Weed Control: Crop Yield Losses Due to Weeds

| Corn | Soybeans | Forages | Cereals | Dry Edible Beans |
| Spring and Winter Canola | Other Crops | Soil Management |
| Soil Fertility and Nutrient Use | Field Scouting |
| On-Farm Stored Grain Management | Weed Control |
| Insects and Pests of Field Crops | Diseases of Field Crops | Appendices |

Pub 811: Agronomy Guide > Weed Control > Crop Yield Losses Due to Weeds

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

 

• Introduction
  • Table 12-1. Critical Weed-Free Periods for Common Ontario Field Crops
  • Table 12-2. Corn and Soybean Yeild Losses From Weeds Under Adequate Soil Moistuer vs Inadequate Soil Moisture
• Mechanical Weed Control
  • Table 12-3. Soybean and Corn Yield Losses Due to Weeds at Known Populations
• Herbicide Resistance
• Herbicide Injury
  • Table 12-4. Classes of Herbicide Injury According to Their Location on the Plant

Introduction

Yield losses due to weed competition will be greatest when:

• weeds emerge with or prior to crop emergence
• weeds are at high densities
• broadleaf weeds are present since they are more competitive than grasses
• there is limited soil moisture

Weed control is an important part of crop production. In general, agronomic practices that produce a healthy, fast-growing crop will provide the best competition against weeds. Consider cultivation, rotation and other effective cultural practices for weed control along with herbicide treatments, when developing a weed control program. Any single method of weed control or the continuous use of the same herbicide program will lead to the build-up of weeds resistant or tolerant to that control method.

An integrated approach to weed management that uses all available weed control strategies to manage weed populations can reduce herbicide use and optimize economic returns.

Integrated weed management strategies include:

• Field scouting to determine the weed species present, as well as density and population of weeds. Additional field scouting following application of control measures also helps to evaluate the effectiveness of the weed management programs used.
  
  
• Crop rotations - effective in reducing weeds, especially perennials that grow best in a particular crop. Rotating crops can allow for different herbicides or tillage practices to be used, which may be particularly effective on specific weed species. For example, forages are known to reduce the populations of annuals in the first year of the next crop, but annual weeds can be a problem when establishing forages.
  
  
• Crop and variety/hybrid characteristics - The early-season harvest of cereal crops creates an opportunity to improve weed control in the field during the late summer. Leaving weeds to grow following cereal harvest can result in a high weed seed return to the soil seed bank, creating weed problems in following crops. Do not leave weeds to grow following harvest when they can reach maturity and produce seed. Following cereal harvest, start control practices for annual weeds earlier than for perennial weeds, such as perennial sow thistle and field bindweed.
  
  
• Cover crops such as rye, red clover, buckwheat, oilseed radish, oat or over-wintering crops such as winter wheat or forages can suppress weed growth. When choosing a cover crop, consider how the cover crop and its management will affect the succeeding crop.
  
  
• Fertilizers (especially nitrogen) tend to stimulate the germination of some plant species and can affect the competition between crops and weeds in current and subsequent crops. Using banded phosphorus and potassium tends to concentrate the nutrients most where the crop has access to them. Side-dress nitrogen applications disturb the soil, which may stimulate the germination of weeds but also places nitrogen in a narrow band below the depth from which most weeds germinate and grow.
  
  
• Population and row width can affect weed growth. Narrow rows, high populations and fast-growing cultivars can have a competitive edge over weeds. Using vigorous, high-quality seed to achieve uniform stands at the recommended plant populations combined with early planting gives the crop a head start to compete with weeds. Deep planting can delay emergence and favour weed development but alternatively can be effective if a shallow tillage is utilized prior to crop emergence to remove the initial flush of emerging shallow-rooted annual weeds.
  
