Back to Basics: Root rots, Leaf blights and the Disease Triangle. Drs. Anne Dorrance and Pierce Paul

Transcription

1 Back to Basics: Root rots, Leaf blights and the Disease Triangle Drs. Anne Dorrance and Pierce Paul

2 Key Root Rot Pathogens

3 Factors that favor development AND Moist conditions after planting.

4

5 SCN populations are expanding and Increasing 12% of samples > 5,000 eggs/cup of soil - -

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7 Factors that favor development Soybean cyst nematode populations present Continuous corn or several years of corn Compaction, poor drainage Susceptible varieties

8 Information on Resistance Variables reported for each variety are: Entry: Check varieties are listed with the following designations. sus: Susceptible res: Resistant int sus: intermediately susceptible int res: intermediately resistant Foliar DX: SDS disease index is calculated as (DI*DS)/9. DX ranges from Plots were rated at R6 (full seed) growth stage. Numbers represent the mean of 3 replications. DI: SDS disease incidence scored as % of plants with symptoms. DS: SDS disease severity scored on a 1 9 scale with 1=mild chlorosis, 5=severe leaf scorch, and 9=premature death of plant. Relative DX: Every trial contains at least one susceptible check that is appropriate for that MG and herbicide compatibility trait. Relative DX allows comparison of a particular entry against the susceptible check. It is the DX percentage of the susceptible check's DX. Suggested Category: Entries are classified according to Relative DX. R: Resistant with a Relative DX of 0 20 MR: Moderately Resistant with a Relative DX of MS: Moderately Susceptible with a Relative DX of S: Susceptible with a Relative DX of 61+

9 SCN Population changes - Non- host Crop Rotation soy- wheat- corn- soy soy- corn- soy- corn soy- Rsoy- soy- Rsoy Column Year 1 Year 2 Year 3 Year 4

10 Crop Rotation at Higher numbers Soy- Wheat- Corn- Soy Soy- Corn- Soy- Corn RSoy- Rsoy- RSoy- Rsoy Year 1 Year 2 Year 3 Year 4 Year 5

11 General Pattern of SCN Population End-of-season SCN numbers (eggs/100 cc soil) Density Decline when Growing Several Years of Corn soybeans 1st year 2nd year 3rd year corn corn corn G. Tylka, Iowa State University, personal communication

12 Soybean Cyst Nematode (SCN) in Ohio Field Studies in Ohio: Impact of planting varieties from different sources of resistance o PI88788/Peking/CystX Impact of Nematode Seed Treatments Changes in SCN populations among varieties in later maturities

13 Changes in SCN egg counts Putnam08 Sandusky08 SCN eggs/100cc of soil None PI88788(S) PI88788(G) Peking CystX Source of resistance

14 SCN Study PI88788 SCN resistant Name Brown (bu/a) Pike (bu/a) Wood(bu/A) ApronMax Avicta Complete Trilex Trilex 2000 plus Poncho VoCvo Nontreated Seed Mean P Value Egg counts (eggs/100 cc of soil) Spring SCN 3,733 1,304 9,232 Fall SCN 6,389 3,080 3,299

15 Evaluation of SCN population changes when planted with resistant varieties Highland- SCN counts Spring distribution ,115 eggs/cup of soil Fall distribution eggs/100 cc rep not collected in fall sampling due to excessive grass weeds.

16 Be sure to look!

17 Possible SDS Look- A- Like Brown Stem rot Low ph - <6.0 Notice the brown pith

18 The Emergence Problem Googleearth.com

19 Soybeans Soilborne diseases Poorly drained fields 1/3 acreage continuous soybean Large % no-till

20 No- till or reduced tillage production systems Conservation Technology Information Center, 2006 Longer to warm up in spring Stays wetter longer o increases infection period Workneh et al., 1998 Plant Dis. 82: Inoculum builds up in top 3 in (7.5 cm) of soil

21 Pythium Pathogenicity 4 Corn (LG2540) Soybean (Sloan) Importance of species diversity in pathogenicity Pathogenicity Score P. attrantheridium P. dissotocum P. echinulatum P. graminicola P. helicoides P. inflatum P. irregulare Species P. sylvaticum P. torulosum P. ultimum var. ult. P. ultimum var. spor. Group 3 Group 6 Broders et al Plant Disease

22 Fusarium graminearum Head scab fungus Gibberella stalk rot of corn Also soybean seed and seedling pathogen South America o Pod and stem blight

23 It s about inoculum Soybean cyst nematode Pythium spp. Fusarium graminearum Corn:Soybean Compacted, poorly drained soils Variety Selection is critical

