Basic Plant Pathology (Continued)

Biotic Plant Diseases and Their Causes

As mentioned previously, biotic or infectious plant diseases are caused by living organisms. While most biotic plant diseases are caused by fungi (around 70% of them), other types of plant pathogens exist. 

Causes of Biotic Diseases in Plants

There are five common groups of plant pathogens.

Fungi Pathogens in Plants

Fungi are a group of eukaryotic, non-phototrophic organisms that have rigid cell walls.  Eukaryotic means that fungi cells have a nucleus like those of plants and animals (unlike bacteria and archaea).  Non-phototrophic means that they cannot use light energy to produce food through photosynthesis because they do not contain chlorophyll making them different from plants.  The cell walls are rigid because they contain large amounts of chitin which makes them hard.  They form long, fine filaments or threadlike structures called hyphae. Some common names for fungi include mold, mildew, mushrooms, and yeast though other types of fungi also exist.

Fungi are often both obligate parasites and saprophytes.  They will often infect and feed on a living host until it dies.  Once the host is dead, the fungi pathogen may continue to feed on the dead organic matter.  Therefore, it is important to remove all diseased portions of host plants from their environment once disease has been detected.

How Fungi Reproduce

Fungi reproduce by forming spores, mycelial fragments, and sclerotia.  Some fungi require mating and fertilization (i.e. they reproduce through sexual reproduction) and are called perfect fungi.  Most, however, do not require fertilization and reproduce through asexual reproduction.  Those are called imperfect fungi.

A spore is a sexual or asexual reproductive structure (single cell or multicell) adapted for dispersal and survival often in unfavorable conditions and sometimes for long periods that is capable of germinating and growing into a new individual.  Spores are to fungi as seeds are to plants.  Spores come in many shapes, sizes, structures, and colors.  Typically, they are different enough that they can be used to identify the pathogen. Once a spore germinates, it can infect a host plant by gaining access to the interior plant tissue either by digesting the outer cell walls of the plant or through natural opening between the host plant cells.  Then they grow hyphae through the openings and into the cells to feed off the nutrients they contain.

The vegetative part of a fungus made of a complex network of fine filaments (hyphae) is called mycelium.  Some fungi reproduce when fragments of the mycelium separate into pieces, detach, and grow into new colonies.  This is like vegetative reproduction in higher level plants.

Some fungi produce sclerotia which are compact masses of hardened fungal mycelium containing food reserves.  Sclerotia are designed to survive environmental extremes and often remain dormant for several years until favorable growth conditions exist or until a susceptible crop is planted.  They germinate at the soil surface and grow into the stems of their host plants.

Bacteria Pathogens in Plants

Bacteria are single-celled microorganisms surrounded by a cell wall which do not contain nucleus or membrane surrounding the DNA material for the cell.  Bacteria are typically rod-shaped organisms.  They reproduce through a process called fission in which one cell splits into two identical cells after the DNA has replicated.  This process can take place in 30 minutes or so when conditions are right.

Some bacteria are nonpathogenic to plants while others called plant pathogenic bacteria cause diseases in susceptible plant hosts. These plant pathogenic bacteria derive nutrients from living and nonliving plant tissue.  They infect the host typically in low numbers by entering the plant through natural opening such as stomata or by entering through areas where the plant has been injured in some way.  Once they have colonized the host, plant pathogenic bacteria multiply profusely in living plant tissues.  They cause many kinds of symptoms like galls, wilts, leaf spots, blights, soft rots, and cankers to name a few. Bacteria can be spread from plant to plant by rain, insects, or humans.

Phytoplasmas and spiroplasmas are obligate parasites usually found in water that invade the food carrying phloem in vascular plants.  They are like bacteria in the way they reproduce, but they lack cell walls.  Some symptoms caused by these microorganisms are fruit deformities, stunted growth, yellowing of leaves, rolled foliage, and even dieback or death of the plant. 

Nematode Pathogens in Plants

Nematodes are roundworms classified in the phylum Nematoda and are the most abundant of all animals on earth. They exist in almost every environment including soil, salt water, and freshwater. They are non-segmented, generally transparent without color, and slender with bodies that are typically almost microscopic in length, though the largest nematode is the Placentonema gigantissima which parasitizes the placenta of sperm whales and can grow to a length of 28 feet with a diameter of about an inch.

All nematodes lay eggs to reproduce. Some terrestrial nematodes found in soil deposit their eggs in the soil or tissue of plants.  Others female nematodes keep their eggs in a jelly-like mass either attached to their body or inside their body which hardens into capsule called a cyst when the female dies.  Eggs of some nematodes can lay dormant without hatching for years waiting on a susceptible host plant to grow near them before hatching.

Once they hatch, the rate of growth and reproduction for nematodes increase as the temperature increases.  Nematodes like temperatures between 50° and 95° F, with 80° to 86° being the optimal temperature range where they can complete their entire life cycle of hatching from an egg to laying their own eggs in as little as 4 weeks.

Most terrestrial nematodes found in soil are beneficial, though a small percentage are plant parasitic nematodes often referred to as eelworms.   These plant parasitic nematodes are obligate parasites (i.e. they feed on living plant tissue).  They feed on plant cells by using an oral stylet much like a hypodermic needle to puncture the cell wall of a host plant’s cell.  The stylet can be used to inject saliva containing enzymes into the host cell to partially digest its contents before they are sucked out of the cell and into the digestive track of the nematode.

