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Selecting for Resistance to Plant Disease: Part I
Martha Rosemeyer
Organic Seed
May 20 2004
Outline
Importance of disease resistanceIdentifying symptoms of disease (those
plants that are not resistant!)Types of pathogensKoch’s postulatesThe disease triangleImplications for selection
Most plants are resistant to most pathogens Most plants grow relatively unhindered by
diseases Why?
One of most powerful methods for organic farmer, and coming back in importance
What is a disease?
“A harmful deviation from normal functioning of physiological processes”
“Dis-Ease” Plant diseases cause loss of 12% of food
crop worldwide (Science 257: 482, 1992)
What is a plant pathogen?
Any organism which can cause plant disease
What are the mechanisms of resistance to disease?
Structural- pathogen cannot enter- sticky or tough
Chemical- pathogen enters but is hindered by “plant secondary chemicals” of plant used in plant defense can inhibit insect growth, e.g. tannins are protein-
binding Organic foods higher in antioxidants because
insects bite and induce formation
Mechanisms of resistance to plant disease cont.General systemic PR (pathogenesis-related)
proteins act like immune system Example- pathogen cell wall degrading
Specific antibody-like resistance genes (vertical) Genetic engineering moving them between species Gene cloned “osmotin” that inhibit fungal
reproduction by bursting spore cases of Phytopthora infestans
Other resistance mechanisms
Hypersensitive reaction- immediate death of tissue (necrosis) around the point of entry of the pathogen
Systemic induced resistance (SIR) heightened immunity to plant pathogens
Usually mechanism of resistance is not apparent
However we can still select plants that are resistant
Let’s now turn to susceptibility, so we know which plants to eliminate in our screening for disease resistance (whether vertical or horizontal)
In selection the observation of symptoms is key!
Many plant problems caused by non-pathogens in PNW, like unfavorable weather and poor growing conditions or herbicide
Only 1/3 caused by insect pest or pathogen (WSU extension bull. 1247)
Non-pathogens may be identified by regular pattern, more than one crop affected
General steps to diagnosisExamine leaf or affected plant part and find the time
course of symptoms Is it environmental condition, nutritional problem, pest or
disease?
Examine pattern of symptoms in plot- random or regularFind appropriate resources
Insect references or on-line Web databases Host Disease Index; Web: Cornell site
Take to plant disease diagnosticianPerform Koch’s postulates
General bacterial vs. fungal leaf symptoms
Bacterial, usually Fungal, usually
Mildew vs. Virus or nutrient deficiency
Non-pathogenic conditions
Chemical spray injury Salt or chemical injury
Nutrient symptoms can also be confused
Insect damage can cause disease like symptoms
Look for the insectCan cause chlorosis, leaf curl, distorted growth
Blue aphid symptoms on alfalfa
Red currant in OR
Aphid damage can manifest in various symptoms AND most importantly they can vector virus
Sooty mold
Fungus complex living on aphid“honeydew” (excrement)
Other: Walnut Wilt of Tomato (allelopathy caused by juglone)
MacNab et al. 1983
Lightning damage
MacNab et al. 1983
Types of plant pathogens and their symptoms
Viruses (NA plus protein shell)Bacteria (single cell, no nucleus)Fungi (most multicellular, have a nucleus)Nematodes (multicellular organisms)
Most organisms do not cause disease-- less than 0.5%
VirusesParry 1990
MacNab et al. 1983 MacNab et al. 1983
Cucumber mosaic virus
Wide host rangeSymptoms:
chlorosis distortion of leaf
Tulip Breaking Virus
•First plant disease recorded in 1576 byCarolus Clusius in Netherlands •Realized viral nature in 1900s•Potyvirus- forms filaments strands of protein coat with nucleic material inside
Bacteria
Bacterial Spot of Tomato Xanthomonas vesicatoria
MacNab et al. 1983
Angular leaf spot of strawberryXanthamonas frageriae
Symptoms: watersoaked lesions leading to necrosis and chlorosisAvoid contaminated plant material when planting (crowns)
Fungi
MacNab et al. 1983
Late blight of tomato and potato
Late Blight of Tomato and Potato
MacNab et al. 1983
Check disease cycle to know when crop may be susceptible so select at correct time
Schumann 1991
Disease started plant pathology as a discipline
Potato famine of 1846 in Ireland (8 million population) caused by Late Blight of Potato, pathogen is fungus Phytopthora infestans
1 million people die, 1.5 million emigrate to US or Canada
Observations of these organisms were previously seen as result not cause of the disease. Accepted Pasteur’s “germ theory of disease” originally proposed in 1863.