24 Key to High Yields Variety Selection Drainage Optimum planting conditions Seed treatment fungicides

25 Northern corn leaf blight: Increasing prevalence in the Midwest

26 Northern corn leaf blight Exserohilum turcicum Ø Large cigar shaped necrotic lesions. v 2-8 long by wide v Tan in color v Older lesions dark gray

27 Disease Cycle C (66-80 F) 6-18 hr of water 7-12 d

28 Why are we seeing more NCLB? The plant Are we planting more susceptible hybrids? NCLB The environment Are conditions becoming more favorable? The Pathogen Is there more inoculum available? Has there been a shift in the pathogen population? Is the pathogen becoming more adapted to changes in the environment?

29 20- year Tillage Trend Percentage of Corn Acres Under No-Till %/Yr %/Yr 2.4%/Yr Y_1989 Y_1990 Y_1991 Y_1992 Y_1993 Y_1994 Y_1995 Y_1996 Y_1997 Y_1998 Y_2000 Y_2002 Y_2004 Y_2006 Y_2007 Y_2008

30 20- Year Weather Trend Mean Temperature (June-August) Total Rainfall (June-August) in in in

31 Race- Specific Resistance Ø HT Genes: Ht1, Ht2, Ht3, Htm1 and Htn1, v Expressed as a qualitative form of resistance, characterized by chlorotic rather than necrotic lesions when challenged by an avirulent race v THE EXCEPTION: Htn1 acts quantitatively by delaying lesion development until after flowering (Gevers, 1975). Leonard, K.J., Levy, Y. and Smith, D.R. (1989)

32 Resistance and Lesion Characteristics Northern Corn leaf blight resistant-type lesions Northern Corn leaf blight susceptible lesions Courtesy: J. K. Pataky

33 Partial Resistance Ø Highly heritable, in many cases controlled by several genes. Ø Considered durable relatively effective against all races of the pathogen less selection pressure. Ø Expressed as quantitative resistance through different components of the infection cycle. v Number and size of lesions v Infection efficiency v Rate of lesion expansion v Latent period v Sporulation capacity

34 Partial Resistance and Disease Progress Low Partial Resistance 1.07%/day High Partial Resistance 0.45%/day Pataky, J. K., Perkins, J. M., and Leath, S. (1986)

35 Partial + Race- Specific Resistance Percent leaf area affected LOC1 LOC2 LOC3 Number of lesions Group1 Group2 LOC1 LOC2 Group3 Group4 Group5 Ht Partial Group1 S S Group2 S R Group3 R S Group4 S R Group5 S R Group6 R R Check S R Group6 Check 0 Group1 Group2 Group3 Group4 Group5 Group6 Check Lipps P.E., Pratt R.C. and Hakiza J.J. 1997

36 SO, why are we seeing more NCLB? Ø Current conservation tillage practices favor recent increases in NCLB intensity in the Midwest. Ø Summer temperature have been consistently within the NCLB- favorable range over the last 20 years. Therefore temperature or adaptation of the pathogen to it are less likely to be responsible for recent changes in NCLB intensity. Ø Higher amounts of rainfall during the last 7 years likely is a key driver of recent changes in NCLB intensity. Ø The possibility of a race shift cannot be ruled out. Further investigation is needed. Ø Widespread cultivation of hybrids without resistance to NCLB is probably the single most important contributor to the recent changes in NCLB intensity

37 37 Disease Quantification

38 Common measures of disease Ø Incidence (% or proportion) v Proportion of plants or plant units diseased (6/10) x 100 = 60% (6/10) x 100 = 60% Chapter 2: Madden, et al

39 Common measures of disease Ø Severity (% or proportion) v Area of plant tissue diseased Chapter 2: Madden, et al

40 Common measures of disease Ø Prevalence v e.g. Number of field in a county with the disease or interest. Ø Density v e.g. Number of lesions on a leaf. Chapter 2: Madden, et al

41 Assessing severity 41

42 Assessing severity 42

43 Assessing Leaf Rust 5 % 15 % Nutter et al Severity Pro. Iowa State University 43

44 Assessing Powdery Mildew 15 % 25 % Nutter et al Severity Pro. Iowa State University 44

45 Assessing Leaf Rust 50 % 25 % Nutter et al Severity Pro. Iowa State University 45

46 Assessing Powdery Mildew 5 % 50 % Nutter et al Severity Pro. Iowa State University 46

47 More Information

48 Contact Information Anne Dorrance, Ph.D., Professor