off to early postemergence damping-off. Affected tissue develops a soft, watery brown rot. Pythium damping-off is most likely to occur in cool (50-55 degrees), wet soils. Phytophthora can cause seed rot, preemergence damping-off and early postemergence damping-off. Initially affected tissue develops a soft, watery brown rot. Within several days the affected plant parts may dry out and shrivel up becoming dark, dry and brittle. This early stage Phytophthora is difficult to distinguish from Pythium damping-off; it may be necessary to submit a sample to the Plant Disease Clinic for an accurate diagnosis Phytophthora can also cause a seedling blight in which established seedlings turn yellow, wilt and die. Generally the entire seedling is affected and roots may be poorly developed and rotted. Phytophthora root rot is more likely to occur in heavy, wet soils, low areas or compacted areas, but it may occur in light soils or better drained areas if heavy rains occur after planting. Rhizoctonia can cause seedling blight and root rot of soybean. Affected stands may have an uneven appearance and seedlings appear pale green in color and stunted in growth. The identifying feature of this disease is a small, reddish lesion on one side of the stem at or just below the soil line. This lesion develops into a sunken, cankered area a the point of infection. Sometimes the lesion will expand to completely girdle the stem. On severely infected seedlings, the entire hypocotyl may be discolored and shriveled into a dry, stringy or wiry stem. Fusarium can also cause root rot of soybean. Infection is usually confined to roots and lower stems. The lower part of the taproot and the lateral root system may be discolored, deteriorated or completely destroyed. General roots show a nondescript brown discoloration and a dry, shrunken rot. Above ground portions of plants may appear off-color and stunted. Plants with severe Fusarium root rot may die prematurely. Charcoal rot, caused by Macrophomina phaseolina, may be more commonly recognized as a mid- to late-season disease on maturing soybean plants, but it can also occur early in the season on seedlings. Infected seedlings tend to show a reddish brown discoloration from the soil line up the stem. The discolored area changes from reddish brown to dark brown to black. Foliage may appear off color or begin to dry out and turn brown. If the growing point is killed, a twin stem plant may develop. Under hot, dry conditions, infected seedlings may die. Under cooler, wetter conditions, infected seedlings may survive but carry a latent infection. Then symptoms may reappear later in the season with hot, dry weather. Once the crop has been planted, there is little that can be done to reduce incidence or severity of soybean seedling diseases. Additional stress from poor growing conditions, herbicide injury or other factors may compound problems with soybean seedling diseases. Prior to planting it is important to consider variety selection (especially in fields with a history of Phytophthora), fungicide seed treatment, crop rotation, seedbed preparation and conditions at planting. Laura Sweets SweetsL@missouri.edu ********************************************************************** Grasshoppers Nymphs Numerous Central and Southern Missouri By Wayne Bailey Moderate to high numbers of small grasshopper nymphs are common in grass pastures, grass field margins and waterways, and to a much lesser extent in no-till soybean fields. Grasshopper nymphs may continue to emerge from eggs for another two weeks. Although typically a late summer pest on field crops, this year high numbers of grasshopper nymphs have emerged in field borders, grass pastures and no-till soybean fields. They have the potential to cause serious defoliation and some stand loss as they grow in size. At present, most nymphs are newly emerged from eggs and currently range from 1/4 to 1/2 inch in length. They will rapidly increase in size over the next few weeks. Typical grasshopper damage consists of irregular shaped holes extending from the leaf margin to the center of the leaf. Grasshopper damaged foliage often has a thin border of dead tissue surrounding the feeding site, unlike caterpillars which leave a smooth green surface at the feeding site. The following table lists the economic threshold for grasshopper infestations in non-cropland areas, grass pastures and soybean. Also included is a listing of recommended insecticides and a range of use rates for formulated material per acre. It is generally accepted that small grasshoppers are more readily killed than older hoppers by the same rates of insecticides. Regardless of size, grasshopper control is best achieved with the use of high rates of water and moderate to high rates of insecticides. Be sure to read and follow all pesticide label directions and precautions. ---------------------------------------------------------------------- | Insecticides recommended for Grasshopper management in Soybean: | |--------------------------------------------------------------------| |Economic Threshold: Treatment is justified if 30% or more | |defoliation prebloom or 20% or more defoliation bloom to pod or 5 | |to 10 % of pods damaged. | |--------------------------------------------------------------------| | Insecticide | Rate of formulated material/acre | |-----------------|--------------------------------------------------| | *Asana XL | 5.8 to 9.6 fl oz | | *Baythroid XL | 2.1 to 2.8 fl oz | | dimethoate | see label | | *Furadan 4F | 1/4 to 1/2 pt | | *Lorsban 4E | 1/2 to 1 pt | | *Penncap-M | 2 to 3 pts | | *Proaxis | 3.2 to 3.84 fl oz | | *Warrior | 3.2 to 3.84 fl oz | |--------------------------------------------------------------------| |*designates a restricted use product | |Be sure to follow all label directions, restrictions and | | precautions. | ---------------------------------------------------------------------- ---------------------------------------------------------------------- | Insecticides Recommended for Grasshopper Management in | | Non-Cropland Areas: | |--------------------------------------------------------------------| |Economic Threshold: Treatment is justified if 15 or more nymphs per | |square yard are present in non-cropland areas. | |--------------------------------------------------------------------| | Insecticide | Rate of formulated material/acre | |-----------------|--------------------------------------------------| | *Asana XL | 2.9 to 5.8 fl oz | | Malathion 57% | 1.5 to 3 pts | | Orthene 75S | 0.33 lbs | | *Proaxis | 2.56 to 3.84 fl oz | | *Warrior | 2.56 to 3.84 fl oz | |--------------------------------------------------------------------| |*designates a restricted use product | ---------------------------------------------------------------------- ------------------------------------------------------------------------- | Insecticides Recommended for Grasshopper Management | | in Grass Pastures | |-----------------------------------------------------------------------| |Economic Threshold: Treatment is justified if 8 or more nymphs per | |square yard are present in grass pastures | |-----------------------------------------------------------------------| | Insecticide | Rate of formulated material/acre| Preharvest Interval | |---------------|---------------------------------|---------------------| | Malathion 57% | 1.5 to 2 pts | 0 days | | *Mustang Max | 2.8 to 4 fl oz | 0 days | |-----------------------------------------------------------------------| |*designates a restricted use product | ------------------------------------------------------------------------- Wayne Bailey BaileyW@missouri.edu ********************************************************************** True Armyworm and Black Cutworm Numbers Decline By Wayne Bailey True armyworm and black cutworm larval numbers are on downward trends in most areas of the state. Except for northeast Missouri, numbers of larvae of these pest insects are being reduced by parasites, predators, fungal pathogen and by many mature larvae pupating in order to change into the moth form. As more larvae pupate, numbers of true armyworms and black cutworm moths will increase at lights during early evening and night periods. Moths of the second generation of true armyworm will search for new host plants and generally lay eggs in lawns and turf type environments. Black cutworm moths of the second generation tend to seek out many horticultural and non-crop plants on which to lay their eggs. In northeast Missouri some scattered problems with true armyworm infestations in grass pastures, wheat, and seedling corn are still being reported. Late moth flights into this region of the state account for most of these infestations. Wayne Bailey BaileyW@missouri.edu ********************************************************************** Weather Data for the Week Ending May 28, 2007 By Pat Guinan -------------------------------------------------------------------------------- | Monthly | Growing Weekly Temperature (deg. F) |Precip (in.)|Degree Days^ -----------------------------|------------|------------ Ext- Ext- Depart| Depart|Accum Depart Avg.Avg. reme reme from |May 1 from |since from Station County Max.Min. High Low Mean avg. |May 28 avg |Apr 1 avg. ------------------------------------------------------|------------|------------ Corning Atchison 77 59 83 48 67 2 | 8.18 4.23 | 716 281 St. Joseph Buchanan 76 59 80 50 67 2 | 7.46 3.15 | 679 201 Brunswick Chariton 77 62 83 58 69 3 | 2.81 -2.05 | 689 197 Albany Gentry 77 58 82 46 67 2 | 9.31 5.15 | 641 196 Auxvasse Audrain 79 62 84 56 69 3 | 3.14 -1.54 | 708 217 Columbia Boone 79 62 84 57 70 4 | 2.74 -1.9 | 713 177 Sanborn Field Boone 79 63 85 56 71 4 | 2.48 -2.22 | 770 213 Williamsburg Callaway 81 62 86 57 70 5 | 2.71 -2.2 | 725 248 Novelty Knox 77 61 82 56 68 3 | 4.13 -0.27 | 611 146 Linneus Linn 78 60 85 52 68 4 | 3.97 -0.78 | 651 206 Monroe City Monroe 78 61 84 57 70 5 | 2.12 -2.47 | 658 161 Versailles Morgan 80 63 86 58 70 3 | 4.74 -0.12 | 766 180 Green Ridge Pettis 78 62 84 58 69 4 | 2.36 -2.72 | 704 238 Lamar Barton 76 62 81 60 69 1 | 4.77 -0.57 | 714 107 Cook Station Crawford 82 61 84 55 71 4 | 2.84 -1.68 | 703 92 Alley Spring Shannon 83 57 84 54 68 2 | 6.32 1.67 | 662 101 Round Spring Shannon 84 58 86 54 69 3 | 4.6 -0.03 | 682 121 Delta Cape 87 62 89 60 74 4 | 3.45 -1.12 | 803 88 Girardeau | | Cardwell Dunklin 89 63 90 61 76 4 | 1.09 -3.76 | 917 89 Clarkton Dunklin 89 63 90 62 76 5 | 2.94 -1.07 | 872 66 Glennonville Dunklin 88 64 89 62 76 5 | 4.47 0.68 | 872 66 Charleston Mississippi87 63 89 61 75 6 | 2.65 -1.76 | 854 158 Portageville- 88 66 90 65 77 6 | 1.91 -2.42 | 928 123 Delta Center Pemiscot | | Portageville- 88 66 90 65 77 6 | 2.44 -1.99 | 933 138 Lee Farm Pemiscot | | Steele Pemiscot 90 67 91 65 78 7 | 0.68 -4.21 | 975 166 ‡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 GuinanP@missouri.edu