Plant Pathogens

Group 1 - Seeds (Diplodia Sapinea)

  • fungus belonging to the family Botryosphaeriaceae (diverse fungi that are either pathogens, endophytes or saprobes, mainly on woody hosts.)

  • previously known as Sphaeropsis sapinea

  • has an endophytic mode in its lifecycle, making it difficult to detect before disease outbreaks

  • INFECTION: affects the seed through the process of seed cone blight

  • spread by wind or rain, and when conditions are favorable, the spores will land on the seed cones

  • spores germinate and penetrate through the wounds and natural opening.

  • COLONIZATION: Once inside the plant tissue, it colonizes the host by growing and spreading through the infected needles. It produces fungal structures called pycnidia, which contain spores that can be released to initiate new infections.

  • SPREAD: The disease can spread within a tree and to neighboring trees, especially during wet conditions when spore production and dispersal are most active.

  • SYMPTOMS: Four D’s = Damping off and root rot of seedlings, Decay, Discoloration, Death of seedling

  • Favorable Conditions: Warm and wet weather conditions are conducive to the development and spread of Diplodia tip blight.

Group 2 - Xylem (Xylella Fastidiosa)

  • a Gram-negative bacterium that belongs to the family Xanthomonadaceae

  • has a slender, rod-shaped morphology and is non-motile

  • colonizes the xylem vessels of infected plants, where it forms biofilm-like structures that aid in adhesion and protection against host defenses

  • ability to block the xylem vessels, which are responsible for water and nutrient transport in plants

  • transmitted from plant to plant primarily by xylem-sap feeding insects, known as vectors

  • In United States, Pierce's disease, caused by Xylella fastidiosa, affects grapevines and is associated with significant economic losses in the wine industry

  • Control Strategies: 1. Use of insecticides to target vector populations

    2. Removal and destruction of infected plants 3. Quarantine measures to prevent the spread of the bacterium to new area

  • Alternative Strategies: 1. Breeding resistant plant varieties, using bacteriophages to control bacterial populations 2. Utilizing biological control agents

Group 3 - Flower (Ustilaginoidea virens)

  • Rice False Smut Fungus, a severe and widespread disease in major rice growing areas of Asia, Africa and America

  • inhibits pollen germination, and pollen tube elongation

  • Infection induces elevated levels of ROS and Enhanced Callose Deposition

  • invades both pistil and stamens

  • specifically digests the integuments and is not a systemic pathogen

  • The hyphae colonize the inner upper spaces of spikelets from the apex and grow downward to infect the pistil and stamens.

  • The hyphae reach the ovary and destroy the integuments of ovule.

Group 4 - Fruits (Alternaria tenuis and Botrytis cinerea)

  • Alternaria tenuis - Named by Nees, 1816 but the accepted name is Alternaria alternata by Keissl on 1912.

  • causes fruit rot in many fruits and vegetables in the form of black spot

  • regarded as the major mycotoxin producing species; also caused blights in many parts.

  • Cause severe problems in agriculture; deteriorate food quality and quantity

  • needs a moist warm environment; found in areas with humid climates, or where there has been significant rainfall

  • Infection: Wound, Natural Openings, Insect damage

  • After infection, the fungus grows and produces spores, which can spread to other parts of the fruit or to other fruits.

  • Botrytis cinerea - causes gray mold on fruits

  • produces fuzzy gray-brown spores that spread by wind, water, or insects

  • found worldwide, especially in temperate and subtropical regions, where humiditiy and moisture are high

  • Infection: Wounds, Senescence, stressed by environmental factors

  • The infected tissues become soft, watery, and rotten, and may develop dark lesions or spots.

Group 5 - Leaves (Puccinia Triticina)

  • Wheat leaf rust is a fungal disease that affects wheat, barley, rye stems, leaves and grains. Infections can lead up to 20% yield loss, which is exacerbated by dying leaves, which fertilize the fungus.

  • Hosts:

    • Telial/uredinial (primary) hosts: common wheat, durum wheat, cultivated emmer wheat, wild emmer wheat, goatgrass, and triticale.

    • Pycnial/aecial (alternative) hosts: meadow rue and Isopyrum fumaroides

  • Life Cycle:

    • Asexual Stage: Occurs primarily on wheat leaves. Starts with uredospores, orange-brown pustules containing spores that are readily dispersed by wind. Urediniospores germinate on wheat leaves, forming haustoria (root-like structures) that absorb nutrients. New uredospores are produced in pustules, repeating the cycle and causing further infection on wheat.

    • Sexual Stage: Occurs on an alternate host, a flowering plant called meadow rue (Thalictrum). During late summer, teliospores are produced on wheat leaves. Teliospores germinate under cool, moist conditions, forming basidiospores, which are transported by wind. Basidiospores infect meadow rue, leading to the formation of pycnia (flask-shaped structures) and aecia (cup-shaped structures). Aecia release aeciospores, which can infect wheat leaves, completing the life cycle.

  • Infects through small leaf openings called stomata

  • When faced with plants they don't typically infect, these fungi struggle to penetrate through these openings. This is because they rely on specific cues that initiates infection, when these signals are absent, the fungi fail to invade the plants.