  
• Tillage practices and mechanical weed control
  • No-till - 75% of the weed seed bank is in the upper 5 cm (2 in.) of soil. The use of burndown herbicides has been effective for controlling many perennial weeds such as quackgrass or thistle.
  • Moldboard plow - the seed bank is more uniformly distributed over the depth of the plow layer.
  • Blind harrowing - kills small weed seedlings just before crop emergence.
  • Rotary hoe - at 10-20 km/hr, it has "fingers" that lift and mix soil, uprooting small weeds just before or shortly after crop emergence.
  • Inter-row cultivation - or scuffling, of row crops uproots small weeds and cuts off larger ones and smothers weeds in the crop row. Relative size of crops to weeds and timing of cultivation will determine success.
  • Mowing can help control weeds in crops such as newly established forages, cereal crops or cereal stubble

   

Figure 12-1. Typical Yield Losses of Different Field Crops Due to Weed Competition

Adapted from Several sources*

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* Figure 12-1, Typical Yield Losses of Different Field Crops Due to Weed Competition, is adapted from:
Weed Science Research Program, Department of Plant Agriculture - University of Guelph (1986-2008).
Wall & Smith, 2000. Canadian Journal of Plant Science, Vol. 80, No. 2, pp 411-47.
Milberg and Hallgren, 2004. Field Crops Research, Vol. 86, pp 199-209.
Martin et al., 2001. Critical Period of Weed Control in Spring Canola, Vol. 49, pp 325-33.
Varga et al., 2006. Cereal Research Communications, Vol. 34, No. 1, pp 701-04.

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Table 12-1. Critical Weed-Free Periods for Common Ontario Field Crops

Crop	Critical Weed-Free Period	Source
Corn	3-10 corn leaf tips	Swanton (University of Guelph)
Soybean	1st-3rd trifoliate-leaf stage (V2-V3)	Swanton (University of Guelph)
Spring cereals	1-3-leaf stage (Zadok's 10-13)	Van Dam, Swanton (University of Guelph)
Winter wheat	500-1,000 Growing Degree Days (Base Temp. = 0)	Welsh et al., 1999 (University of Reading)
Forages	Year of establishment: 4-6 weeks after planting	Dillehay, Penn State University http://a-c-s.confex.com/crops/2007asa/techprogram/P30284.htm
Canola	Emergence to 6-leaf stage	Van Acker (University of Guelph)

Critical Period for Weed Control in Field Crops

Yield loss caused by weeds can be reduced to less than 5% by maintaining good weed control during the critical period. Later-germinating weeds have a minimal impact on yields and have relatively low weed seed production.

The product label for postemergent herbicides indicates the growth stage of the weeds when optimum control can be obtained. Time weed control to optimize the critical period for the crop and the optimum growth stage of the weed.

The critical periods highlighted in Table 12-1, Critical Weed-Free Periods for Common Ontario Field Crops, are a range. The critical point within the period will vary yearly and by site due to variations in climate, soil type, weed species and density. For example, the critical period will be earlier in the window for fields with light-textured soils under moisture stress conditions when weed densities are very high. Delaying control measures to the later part of the critical period in this situation would likely result in significant yield losses.

Impact of Soil Moisture on Weed Competitiveness

When soil moisture is abundant, the impact of weeds on crop yield loss is limited. Table 12-2, Corn and Soybean Yield Losses From Weeds Under Adequate Soil Moisture vs. Inadequate Soil Moisture, compares observed yield losses due to weeds in corn and soybeans at the Elora Research Station in a season with more than adequate moisture compared to a "dry" season.

Table 12-2. Corn and Soybean Yeild Losses From Weeds Under Adequate Soil Moistuer vs Inadequate Soil Moisture

Precipitation (mm - May to August)	Corn Yield Losses from Weeds (%)	Soybean Yield Losses from Weeds (%)
458	18	23
218	96	84

Source: Weed Science Research Program, Department of Plant Agriculture - University of Guelph (1986-2008).

Impact of Weed Species on Crop Yield Losses

Crop scouting determines the weed species present and their respective densities in the field. Some weeds are more competitive than others. Table 12-3, Soybean and Corn Yield Losses Due to Weeds at Known Populations, shows the comparative yield losses caused by different weed species.

Consider weed competitiveness when deciding whether to treat escapes. The estimates in Table 12-3 are based on normal weather conditions and adequate soil moisture. They also assume that the weeds emerged with the crop. Yield losses may increase under drier soil conditions and may be variable under conditions of plant stress.

Also consider the effects of weed populations on crop quality and harvest procedures. For example, Eastern black nightshade is not a big threat to yield but can have a severe effect on crop quality of identity-preserved (IP) soybeans.

