ISSUE 7 June 24, 2010
SMALL GRAIN HEAD SCAB RISK
The scab risk model on June 22, 2010 showed that the highest risk of scab infection was in the Northeast corner of the state, if wheat is flowering.
NDSU illustration of Scab Risk Model for
June 22, for Susceptible Wheat Variety
As of June 22, the model indicated that scab risk diminished for most of the rest of the state over the weekend (June 18-20), primarily because of some strong winds that dried the wheat canopies. However, widespread rains on June 21 and June 22 are likely to raise the risk back up again in many areas, because of increased durations of high relative humidity. Brief periods of drier or sunnier days may not be sufficient to eliminate risk. The plant pathologist who manages the scab forecasting model indicated to me that these interrupted periods of moisture may not be adequately considered by the mathematical models in the current scab risk forecast. The short periods of dry weather may be making the risk levels go too low under the conditions that we’ve had in the region.
A clarification on the scab model risk levels: When the model indicates a moderate risk (yellow on map), this means there is a 50 to 80% probability of a scab epidemic having severities greater than 10%; so moderate risk is still higher than we would like to see and would indicate a fungicide may be warranted. A red color or high risk means that there is an 80% probability of an epidemic with greater than 10% severity. These risk levels depend on the susceptibility of the variety and only apply to wheat in the flowering stage or barley that is fully headed.
Fungicide application timing: (see article by Joel Ransom): Fungicide applications for scab suppression should be applied at early flowering in wheat, while in barley, they should be applied once the full head emergence stage has been reached. Application to partially emerged grain heads is not effective. Generally, there is about a 5-7 day window when fungicides may be applied, but this window is shortened if temperatures are over 80 degrees F.
NDSU IPM SURVEY RESULTS (as of June 18)
The NDSU IPM field scouts surveyed 100 wheat fields and 10 barley fields last week. The most common disease observed in wheat was tan spot, with 80% of fields showing symptoms, and they had average severity of 1.7% and a range of severity from 1-15%.
Stripe rust was observed in four commercial wheat fields by the scouts, three of which were spring wheat. In winter wheat, the stripe rust was on the flag leaf, but in spring wheat, generally the rust was observed in the canopy. Other observations of low levels of stripe rust were made this past week by area and county extension agents, crop consultants, and farmers. In all cases, the levels still remain low. Stripe rust observations have been made across the whole state now.
Leaf rust detections are still rare, and primarily in winter wheat. Leaf rust likes warmer temperatures than stripe rust, so we may start seeing more of this rust if temperatures remain warm.
In barley, disease levels are low and consist of some fungal leaf spot infections, including spot blotch and net blotch. In my barley plots in Fargo, I am seeing some net blotch on lower leaves, and the crop has now fully headed.
BARLEY YELLOW DWARF VIRUS
Cereal grain aphids are starting to be picked up by the field scouts, (in 5% of fields scouted), but numbers are generally too low at this time for insecticide treatment. Some reports have indicated that the virus transmitted by grain aphids, Barley yellow dwarf virus (BYDV), is very evident in some winter wheat. These aphid transmissions of the virus may have occurred last fall, as this year’s aphid arrival was only recent.
If barley yellow dwarf virus symptoms (golden yellow leaf tips) are evident in the crop, the following is an indication of possible yield losses:
Losses when infection occurs at flag leaf stage: ~ 5%
Jointing stage infection: losses average 19%
Tillering stage infection: losses average 23%
Seedling stage infections: losses average 55%
Losses from barley yellow dwarf depend on various factors, including the cultivar (we assume all of ours are susceptible), the time of infection (see above), the number of aphids present and vectoring BYDV, and environmental conditions that favor aphid reproduction.
NDSU Extension Plant Pathologist
MANAGING AND PLANNING FOR DISEASES OF SUGARBEET
Diseases found in sugarbeet fields
Sugarbeet fields were found that had some seedling damping off caused by Pythium. Sugarbeet seeds and seedlings are susceptible to Pythium species in wet soils that are cold or warm. Fungicide seed treatments typically provide adequate protection against Pythium, but prolonged wet periods may result in reduced efficacy of the fungicides. Keeping fields well drained will assist in reducing disease incidence.