In addition to digestive enzymes, the nematode saliva injected into the host cell can contain toxins that kill the cell and sometimes neighboring cells that were not injected which can lead to a variety of root deformities when it occurs below the soil surface.    The saliva can contain growth-regulating hormones that affect how the roots grow which can produce knotting, galling, or other changes in root growth.  The saliva of some nematodes can contain and transmit viruses to the host plant.

As nematodes eat their way through root tissue, they wound the plant.  These wounds can make the plant more susceptible to other plant pathogens.

Below the ground (primary) symptoms that might lead one to suspect nematode infestations include:

  • Stunted root growth
  • Discolored roots
  • Galls or knots in the root system (indicative of a root-knot nematode infestation)
  • Shortened roots frequently with swollen tips or lots of lateral roots near the root tip
  • Very small round cysts attached to the roots ranging from white to yellowish or golden to brown (indicative of cyst nematode infestation)

Above the ground (secondary) symptoms that might lead one to suspect nematode infestations include:

  • Stunted stem growth
  • Yellowing of leaves known as chlorosis
  • Premature wilting of leaves or other sensitivity to heat or moisture stress
  • Premature loss of leaves and fruit
  • Thinning out of turf and the plant’s failure to compete with weeds

Such damage is typically most severe in course-textured sandy soils.

If you suspect based on symptoms that nematodes are damaging plants, the only way to be sure of the type of nematodes, their population, and the risk to the plants is through laboratory analysis.  This can typically be performed by the university in your state which administers the state’s Master Gardener program.

When nematodes have been a problem in the recent past, the likelihood of the problem reoccurring is high if susceptible host plants are planted in the same area again.  Since controlling them with chemicals is limited, especially in residential settings, the best defense is a good offense. 

The best way to minimize nematode damage is to grow healthy plants, but do not overdo it.  This includes fertilizing as you would normally to grow healthy plants, watering as you would normally to promote a large, deep root system, and mulching to keep the roots cool and moist.  Mulching also adds organic matter to the soil which promotes the growth of natural enemies of nematodes like fungi and predatory nematodes which may keep the pathogenic nematode populations at levels where the plant can withstand their damage. 

Viral Pathogens in Plants

Viruses are intracellular (inside the cells) pathogenic particles consisting of an inner core of nucleic acid (either ribonucleic acid/RNA or deoxyribonucleic acid/DNA) surrounded by an outer layer of protein called the capsid.    Because cells in plant tissue are surrounded by rigid cell walls, plant viruses require a wound through which to enter a plant cell.  These wounds can be natural as from the branching of lateral roots or they could be caused by gardening practices such as budding or grafting.  They can also be caused by pests like insects and other pathogens such as fungi, nematodes, or parasitic plants.  In some cases, the insect or pathogen causing the wound can carry and transmit the virus to the host cell making them vectors.

Once inside the living plant cell, the virus takes over the cell and uses it to reproduce.  The virus sheds its protein coat and then produces copies of itself and related proteins.  This leads to the development of new virus particles which then move cell-to-cell through cytoplasmic bridges between cells called plasmodesmata and eventually throughout the entire plant via its phloem.  Essentially, plant viruses cause disease by using photosynthates and disrupting the plants’ normal cellular activity as the virus multiplies.  Because they require a living host cell to grow and multiply, viruses are obligate parasites. 

Parasitic Vascular Plant Pathogens

Parasites are organisms that live on or in another organism called the host and benefits by deriving its nutrients at the host’s expense.  Some parasites called obligate parasites derive their nutrients only from living tissue while others called saprophytes can derive their nutrients from dead organic material.  If a parasite harms its host in the process of feeding off it, it is said to be a pathogen and causes diseases. 

There are a number of flowering or seed plants that parasitize other plants.  These parasitic plants cannot photosynthesize food on their own and require a host plant for food or for one or more ingredients needed to make their own food.  They connect themselves to the host plant via a specialized structure called a haustorium.  It should be noted that a plant can be parasitic and not be a pathogen.  Only if the parasitic plant creates disease symptoms in the host is it also a pathogen.

Parasitic plants can be classified based on where they are located on the host plant. Some like mistletoes and dodder are stem parasites.  But most are root parasites like broomrape.

Parasitic plants can also be classified based on the degree of dependency on the host plant.  Hemiparasites or half-parasites are the least dependent on the host plant.  These parasitic plants contain chlorophyll and are photosynthetic (can produce their own food needed for growth), but they obtain water and dissolved nutrients by connecting their haustorium to the host xylem.  Others are holoparasites which are fully depending on the host plant.  These parasitic plants do not contain chlorophyll and are non-photosynthetic.  They not only depend on the host plant for water and dissolved nutrients, but also food necessary for growth which they obtain by connecting their haustorium to both the host’s xylem and phloem.  All holoparasites are obligate parasites because they require a host to complete their life cycle.

When parasitic plants are discovered, you can usually prune the infected areas and destroy them to eliminate the threat.  Then reduce humidity around the treated area by thinning it or cutting limbs off neighboring plants to promote airflow.