Mechanical Weed Control

Small annual weed seedlings can be partially controlled by blind harrowing prior to crop emergence. Use a set of light harrows operating at a shallow depth. Once the crop has emerged, a weeder-harrow (with L-shaped flexible tines) can be used until the crop is 5-10 cm (2-4 in.) tall. Timing of harrowing operations is critical to achieve success, since the weeds must be small and the soil surface dry and easily worked. Cultivation with the rotary hoe at high speeds (10 km/h) and at shallow (2.5-3 cm or 1-1.5 in.) depths when corn is 7-8 cm (3 in.) high or when beans are in the one-to-two leaf stage will help control small weed seedlings. These techniques will not reduce herbicide action and may in some years enhance chemical weed control. Under dry soil conditions, rotary hoeing dry edible beans 7-10 days following planting will help control emerging weeds but can also help activate soil-applied herbicides by mixing the chemical with moist soil. Rotary hoeing is unlikely to remove weeds that are past the two-true-leaf stage.

Table 12-3. Soybean and Corn Yield Losses Due to Weeds at Known Populations (Crop losses assume that the weeds have emerged with the crop)

Crop	% Yield Loss
	(1 plant/m2)	(5 plants/m2)
Corn
Annual Broadleaves
Giant ragweed	13	36
Lamb's-quarters	12	35
Pigweed	11	34
Cocklebur	6	22
Ragweed	5	21
Wild mustard	5	18
Velvetleaf	4	15
Lady's thumb	3	13
Wild buckwheat	2	10
Eastern black nightshade	2	7
Annual Grasses
Giant foxtail	2	10
Proso millet	2	10
Fall panicum	2	10
Barnyard grass	2	7
Green foxtail	2	7
Yellow foxtail	1	5
Old witch grass	1	5
Crabgrass	1	3

 

Crop	% Yield Loss
	(1 plant/m2)	(5 plants/m2)
Soybeans
Annual Broadleaves
Cocklebur	15	41
Eastern black nightshade1	14	40
Giant ragweed	14	40
Lamb's-quarters	13	38
Pigweed	12	36
Ragweed	10	33
Velvetleaf	6	23
Wild mustard	5	20
Lady's thumb	4	15
Wild buckwheat	4	15
Annual Grasses
Volunteer corn	4	15
Giant foxtail	3	12
Proso millet	3	12
Barnyard grass	3	12
Fall panicum	2	10
Green foxtail	2	8
Yellow foxtail	1	5
Old witch grass	1	4
Crabgrass	1	4

Adapted from www.wedpro75.com

¹ Eastern black nightshade in soybeans reduces its quality.

 

Use inter-row cultivation to complement other weed control measures; it is most effective when weeds are small. Cultivate to a shallow depth to reduce germination of new weed seeds, soil moisture loss and corn root injury. Inter-row cultivation may be required when weeds escape a herbicide treatment. Consider weeds as escapes when they are 5-7 cm (2-3 in.) high. Since cultivation is less successful on larger weeds, cultivate quickly after determining a herbicide failure. If weeds are too large, consider alternative herbicide choices.

Band treatment of chemical over the row will reduce herbicide cost by half to two-thirds, depending on the row spacing and the width of the band. Control weeds between the bands with shallow inter-row cultivation. Consider the combination of the two operations when evaluating the economics of treating weeds in this manner.

Herbicide Resistance

There are over 18 herbicide-resistant weed species in Ontario affecting six different herbicide modes of action. Herbicide-resistant weed species will dominate a field's weed population when herbicides from a single chemical mode of action are used repeatedly. The speed at which herbicide-resistant weed populations are selected will depend on the complexity of the crop rotation and the herbicide modes of action repeatedly used. Applying the principles of integrated weed management will delay the onset of herbicide-resistant weed populations. To prevent or slow the development of resistant weeds, use the following approaches:

• identify, monitor and keep records
• rotate crops and herbicides
• prevent spread of weeds
• use alternatives to chemical weed control

For the latest on herbicide-resistant weed management and testing, see www.plant.uoguelph.ca/resistant-weeds.