Result of the Irish Potato Famine
30% of ppn died or emigratedSchumann 1991
How do you know any pathogen causes your disease?
Koch’s postulates German 1843-1910
Grew microrganisms on potato slices and then gelatin
Student was Petri
Isolated cholera and anthrax
Koch’s postulates first demonstrated with anthrax
Anthrax disease of herbivores caused by the bacteria, Bacillus anthracis
Wade, N. New York Times. October 23, 2001
Koch’s postulatesThe disease must always be associated with the
pathogenThe pathogen is isolated from the diseased tissue
in vitroThe pathogen is inoculated onto susceptible host
with resulting symptoms of disease as was originally found
The pathogen is re-isolated from the host tissue and found to be the same
Control of Late Blight of PotatoMajor problem for organic farmers
yields are about 30% lower and tuber size small
Major control is resistant varieties problem is keeping up with the pathogen
Irrigate using drip, hill soil, kill foliage before harvest
Fungicides Org: Copper sulfate and lime (Bordeaux) Conv: copper, Maneb, Mancozeb, et al.
Sclerotinia Rot of Tomatofungus Sclerotinia sclerotiorum
Often disease of Sunflower and Pepper in PNW
MacNab et al. 1983Symptoms vs. signs (the visible organism)
Clubroot of Cabbage(Slime mold)Plasmodiophora brassicae
Serious disease of crucifersworld wide
First see wiltLongevity in soil
Liming may reduce or completely control the disease. The pH of the soil should be raised to 7.2 or higher with hydrated lime
Resistance in cabbage ‘Badger Shipper’ and turnips, radish and rutabaga
Tulip Fire or Tulip blight Botrytis tulipae (Fungus)
Attacks all parts of plantBy far most common and
serious diseaseCan cause complete lossControl- remove diseased
plant and bulbRotate- 2 years minimum
Blue mold of Tulip, Penicillium sp.
Grows especially on damaged bulbsAvoid damaging bulbs
Blue Mold of Onion and GarlicPenicillium sp. (Fungus)
Appears during harvesting and storage
Watersoaked lesions to green/blue powdery mold
Dry immediately store at 41°F with low relative humidity
Blue mold of onion
Early Blight of Potato and TomatoAlternaria solani (Fungus)
Early in seasonSame disease affects
tomato, potato, eggplant, pepper
Look for target like spots
Can infect tuber when lifted out of the ground through infested ground
Can affect crown of stemControl: Minimize leaf
wetnessRotate 3-4 yearsRemove infected materialResistance
Tomato Varieties Resistant to Early Blight
Mountain
Mountain Fresh
Plum Dandy
•Supreme
•Big Beef
•Floramerica •Juliet
Potato Varieties Less Susceptible to Early Blight
Kennebec
Root Knot Nematodes(there are other pathogenic ones as well)technically a parasite
Root knot nematode of bean
Dindal’s, Foodweb of the Compost PileCardona et al. 1982
Nodules vs. Gall?
Just because the pathogen is there will disease be present?
A susceptible host plantA pathogen capable of
causing diseaseEnvironmental
conditions Need all 3! So if your
plant does not appear susceptible, do you know if it is or not?
If disease is present you can select it out of your breeding population
If no disease present you don’t know if you don’t have the right environment for expression of disease or the pathogen is not present
Summary
Know your germplasm and know your potential plant disease, pest and abiotic problems, so you can select appropriately
ResourcesAmerican Phytopathological Society series for specific crop
plants.DeAngelis J., et al. 2000. 2000 Insect Management
Handbook for the PNW. OSUPscheidt, J. and C. M. Ocamb. 2000 Plant Disease
Management Handbook.OSUhttp://www.ippc.orst.edu/cicp/Vegetable/veg.htmhttp://plantclinic.cornell.edu/FactSheets/http://www.ipm.ucdavis.edu/PMG/
http://vegetablemdonline.ppath.cornell.edu/
ResourcesMacNab AA, AF Sherf and JK Springer. 1983. Identifying Diseases of Vegetables. Penn. State Press. and the newer Sherf and MacNab. Vegetable Diseases and their Control
Englehard, AW (Ed.) 1989. Management of Diseases with Macro- and Microelements. APS. Huber, Take-all and nutrition, pp 46-74
Agrios, G. Plant Pathology. Several EditionsHoward (Ed.). Diseases and Pests of Vegetable Crops in CanadaBio-Integral Resource Center (BIRC). Berkeley CAKF Baker and RJ Cook. 1982. Biological Control of Plant
Pathogens. APS
Brassica flower