Warm wet soils in May also resulted in some seedling damping-off by Rhizoctonia solani. Fields with a known history of severe Rhizoctonia should be planted to a Rhizoctonia tolerant variety and the use of a fungicide such as Quadris or Proline is strongly recommended to provide additional protection. Fungicides should be applied in a band application before the average daily soil temperature for bare soil at the four inch depth reaches 65 F.
Fusarium yellows have been observed in some fields, especially in the Moorhead factory district in the Glyndon and Sabin townships. Symptoms include interveinal yellowing and death of older leaves, sometime distinct death of half the leaf on one side of the midrib, followed by death of the younger leaves. When the roots of infected plants are cut in a cross section, there is a distinct darkening of the vascular system. Symptoms were first observed in early June on plants with two to six leaves; many of the younger plants were killed. Roots of infected plants will not store well in piles. The best and only way to manage Fusarium yellows is to plant tolerant varieties, several of which are available.
Plants with symptoms of Rhizomania have been reported by Allan Cattanach at American Crystal Sugar Company. Rhizomania is caused by the beet necrotic yellow vein virus which is transmitted by Polymyxa betae. Plants infected with Rhizomania have distinctly fluorescent yellow-green leaves with hairy adventitious roots. The best way to manage Rhizomania is to plant resistant varieties. Therefore, in fields with a history of this disease, use resistant varieties approved for your factory district.
Look out for other diseases
Fields are currently very wet and warm weather is expected. Aphanomyces cochlioides, the pathogen which causes Aphanomyces root rot, becomes damaging in warm, wet soils. Many high yielding Aphanomyces tolerant varieties are available and should be used in fields with a history of this disease. Tachigaren seed treatment is recommended to provide additional protection, especially during the seedling stage, for fields with high inoculum pressure.
Many sugarbeet fields have closed rows already, or rows will be closed in another 7 to 10 days. This means that the sugarbeet crop has a great potential for high yields. However, closed rows, wet leaves by rain or dew, and warm temperatures (more than 60 F in the night more than 80 F in the day) provide favorable conditions for the development of Cercospora leaf spot caused by Cercospora beticola. Cercospora leaf spot is the most damaging leaf disease of sugarbeet in our area. The fungus destroys the leaves that are responsible for making the sugar in the plant resulting in reduced yield and lower sugar concentration. Growers have done an excellent job over the past 6-8 years in controlling leaf spot by using tolerant varieties, crop rotation, incorporating infected residue by tillage, and timely application of fungicides. As such, the inoculum level of C. beticola is very low in fields. However, in favorable conditions, the fungus can multiply rapidly completing one sporulating cycle in less than two weeks. The best time to reduce the population of the pathogen is early in the disease cycle when the population is still low.
The best way to control Cercospora leaf spot is to apply fungicides in a timely manner. We have a number of fungicides – namely Proline, Eminent, Inspire, Headline, Gem, Tin, and Topsin that will provide effective disease control when used in alternation or recommended mixtures. The first fungicide application should be made when leaf spot symptoms are first observed. Subsequent applications, typically at 14 days in high disease pressure, should be made when symptoms are present and two consecutive daily infection values are high (>6). For ground application, apply fungicides in 15-20 gallons of water per acre at 100 psi pressure; aerial applicators should use 5 to 7 gallons of water per acre for best results. Use the recommended rate of the fungicides; cutting rates will result in poor disease control and will quickly lead to the development of resistant isolates. Fields should be scouted regularly to determine the presence of symptoms.
Extension Sugarbeet Specialist
BLACKLEG APPEARING IN CANOLA
We are getting reports of severe blackleg appearing in canola, primarily from the Northeast part of the state. Blackleg can be very destructive under the right environment. Although it is unlikely that anything can be done to mitigate the infection, it is critical to pay attention to your fields for the disease for future years.
Blackleg is caused by two fungal pathogens. The first pathogen causes superficial lesions only (Leptosphaeria biglobosa) and is, frankly, not very important. The second pathogen (Leptosphaeria maculans) causes deep cankers that result in yield reduction and/or lodging. This article will only focus on the primary pathogen, L. maculans.
