Plant DiseasesPresented by Terry Ferriss, PhD

University of Wisconsin—River Falls

Causes or Agents of Plant Diseases

• Abiotic – non-pathogenic / physiological• EX: environmental, cultural, air

pollution, etc.

• Biotic --- caused by living organisms (pathogens)– Ex: Primarily: Bacteria, Virus, Fungi

Ozone damaged potatoPhoto by Gerald Holms

Plant Disease TriangleSusceptible Host Plant

Pathogen Favorable environment

Disease

Identifying Diseases• Symptoms: – Reaction of host plant to the organism or agent• Ex: Leaf spots, wilting, chlorosis, galls

Crown Gall on PeachPhoto: Turner Sutton

Alternaria blotch on applePhoto: Turner Sutton

Identifying Diseases (cont.)• Signs:– Physical evidence of the presence of the disease

• Ex: Bacterial ooze, fungal spores, fungal hyphae

Bacterial ooze on apple (fire blight) Green mold on orange (Penicillium)Photos: Turner Sutton

Bacteria• Microscopic / Single cell organism• Reproduces by dividing• Signs/symptoms

– Water soaked spots – papery as ages– Chlorotic spots with “HALOS”– Angular spots & chlorosis are between veins– Soft rots– Discoloration of vascular system– Fasciation / Galls

Wildfire bacterium of tobacco (Pseudomonas tabaci) Photo: NCSU Plant Path Dept.

Close-up of Xanthomonas leaf spot of croton shows vein-delimited spots with irregular water-soaked margin. DPI Photo Credit: Jeffrey W. Lotz; Univ FL

Spread of Bacteria• Spread: – Physical spread by humans -

hands / tools, etc– Splashing contaminated water– Contaminated irrigation water– Insects / birds

Foliar symptoms of bacterial leaf disease of aglaonema include hydrosis of infected tissue and leaf spots and blight caused by the bacterial plant pathogen Dickeya chrysanthemi.. (Photo: S. Nelson) Univ of Hawaii

Control of Bacteria“DIFFICULT”

Therefore Prevention is BESTSanitation

Rogue infected plantsInsect control

Clean stock/ seed

Common Bacteria:Xanthomonas;

Pseudomonas (Ralstonia) Erwinia

Viruses

Scanning electron micrograph of Tobacco mosaic virus

Photo: NCSU Plant Path Dept.

•Very small•similar in size and chemistry to DNA

•Spread:• Insects / fungi / nematodes from one infected plant to another

•Control: •Clean stock •Insect control•Rogue out infected plants

NO CHEMICAL CONTROLS !!!!!NO CHEMICAL CONTROLS !!!!!

Viruses• Symptoms: (many and varied)– Stunting– Mottling / blotches / spots– White – yellow – brown rings on lvs– Deformed plant parts– Breaking of the flower color

UGA Plt Path Dpt Archive Photo

Cucumber mosaic virusPhoto: Div. Plt Industry Archives

FL Dept of Ag

VirusNotable Examples: TSWV: Tomato spotted wilt virusINSV: Impatiens necrotic spot virus

TMV: Tobacco mosaic virus TMV can be transmitted via cigarettes Ring spot virus on tropical foliage plants

Fungal Diseases1 = Fungus 2+ = fungi

Hyphae = single thread

Mycelium = mass of hyphae

Spores = fruiting or reproductive structures asexual and sexual spores possible

Root Rot DiseasesWater Mold Fungi

Pythium PhytophoraCharacteristics:• soil born• Favored by cool, wet soil• Encouraged by: low O2, hi NH4

+, hi SS• Low O2 root exudate = stimulates spore germination

Non-water Mold FungiRhizoctonia : drier, brown rot intermediate moistureFusarium: Dk brown - black favored by dry-interm.H2O collapsed roots, stunted pltThielaviopsis: drier, BLACK rot likes hi pH (6.5 +)Stunted plt; excessive lf dropOlder plts / pansy / poinsettia

Root Rot Symptoms/Signs

Pythium on cuttings and pltsChase Horticultural Service

1. Attacks at soil line ( Pythium) 2. Lack of root hairs3. Dark roots / outer root cortex sloughs off4. Lower lvs – chlorosis + abscise 5. Decline in vigor / stunted growth6. When severe = wilt

Control of root rots1. Sanitation2. Well aerated medium3. Avoid excessive moisture in medium4. Biological: Protective fungi (ex: Trichoderma sp in RootShield / PlantShield & others)

5. Chemical: Water molds: Subdue; Benlate, Aliette, etc Non-water molds: Truban, Chipco, Medallion, Daconil, Cleary 3336, Heritage,etc

Botrytis cinerea = “Grey Mold”

Signs/ symptoms:1.Grey “fuzz” 2.Black sunken-lesions3.Tissue blackening/ collapse / rotDevelopment favored by :1.Dead & stressed plant parts2.Poor air circulation3.Any temp 33-850 F

Plant Disease Clinic; Cornell Univ.

Botrytis on PoinsettiaBrian Whipker; NCSU

Controlling Botrytis1. Air circulation

2. Watch stressed plants

3. Chemical : Termil (fumigant) Chipco , Daconil, Medallion, etc

Powdery Mildew1. Mycelia feed via haustoria2.Spread: • spores fly in low RH (day)• Spores germinate in high

relative humidity (RH) + free water at night

Univ. of IL Extension, PM on lilac

PM spores on lower epidermis; NCSU

Controlling Powdery Mildew1. Reduce RH2. Sanitation3. Increase air circulation4. Biological: Horticultural oil potassium bicarbonate (baking soda) Neem oil Bacillis subtilis sulfur 5. Chemical: Copper based fungicides, also: Banner, Eagle, Heritage, Pipron, Strike Terraguard, Compass O

Other Fungal Leaf Spot Diseases

1. Alternaria2. Anthracnose3. Cercospera4. Septoria5. Fusarium6. Many others

Alternaria; Cornell Dept of Plant Path.

Anthracnose leaf spot and twig blight on maple; Iowa State University Extension

Fusarium :U

CDavis

Fungal Leaf Spots• Round spots / sometimes elongated• Not limited by veins• Bulls-eye effect is common

University of Wisconsin—River Falls

Thank you

Terry Ferriss, PhD