Herbicide Injury

When the directions on a herbicide product label are followed correctly, the risk of injury to the target crop is very small. However, under less-than-favourable conditions, all herbicides have the potential to cause crop injury. The primary sources of herbicide injury to crops are:

• herbicide residues persisting from the previous crop year, especially in areas where spray overlaps occurred
• excessive product rate due to a miscalculation or spray overlap
• tank contamination due to fungicide or insecticide application that has herbicide residues in the spray solution when applied (e.g., a Folicur application on winter wheat that contains Ultim residues will cause considerable crop injury and yield loss).
• off-target drift from a herbicide application to a neighbouring crop
• herbicide applications made past the labelled crop stage. In cereals, late applications occurring close to heading time can interfere with pollination and reduce yield.
• adverse environmental conditions around the time of application or crop emergence
• Air temperature fluctuations of more than 20°C or daytime highs exceeding 30°C will dramatically increase the potential for herbicide injury.
• Excessive rain after a soil-applied herbicide application can cause the herbicide to "splash up" on the leaves causing injury.

Crop growth stage, variety, stress, environmental conditions, tank-mix partners and adjuvants will all affect the potential amount and severity of crop injury. When the target crop is under stress, its ability to metabolize a herbicide is reduced, and injury may result. A herbicide's mode-of-action will also influence the severity of crop injury. In general, while contact herbicide injury may look worse, systemic herbicides will have longer-lasting injury, which may be more severe. Each herbicide's product label will have a precautionary section outlining circumstances that may increase the potential for crop injury. Review these sections to minimize the potential of herbicide injury.

See Table 12-4, Classes of Herbicide Injury According to Their Location on the Plant.

Table 12-4. Classes of Herbicide Injury According to Their Location on the Plant

Location on the Plant	Injury Symptoms
Injury to Newly Emerged Seedling Plants (Plates 39, 40 and 41 below this table)
Dinitroanalines (Group 3) (systemic - xylem mobile) e.g., Prowl, Treflan	stunted plants that do not fully emerge from the soil short, thick lateral roots impact on yield will depend on severity of injury and crop stage at time of injury Grasses shoots are short, thick and may appear red or purple Broadleafs may have swollen and cracked hypocotyls (area below cotyledons)
Chloroacetamides (Group 15) (systemic - xylem mobile) e.g., Dual ll Magnum, Frontier Max, Define	stunting of shoots that result in abnormal seedlings that do not emerge impact on yield will depend on severity of injury and crop stage at time of injury Grasses grasses may leaf-out underground shoots may be abnormal when leaves do not properly unfurl Broadleafs crinkled leaves and/or shortened mid-vein, which produces "draw-string" effect or heart-shaped leaves

Location on the Plant	Injury Symptoms
Injury Affecting Older Leaf Tissue (with the potential to move upward) (Plate 42 and Plate 43 below this table)
Systemic Photosynthetic Inhibiting Herbicides (systemic - xylem mobile) Triazines (Group 5) e.g., Atrazine, Sencor, Princep Nine-T Substituted Ureas (Group 7) e.g., Lorox	translocation occurs only in the xylem (upwards movement only) injury symptoms occur after the cotyledons and first true leaves emerge injury begins with yellowing of the leaf margins or tips and yellowing between the leaf veins older and larger leaves are affected first injured leaf tissue eventually turns brown and dies injury is greater on higher pH soils (>pH 7.2) impact on yield will depend on the severity of injury and the crop stage at which the injury occurred

 

Location on the Plant	Injury Symptoms
Injury Limited to Plant Tissue Exposed at the Time of Application and With No Movement to New Plant Growth (Plates 44-48, below this table)
Non-Systemic Photosynthetic Inhibiting Herbicides (Group 6) (contact) e.g., Basagran, Pardner	injury is confined to foliage that has come in contact with herbicide typical symptoms include leaf speckling, blotching or bronzing and leaf tip burn crop oil concentrates and other additives may intensify injury symptoms injury is typically cosmetic with little to no impact on yield Grasses grass plants are generally tolerant to the non-systemic photosynthesis inhibitors
Phosphorylated Amino Acids (Group 10) (contact with limited phloem & xylem mobility) e.g., Liberty, Ignite	chlorosis and wilting usually occur within 3-5 days followed by necrosis within 1-2 weeks symptoms occur faster in bright sunlight and high humidity impact on yield is typically significant
Diphenylethers (Group 14) (contact) e.g. Reflex, Blazer	reddish-bronze spotting of the leaf surface may appear shortly after application spotting is highly correlated to the spray application pattern plants that do not die may be stunted for a week or so crop oils and other additives may increase plant injury injury is typically cosmetic with little to no impact on yield
Bipyridiliums (Group 22) (contact) e.g., Reglone, Gramoxone	injury occurs very quickly (1-2 days after application) plant leaves will have a limp, water-soaked appearance, which is followed by browning of the leaf tissue drift injury appears as blotching necrotic regions on leaf tissue impact on yield can be significant perennial plants affected will grow back
Additives (No specific group) Surfactant or 28% UAN injury	severe browning or leaf tissue new leaf tissue will be unaffected most common with 28% UAN used as a carrier to apply herbicides in cereals or when an excessive rate of surfactant is used largely cosmetic injury with negligible yield loss, provided visual injury is not severe

