Microbiology, Virology, and Immunology Department

Phytopathogenic microorganisms

History of plant pathologyCauses of plant diseaseBacteriaFungiViruses and viroidsDisease cyclesControl

Epiphytes – microbes which are on all plant surfaces.

Endophytes – microbes live inside plants.

Phytopathogens  – organisms that cause infectious disease 

Plant associated bacteria may be beneficial or detrimental.

Populations of 106 CFU (colony-forming units/milliliter) or higher are normally required for bacteria to function as biological control agents or cause infectious disease.

Plant pathology (also phytopathology) is the scientific study of plant diseases caused by pathogens (infectious organisms) and environmental conditions (physiological factors).

Organisms that cause infectious disease include fungy, bacteria, viruses, viroids, phytoplasmas, protozoa, parasitic plants

Once inside the plant tissue, bacterial plant pathogens usually grow intercellularly, that is, they grow between cells.

Physiology of phytopathogenic bacteria

Most phytopathogenic bacteria are aerobic (live in the presence of oxygen) and some are facultative anaerobes which can grow with or without oxygen. 

Plant pathogenic bacterial generaPlant pathogenic bacterial genera

ErwiniaPseudomonas

RalstoniaXanthomonas

Agrobacterium(Rhizobium)

ClavibacterBacillus

Streptomyces

XylellaPhytoplasma

There are several ways by which bacterial plant pathogens may be spread: by wind, water, and soil movements;by insect vectors; by infected seeds; by contaminated tools.

The bacteria enter the plant othrough natural openings, like the stomata of the leaves, oor through wounds in the plant tissue. 

Methods of penetration and invasion

by bacteria

Infection is generally considered to be passive, i.e. accidental, although a few cases of plant chemoattractants have been reported.

Artificially, bacteria are most commonly introduced into plants by wounding.

Types of bacterial plant diseasesTypes of bacterial plant diseases

Foliar diseases (e.g., spot, speck,

streak, blight)

Vascular diseases (e.g., wilt, blight)

Soft rots

Developmental diseases (e.g.,

canker, scab, gall)

are characterized by abnormal outgrowths of plant tissue.

Some of the abnormal outgrowths of plant tissue produced in galls are large and swollen, while others are small and divide rapidly.

GALLS

Infected plants will develop smooth, light-colored galls on its roots and stems. As the galls age, they develop into hardened, discolored galls that eventually slough off to make room for new, secondary galls. These formations inhibit the plant's ability to transport nutrients and water throughout the plant. This lack of transport results in the plant's loss of vigor which is also accompanied by growth stunt and branch and twig dieback.

 Crown gall, in particular, may develop in sugar beets, fruit trees, and other plants with broad leaves and stems that come out of the ground.This disease is soil-borne.

Xanthomonas campestris

Bacterial galls can be produced by the genus Agrobacterium and certain species of Arthrobacter, Pseudomonas, Rhizobacter and Rhodococcus. 

Slime-producing bacteria are the causative agents of bacterial wilts. These bacteria plug up the plant’s vascular system (the vessels that carry water and nutrients) and cause the plant to wilt. 

Bacterial wilts

Infected plants wilt rapidly and often die.

There are four main wilt-causing bacterial plant pathogens : Corynebacterium insidiosum causes wilts in alfalfa; Erwinia tracheiphila causes wilts in cucumbers; Erwinia stewartii produces wilts in sweet corn; and Pseudomonas solanacearum causes wilts in tobacco.

Bacterial wilts

The bacterial plant pathogens that produce leaf spots may also infect the stems and fruits of the diseased plant. The spot-causing bacterial plant pathogens are: Pseudomonas spp., Xanthomonas spp.

Leaf spots

Spot-causing bacterial plant pathogens: Pseudomonas striafaciens, Xanthomonas pruni.

- the most common symptom of bacterial disease

Blights or deadening of tissue on leaves, stems or tree trunks, and rots

Fire blight - Erwinia amylovora

Fungi can grow on living or dead plant tissue and can survive in a dormant stage until conditions become favorable for their proliferation. They can penetrate plant tissue or grow on the plant’s surface. Fungal spores, which act like seeds, are spread by wind, water, soil, and animals to other plants.

Warm, humid conditions promote fungal growth.

Fungal infections

Methods of penetration and invasion by fungi

While many fungi play useful roles in plant growth, especially by forming mycorrhizal associations with the plant’s roots, others cause such common plant diseases as anthracnose, late blight, apple scab, club root, black spot, damping off, and powdery mildew. Many fungi can attack are variety of plants, but some are specific to particular plants.

Fungal infections

While more than 300 plant viruses have been identified. The symptoms of viral infection include yellowing, stunted growth in some part of the plant, and plant malformations like leaf rolls and uncharacteristically narrow leaf growth. The mosaic viruses can infect many plants. Plants infected with this virus have mottled or streaked leaves; infected fruit trees produce poor fruit and a small yield.

The viruses and viroids that attack plants are the hardest pathogens to control.

Viruses and viroids

Effective treatment of bacterial infections in plants may involve radical treatment or removal of the infected plant parts or removal of the diseased plant altogether. A more drastic measure may mean the removal of entire communities of susceptible or diseased plants.

Mechanical inoculation and early stages in the systemic distribution of viruses in plants

Forms and locations of survival of fungi and bacteria between crops

control of plant disease begins with good soil managementselection of disease-resistant plants disposal of infected plants plan crop rotation to eliminate over-winteringdisinfect pruning toolsprevent surface wounds on plantsexpose plant material you wish to compost to dry air, heat and sunlight to kill any disease-causing bacteria

Control

control insects that might vector bacterial diseasesif the disease is systemic, affecting the stem as well as leaves, the plant cannot recover. Destroy it to prevent the spread of the disease to other plants.Make sure the soil is healthy with plenty of organic nutrients and beneficial microorganisms.

Control