Vascular Wilt Diseases 

Overview

Symptoms

1. Sudden wilt and death of plants
2. Premature yellowing of lower leaves
3. Vascular discoloration (for fungal wilts)
4. Viscous ooze in vascular system (for bacterial wilts)

Vascular wilt diseases

Overview

Common denominators

1. Most cause sudden wilt by pluggin the xylem
2. Most are soil-borne
3. Some need vectors (Dutch elm, cucumber wilt)
4. Most have restricted host ranges

Vascular wilt diseases

Fusarium oxysporum

vs.

Verticillium

• both attack the xylem
• Fusarium thrives under warm temperatures
• Verticillium thrives under cool temperatures, high pH

Vascular wilt diseases

Fusarium oxysporum

Similar to Verticillium

Trinomials

• F. oxysporum f.sp. lycopersici (tomatoes)
• F. oxysporum f.sp. cubense (banana)
• Others infect cocoa, cannabis

Vascular Wilt Diseases

Fusarium oxysporum (Deuteromycete)

1. Macroconidia
2. Microconidia
3. Chlamydospores
4. Teleomorphs are Ascomycota

Vascular wilt diseases cause by bacteria

Erwinia Tracheiphila (Cucumber wilt)

• Very narrow host range (certain cucurbits)
• Transmitted by flea beetles (disease forecasting)

Vascular wilts caused by bacteria

Pseudomonas solanaceous (Southern bacterial wilt)

• Some races infect solanaceous plants
• Mostly tropical, subtropical disease
• Survives in plant debris, tibers, wild hosts

Vascular wilt diseases

Dutch elm disease (Ophiostoma ulmi) Ascomycota

vs.

(Chestnut blight) Cryphonectria parasitica

1. Perithecia
2. Conidia
3. Both introduced in the USA
4. Both exterminated respective native hosts

Vascular wilt diseases

Dutch elm disease (Ophiostoma ulmi) Ascomycota

vs.

(Chestnut blight) Cryphonectria parasitica

• Dutch elm disease true vascular wilt
• Chestnut blight is a canker disease

Vascular wilt diseases

Dutch elm disease (Ophiostoma ulmi) Ascomycota

vs.

(Chestnut blight) Cryphonectria parasitica

• Quarantines for dutch elm disease failed because the entire life cycle not understood
• Fungus thrives beneath bark

Vascular wilt diseases

Dutch elm disease (Ophiostoma ulmi) Ascomycota

vs.

(Chestnut blight) Cryphonectria parasitica

Mechanism of attack (Dutch elm)

• Fungus mostly a saprophyte
• Colonizes bark beetle galleries
• Bark beetle emerge, feed on spring xylem

Lesser European elm bark beetle efficient vector

(Native American elm bark beelte inefficient vector because it feeds too early in the season)

Vascular wilt diseases

Cucumber wilt

Control

Disease forecasting (Spray for beetles when winters are mild)

Vascular wilt diseases

Southern Bacterial wilt

Control

• Use clean tools
• Crop rotation
• Fallow
• Use clean planting stock

Vascular wilt diseases

Dutch Elm disease

Control

• Destroy infected trees
• Remove bark from dead trees
• Genetic resistence (oriental elms)

Leaf spot diseases of foliage and flowers

Early Blight

Alternaria diseases

Leaf spot diseases of foliage and flowers

Leaf spots

Caused by: Cercospora

Leaf spot diseases of foliage and flowers

Corn leaf blight

Caused by: Bipolaris (Helminthosporium)

Leaf spot diseases of foliage and flowers

Black rot of grape

Caused by: Guignardia bidwellii

Leaf spot diseases of foliage and flowers

Gray mold

Caused by: Botrytis

Leaf spot diseases of foliage and flowers

Symptoms

Alternaria

Spots and occasionally blights

• Zonate lesions

Leaf spot diseases of foliage and flowers

Cercospora, Bipolaris/Helminthosporium

Symptoms

Spots and occasionally blights

• With dark borders

Leaf spot diseases of foliage and flowers

Guignardia

Symptoms

Spots and occasionaly blights

• Shothole appearance

Leaf spot diseases of foliage and flowers

Botrytis

Symptoms

• Diffuse blighting

Leaf spot diseases of foliage and flowers

Signs

• Generally not readily seen for most leaf spotting organisms
• Hyphae and small sclerotia can be seen for Botrytis under high humidity

Leaf spot diseases of foliage and flowers

Alternaria (Deuteromycete)

Life Cycles

1. Anamorphic
2. Various species
3. Distinctive, dark-colored multicellular spores
4. Can be controlled in greenhouses with UV-absorbing film (inhibits sporulation)

Leaf spot diseases of foliage and flowers

Cercospora (Deuteromycete)

Life cycles

1. Anamorphic
2. Many species, many hosts both dicots and monocots
3. Often lesions have black borders with tan centers

Leaf spot diseases of foliage and flowers

Helminthosporium complex

Life cycles

(rice brown spot, southern corn leaf blight, many others)

1. Anamorphic
2. Like cercospora, except confined to grasses (including corn and rice)
3. Many produce virulent plant toxins

Leaf spot diseases of foliage and flowers

Helminthosporium complex

Life cycles

• Helminthosporium oryzae- rice brown spot
• Cochliobolus miyabeanus- Bengal famine of 1942 which starved 2 million people

 Leaf spot diseases of foliage and flowers

Helminthosporium complex

Life cycles

Southern corn leaf blight (1970s)

1. Bipolaris (formerly Helminthosporium) maidis Deuteromycete name
2. Cochliobolus heterostrophus (Ascomycete Name)

• Sexual stage a perithecium with filamentous ascospores Genetic recombination led to 1970's blight epidemic

Leaf spot diseases of foliage and flowers

Guignardia Bidwellia (Ascomycete)

Life cycles

1. Teleomorph
2. Black rot is Florida's most serious grape disease
3. Sexual stage (Guignardia bidwellia) perithecia air-borne ascospores (1°)
4. Asexual stage (Phyllosticta sp.) pycnidia produces viscous conidia designed for spread by splashing water like anthracnoses (2°)

Leaf spot diseases of foliage and flowers

Botrytis (Deuteromycete)

Life cycles

1. Anamorphic
2. Gray mold most common disease of greenhouse crops and elsewhere
3. Favors cool, humid conditions
4. produces gray mycelium, conidia, and flat sclerotia