ISSUE 16    August 18, 2005

LATE-SEASON DOWNY MILDEW IN SUNFLOWERS: WHATíS THE DEAL?

As reported in previous issues of the Crop and Pest Report (June 30th and July 7th issues), downy mildew was highly prevalent this year due to saturated soils earlier in the growing season. These infections (most likely coming from soilborne inoculum) became systemic which causes the typical stunting and chlorotic leaves (See June 16th issue of Crop and Pest Report). These systemic infections typically occur in young plants, usually not past the 4-leaf stage. On the underside of the leaves of the systemically-infected sunflower plants, spores (sporangia) of the downy mildew pathogen can form. These spores can be spread to other plants by splashing rain and wind, and generally can cause localized symptoms on surrounding plants. These localized symptoms usually appear as angular leaf lesions (see image), and are not generally of economic importance.


Localized lesions of downy mildew on a sunflower leaf.
(Courtesy T. Gulya)

Recently, a few reports of late-season, systemic symptoms of downy mildew have been received. In these cases, middle and upper leaves (not lower) have been affected. In attempt to explain what might be going on, I refer to research conducted in Canada in the early 1970s (Cohen and Sackston, Canadian Journal of Botany 51:15-22 (1973)). In this research, the growing points of sunflower plants of different ages were inoculated with the downy mildew pathogen, and number of days to first symptoms, as well as % plants with systemic infections were measured. Results of this study indicated that plants inoculated at the 8-leaf stage could develop systemic downy mildew symptoms. In these cases (50% of the plants), the average time for symptoms to first appear was 3 weeks. These plants were in or near the flower bud stage when systemic infections became apparent, and only middle and upper leaves were affected. Why donít we see this more often? My guess is that the amount of secondary inoculum produced by the early-infected systemic plants was very large due to the high levels of infection within fields this year. With a large amount of secondary inoculum present, there would be a better chance for late-season systemic infections to occur.

Carl A. Bradley
Extension Plant Pathologist
carl.bradley@ndsu.edu

 

NDSU IPM SURVEY, AUG. 8-12

The last survey of wheat for the year took place the second week of August, in the northwest crop reporting district. Ryan Davis surveyed 25 wheat fields and found 60% of these to have some level of scab, with field severities in symptomatic fields ranging from 2% to 12%. Average severity in symptomatic fields was 6.8%, while across all fields, average scab field severity was 4.1%. These levels are lower than in earlier scouted fields. Flag leaves in most fields were infected with Septoria species.

Many thanks to the field scouts this year for doing such a good job. They had many disease and insect problems to observe in 2005, but frequently had to deal with saturated fields and very rainy days. Field maps of weekly scout observations for the crops they surveyed can be found at the following web site: http://www.ag.ndsu.nodak.edu/aginfo/ndipm/

 

WHEAT FUNGICIDE RESULTS, UNIFORM WHEAT TRIAL, FARGO

All data, except for deoxynivalenol (DON) levels, are now completed for the uniform wheat fungicide trial established in Fargo. Similar trials also were located at Carrington, Minot and Langdon, and that data will be forthcoming from those research extension centers. This trial was supported by funding from the US Wheat and Barley Scab Initiative, and products were supplied by the crop protection companies.

In Fargo, the uniform fungicide trial was established on Reeder hard red spring wheat, with the intent to evaluate fungicides for efficacy in controlling scab (Fusarium head blight = FHB) and leaf diseases. The crop was planted into wheat stubble. Fungicides were applied with a hand held boom, with forward/backward nozzles at 40 psi in 20 gpa. Fungicides were applied at early flowering (Feekes 10.51 ) for all treatments, except for Tilt, which was applied prior to flowering, at Feekes 10.5, the growth stage on the Tilt label. The trial was evaluated at early dough stage, on July 13th. Scab and leaf disease severity did increase substantially after July 13th, so the following disease scores donít reflect the final severity of these diseases in the trial, high severities which contributed to the relatively low yields.

The uniform fungicide results on Reeder HRSW at Fargo:

Product*

Rate/A fl oz**

Feekes Stage of app.

FHB field severity***

Leaf rust -
% on flag

Leaf spot -
% on flag

Yield
bu/A

TWT
lbs/bu

Untreated

----

----

21.1 a

15.2 a

6.9 a

38.8 c

57.0 d

Folicur

4

10.51

4.7 c

0.1 b

1.5 b

46.9 b

57.9 c

Prosaro (Exp.)

6.5

10.51

3.0 c

0.0 b

1.1 b

53.2 a

59.1 ab

BAS555 (Exp.)

13.5

10.51

3.9 c

0.0 b

0.7 b

49.3 ab

59.2 a

BAS555 (Exp.)