Group 6 - Roots (Phytophthora)

  • have a broad host range and are known as soilborne pathogens

  • Phytophthora organisms are often referred to as water molds

  • most commonly associated with root rot disease but can also damage above-ground plant parts

  • Characteristics:

    • Cell walls contain cellulose instead of chitin

    • Growth by means of filamentous hyphae lacking cross-walls

    • Produce sexual spores called oospores and swimming asexual spores called zoospores.

    • Foliar symptoms may not become evident for months or even years

    • Feeder roots are black, decayed and few in number.

    • requires warm, moist soils in order to cause disease

  • Types of Plants it Infects: Irish potato, herbaceous and woody plants, fruit and nut trees, ornamental trees and shrubs, tomatoes, peppers, eggplant, etc.

  • Diseases: rhododendron root rot, sudden oak death, potato late blight, root rot, foot rot or gummosis, fruit brown rot, canopy blight, damping-off of seedlings, buckeye rot in tomatoes and leather rot in strawberries.

Group 7 - Stem (Sclerotinia sclerotiorum)

  • a devastating necrotrophic fungal plant pathogen that infects more than 400 plant species worldwide.

  • causal agent of white mold; host species include lettuce, cabbage, sunflower, canola, and bean

  • Symptoms:

    • First appear on leaf axils and include soft watery lesions or areas of very light brown discolouration on the leaves, main stems and branches 2–3 weeks after infection.

    • Lesions expand, turn to grey-ish white and may have faint concentric markings. The stems of infected plants eventually become bleached and tend to shred and break.

    • When the bleached stems of diseased plants are split open, a white mouldy growth and sclerotia becomes evident.

  • Mechanism: 1. Sclerotia Overwinter in soil 2. Formation of Apothecia

    3. Ascospore Distributes on Petals 4. Germination and Distribution of Infection 5. Distribution of Fungal Lesion 6. Formation of New Sclerotinia

Group 8 - Root Crop (Tuber) (Phytophthora Infestans (Mont.) de Bary)

  • an oomycete (water mold)

  • causes Late blight disease

  • Primary Host: potato and tomato

  • described by Heinrich Anton de Bary and classified by Jean Baptiste Étienne Auguste Montagne

  • Signs and Symptoms:

    • Early signs = Small, light to dark green water-soaked spots

    • Lesions expand rapidly and turn dark brown/black, appearing greasy

    • surrounded by yellow halo around lesions

    • Later signs = During active growth, a white mildew-like area is visible at the edge of the lesions.

    • Brown, dry lesions can develop following warm and dry weather

    • In infection of tubers, it’s characterized by irregularly shaped, slightly depressed areas from brown to purplish of variable size on the skin

    • Late blight causes a tan to reddish-brown, dry, granular rot found under the skin

    • Tissue is marked by a brown fingerlike extension into the healthy flesh of the tuber.

  • Life Cycle: involves both direct germination, indirect germination, and sexual & asexual reproduction

    • The majority of the reproduction happens asexually during the disease cycle. P. infestans develops sporangiospores released through stomata

    • These sporangiospores produce zoosporangia

    • Zoosporangia are airborne and can spread by wind to neighboring plants where they might germinate and directly infect plant cells. When the weather is cool and there is a lack of nutrients, zoosporangia will develop biflagellates which enable them to be mobile. In this process of indirect infection, the zoosporangia will encyst, develop germ tubes, and use them to penetrate plant tissues.

    • In the sexual disease cycle, uses the A1 and A2 mating types. Male and female reproductive organs meet and produce oospores.

    • Oospores are very robust with thick walls and can survive for years in the soil. They are tolerant of cold temperatures but more sensitive to heat. Oospores also form germ tube structures and produce sporangia. The sporangia can then release zoosporangia and infect hosts through indirect or direct germination.

Group 9 - Viroids (Potato Spindle Tuber Viroid/PSTVd)

  • EU (European Union)-listed quarantine pathogen for potatoes

  • causes tuber deformation and cracking in addition to stunting of the plant

  • has a naked, circular, single-stranded RNA. It is a parasite of the host transcription system. It includes 138 species of hosts spread across 10 families, with the Solanaceae family serving as its primary hosts.

  • Symptoms:

    • Spindly, erect foliage with overlapping leaflets with terminal leaflets rolled upward.

    • Display malformations of tubers, cracking, and slower sprouting

    • Prominent eyes equally dispersed across the tuber

  • Mechanism:

    • Transmission = to progeny seeds on potato and tomato plants by aphids and vertically through pollen.

    • It can spread horizontally from one plant to another by way of contaminated pollen.

    • Most commonly found in top leaves and tubers of potato plants.

    • The main vectors for long-distance dissemination are true seed and diseased tubers.

    • Aphids are also vectors but only when Potato Leaf Roll Virus (PLRV) is present does this occur.

    • Control = Within infected plants, it cannot be controlled by chemical or biological means.

    • To prevent PSTVd from infecting infected plants, use virus-free seeds and planting materials, and adhere to agricultural sanitation practices.

    • If infection develops, destroy infected plants and maintain high cleanliness standards to prevent contamination of future harvests.