The pathogen will survive on crop residue for several years. In the spring, spores (ascospores) are released and cause infections in leaves and stems. New infections may occur throughout the season (conidial spores), although the ones that could result in yield losses occur mostly when plants are at the 2 to 4 leaf stage. Temperatures in the 70’s with prolonged wetness favor infection and disease development; temperatures below 50 or above 85 will inhibit disease.
The pathogen has numerous Pg types (races). The most common in North Dakota is PG2, however, new PG’s are being found in increasing frequency. We believe that the current resistance in canola hybrids is exclusively (or nearly exclusively) to PG2, and that the hybrids are most likely susceptible to PG3, PG4, and other races. Thus, fields that have the other PG types will likely be susceptible to infection, no matter what resistance rating the hybrid has.
In fields with other PG types, rotation is the most important management tool we have in this situation. Although a 2-yr rotation is common in certain parts of the state, it is insufficient for blackleg management. A 4-yr rotation is strongly recommended. Fungicides have been shown to reduce disease, but the application must be made at the 2 to 4 leaf stage. Applications after this are unlikely to be effective.
It is too late to manage blackleg this season. But pay attention to the presence of the disease. If blackleg is widespread in the field planted to a resistance hybrid, it is likely that you have a new race of the pathogen. Lengthening rotation is critical before planting canola back into that field, and when done, a fungicide application at the 2-4 leaf stage may be considered.
More information is available about blackleg in 2008 NDSU extension publication, Blackleg of Canola (PP-1367), available at http://www.ag.ndsu.edu/pubs/plantsci/crops/pp1367.pdf or by Googling ‘Blackleg of Canola’.
Extension Plant Pathologist
Cavalier County Agent
Luis del Rio
CONDITIONS FAVORABLE FOR WHITE MOLD IN BROADLEAF CROPS ENTERING BLOOM
Recent persistent and heavy rains have created an environment favorable for the germination of sclerotia, the small and black survival structure of Sclerotinia. Once sclerotia germinate, ascospores will be released. If the soil stays wet, temperatures remain moderate, and the canopies maintain moisture when the plants are beginning to flower, white mold may be an issue.
Most of the broadleaf crops grown in North Dakota are susceptible to sclerotinia, although susceptibility varies. Sunflower, dry edible beans and canola will get hit hard in a favorable environment. Soybeans, pulse crops, and others will get white mold, but most frequently only under high disease pressure. Flax is technically susceptible, but white mold seems to occur rarely in the crop. I am also getting calls on smaller acreage crops; crambe, mustard, radishes, etc… All of these are susceptible and will get disease under high disease pressure, although I don’t know exactly how susceptible each one of them is. Right now, my concern is the crops that will begin to bloom in the next few days.
Plants are only susceptible during bloom. The pathogen needs to colonize senescent flower petals before it can cause disease on living tissue. From the petals, the pathogen will progress into stems and/or branches and cause disease. Because flower petals are so important to the process, plants are only susceptible while the crop is in bloom. An effective fungicide application must be made at EARLY BLOOM to be effective. Applications at the end of the bloom period or later are ineffective.
Fungicides can be a good option for disease management in most broadleaf crops. However, no fungicide will control the disease; rather fungicide will help manage it. Under high disease pressure, white mold will still develop even with an optimal fungicide application(s), although it may be less severe. Notably, fungicide applications for white mold control on sunflower are not recommended.
Bloom is still weeks off for many crops, and the risk may quickly change. Hot temperatures and lack of rain will reduce the risk dramatically. However, for those plants beginning to flower, in area with a favorable environment, a fungicide application could be considered. For canola growers, the sclerotinia risk map and risk calculator is available at www.ag.ndsu.edu/sclerotinia/.
Extension Plant Pathologist
POTATO LATE BLIGHT
Late blight was found in southwestern Michigan yesterday. As our weather continues to be unsettled and late blight severity values are increasing in our area, diligent fungicide applications will be recommended. Late blight severity values are compiled by Gary Secor and available on the Potato Bytes.
Extension Agronomist - Potatoes
North Dakota State University