10

10.51

3.5 c

0.0 b

0.4 b

47.9 ab

58.6 b

Punch (Exp.)

6

10.51

7.9 b

1.5 b

1.0 b

44.5 b

57.5 cd

Punch (Exp.)

8

10.51

4.3 c

0.5 b

1.0 b

45.0 b

57.6 cd

Tilt

4

10.50

7.5 b

0.4 b

0.7 b

44.6 b

57.5 cd

LSD (P = 0.05)

2.1

3.4

1.0

5.4

0.6

* Prosaro (Bayer), BAS555 (BASF), and Punch(DuPont) are unregistered products; all are triazole chemistries.
** Induce non-ionic surfactant was added to Folicur, Prosaro and BAS555 treatments only, at rate of 0.125% v/v.
*** FHB = scab; field severity = % incidence of heads with symptoms x % head severity/100

 

PREVIOUS ISSUE WHEAT CULTIVAR SCAB RATINGS

Last week, the 15th issue of the NDSU Crop and Pest Report contained information from Prosper, ND on scab field severities for spring wheat lines. The statistical difference (LSD = 0.05) for values in this particular chart is 16.2, if youíd like to know which cultivars were statistically different from each other.

 

ERGOT

The 13th issue (July 28, 2005) edition of the NDSU Crop and Pest Report contained a picture of ergot in wheat and indicated that field scouts were finding ergot infection in some wheat and barley fields. Some harvested grain is now showing sclerotial ergot bodies, sclerotia which did not get discarded in the combining process. Ergot may affect the market grade of a grain; wheat or durum is graded as "ergoty" when it contains more than 0.05 % ergot sclerotia by weight; barley is considered "ergoty" when it contains more than 0.1 % by weight ergot bodies. In addition, ergot is toxic to animals and may cause harm to livestock, if present in feed.

Most of the sclerotia can be removed from ergoty grain with modern cleaning machinery, unless broken pieces are present, or the sclerotia are similar in size to the grain kernel. If feeding ergoty grain to livestock is unavoidable, the amount of sclerotia should be reduced to an amount less than 0.1 % (by weight) of the feed, through mixing with clean grain. Ergoty feed should NOT be fed to breeding female livestock. For more information on Ergot, please see NDSU Extension Circular PP-551 at:

http://www.ext.nodak.edu/extpubs/plantsci/crops/pp551.pdf

Marcia McMullen
Extension Plant Pathologist
marcia.mcmullen@ndsu.edu

 

NDSU PLANT DIAGNOSTIC LAB

Sample Summary, Oct. 1, 2004, through Aug. 17, 2005

The NDSU Plant Diagnostic Lab has received 1020 samples since October 1, 2004, nearly 200 samples more than the number received in FY2004 (October 1, 2003 through September 30, 2004). The increase in samples received is probably due to increased disease pressure experienced during the 2005 growing season. Table 1, below, summarizes the types of samples received this year.

Table 1. Types of Samples Received by the NDSU Plant Diagnostic Lab, FY2005

Sample Type

Number received

Field

743

Horticultural (homeowner)

220

Soil Bioassay

8

Home Mold

18

Specimen Identification (plants, insects)

31

Of the 743 field samples, 307 were specifically for seed health testing. Seed health testing includes testing bean seed for bacterial blight pathogens, pea seed for foliar nematodes, potato tubers for the bacterial ring rot pathogen, among others. The NDSU Plant Diagnostic Lab is now USDA-accredited for testing for bacterial ring rot, caused by Clavibacter michiganensis supsp. sepedonicus, for export to Canada. For more information about seed health testing in general and about our new status as a USDA-accredited lab for testing for bacterial ring rot for export to Canada, please call the lab for more information (701-231-7854).

Some of the more common disorders and disease problems diagnosed in the lab include the following (number of samples is in parentheses):

  • herbicide injury (43)
  • Aphanomyces root rot on sugarbeet (2)
  • Aphanomyces root rot ratings from soil bioassays (4)
  • positive for Rhizomania on sugarbeet (3)
  • negative for Rhizomania (20)
  • potato tuber rots (76)
  • Pythium root rot (8)
  • Rhizoctonia root rot on various hosts including a lawn, alfalfa, and sugarbeet (7)
  • Rhizoctonia (black scurf) on potato (2)
  • Spider mite injury (8)
  • Fusarium head blight on wheat (10)
  • Fusarium root rot on various crops (15)
  • Fairy ring on lawns (2)
  • and injury related to unfavorable environmental conditions (11)
  • Kasia Kinzer
    Plant Pest Diagnostician
    kasia.kinzer@ndsu.edu


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