 

Location on the Plant	Injury Symptoms
Injury Affecting New Growth and With the Potential to Move From Leaves to Roots (Plates 49, 50, 51, 52, 53, 54, 55, 56, 57, 59, 60, 61, 62, 63, 64, 65, 66, 67 and 69 below this table)
Lipid Synthesis (ACCase) Inhibitors (Group 1) (systemic - phloem mobile) e.g., Assure ll, Excel, Poast Ultra, Venture	newer leaf tissue typically will be yellow or red, then turning brown; the leaves in the whorl will be decomposed and easy to pull out symptoms develop slowly (7-14 days) impact on yield is significant Grasses injury on grass plants only
Amino Acid Synthesis Inhibitors (Group 2) (systemic - phloem mobile) e.g., Accent, Classic, Pinnacle, Pursuit, Ultim	internodal stunting leaf distortion with interveinal yellowing underside of leaf may have red or purple veins symptoms take 1-2 weeks to develop impact on yield will depend on the severity of injury and crop stage at which the injury occurred
Growth Regulators (Group 4 - phenoxy acids) (systemic - phloem mobile) e.g., 2,4-D, 2,4-DB, MCPA, MCPA/MCPB	broadleaf plants exhibit stem twisting and leaf malformations (cupping, crinkling, parallel veins, leaf strapping) 2,4-D will lengthen petioles of trifoliate soybean leaf, whereas benzoic acid herbicides (i.e., Banvel ll) will not corn plants exhibit rolled leaves (onion leafing), fused brace roots, stalk bending (goose necking) and brittleness, and missing kernels small grains exhibit twisted flag leaves, sterile florets, or multiple florets, twisted awns and head malformation impact on yield will depend on the severity of injury and crop stage at which the injury occurred
Growth Regulators (Group 4 - benzoic acids) (systemic - phloem mobile) e.g., Banvel ll, Distinct	dicamba injury is similar to that caused by phenoxy acid herbicides broadleaf plants may exhibit more cupping than strapping of leaf tissues will cause more goose necking than 2,4-D in corn and lodging in small grain (especially wheat) impact on yield will depend on the severity of injury and crop stage at which the injury occurred
Growth Regulators (Group 4 - pyridine acids) (systemic - phloem mobile) e.g., Lontrel, Milestone	injury similar to phenoxy and benzoic acid herbicides legume crops (soybeans, alfalfa, clovers) are extremely susceptible to the pyridine acids impact on yield sensitive species is significant
Aromatic Amino Acid Synthesis Inhibitors (Group 9) (systemic - phloem mobile) e.g., Roundup, Weathermax, Touchdown Total	plant foliage will first yellow (new leaves first) and then turn brown and die within 10-14 days after herbicide application drift onto corn can cause reddening of leaf tissue impact on yield is significant
Pigment Inhibitors (Bleaching Herbicides) Triazoles (Group 11) e.g., Amitrol 240 Inhibitors of Carotenoid Biosynthesis (Group 13) e.g., Command HPPD Inhibitors (Group 27) e.g., Callisto, Converge, Impact, Infinity	injury begins with new leaf tissue turning white "bleached" colour then progressing to yellow, followed by brown necrotic tissue impact on yield is generally minor, but if injury is severe, it can be significant

 

Plate 39. Prowl (pendimethalin) injury in corn causes short, thick and stunted roots.

 

Plate 40. Frontier (demethenamid) injury to dry edible beans (right), compared to an uninjured plant (left). Yellowing of the leaf margin is an early symptom associated with the herbicide injury, which will later turn necrotic (brown).

 

Plate 41. Dual ll Magnum (s-metolachlor/benoxacor) injury in soybean, showing the characteristic "draw-string" effect that gives a heart-shaped leaf appearance.

 

Plate 42. Soybean response to atrazine residues. Note the lower leaf margins turn yellow. Yellowing then moves to the inner part of the leaf. The yellow leaf tissue will eventually turn brown.

 

Plate 43. Soybean response to Sencor (metribuzin) "Splash. " Note the severe browning that affects more of the lower leaf tissue.

 

Plate 44. Pardner (bromoxynil) leaf tissue burn on corn.

 

Plate 45. Reglone (paraquat) drift onto field corn. Note the severe necrosis (browning) of lower leaf tissue while new leaf tissue is unaffected.

 

Plate 46. Corn leaf tissue response to Reflex (fomesafen). Note the severe necrosis (browning) that causes a fusing of the newest leaf tissue, obstructing normal development of subsequent vegetation.

 

Plate 47. Surfactant injury to soybean.

 

Plate 48. UAN fertilizer burn to winter wheat leaf tissue.

 

Plate 49. Corn response to Assure II off-target drift. Symptoms to the corn plant will include stunting, reddening of leaf tissue and browning of the growing point.

 

Plate 50. Classic (chlorimuron) injury to soybean. The three common symptoms of this class of herbicides are leaf distortion, yellowing of leaf tissue and browning/reddening of leaf veins on the underside of the leaf.

 

Plate 51. Corn response to Pursuit (imazethapyr) drift. Note the distortion and reddening/purpling of the leaf tissue.

 

Plate 52. Ulxtim, Accent or Option injury to corn where symptoms include distortion and yellowing of the new leaf tissue.

 

Plate 53. 2,4-D injury to dry edible beans. 2,4-D injury can often be differentiated from dicamba injury by the elongated petiole of the trifoliate leaf, the "bubbling" of leaf tissue and the narrowing of trifoliate leaves.

 

Plate 54. Twisting and distortion of winter wheat heads from an in-crop application of 2,4-D applied in the fall.

 

Plate 55. Soybean leaf cupping caused by off-target dicamba drift.

Plate 56. Glyphosate drift onto non-tolerant soybeans. Note the newest leaf tissue is yellow, a very characteristic symptom of glyphosate injury to plants.

Plate 57. Soybean response to Callisto (mesotrione) drift with the characteristic "bleaching" of new leaf tissue. This tissue will then turn yellow, followed by brown.

Plate 58. Bleaching of a spring cereal crop due to Command (clomazone) carryover. Leaf tissue will go from white to pinkish-purple, followed by browning. Most of the whitened leaf tissue will not make a full recovery.

Plate 59. European chafer larvae are white with an orange-brown head and dark posterior. They are distinguished from other white grubs by the Y-pattern of the anal bristles (rasters).

Plate 60. June beetle larvae have a raster shaped like an oval, with the two rows of rasters running parallel to each other.

 

Plate 61. Japanese beetle larvae can be distinguished from other grubs by the wide, shallow, V-shaped raster pattern.

 

Plate 62. Japanese beetles have a bright, metallic green head with coppery wings tinged with green edges. Twelve white tufts of hair appear at the wing boundary.

Plate 63. Wireworms have long, cylindrical, copper-brown, hardened bodies. They affect many crops, usually when following grassy crops in rotation.

 

Plate 64. Seedcorn maggots are small, headless, legless larvae that burrow into germinating seeds and weaken the seedling.

 

Plate 65. Slugs are soft-bodied, legless and greyish or mottled. They have a gelatinous (slimy) covering that leaves trails where they have travelled.

Plate 66. Foliar damage caused by slugs. Slugs feed on all parts of the plant, leaving ragged holes. Feeding on the growing point can kill the plant.

Plate 67. Black cutworm larvae are greyish-black with a pale underside. Mature larvae are about 3. 5 cm (1. 25 in.) long. Corn plants are cut at the soil level.

 

Plate 68. Corn flea beetles leave long feeding scratches running parallel to the leaf vein. These beetles are vectors of Stewart's wilt (bacterial leaf blight).