Common Symptoms of Biotic Plant Disease

In some cases, symptoms can be used to accurately identify a biotic plant disease.  In other cases, where symptoms are common across multiple diseases, other signs or tests (display, DNA, etc.) are needed to accurately identify the disease and pathogen which is causing it.

Leaf, Flower, or Fruit Spots

Spots may occur on flowers, leaves, or fruit.  They often start off yellow and then turn tan, brown, or black.  When they occur on leaves, the center of the spot can drop out leaving holes in the leaves.  Over time, spots will sometime kill the entire leaf or cause the entire fruit to rot. 

Blight

Blight is the rapid and complete browning or chlorosis and eventual death of plant tissues in leaves, flowers, fruit, and stems.   Blight is also used to refer to a disease that exhibits this condition as a symptom.  Blight is often caused by bacteria or fungi infecting the plant.  It often starts as lesions which rapidly kill the surrounding tissues.  Spots in advance stages can kill entire leaves, therefor exhibiting blight symptoms.

There are many types of blight.  For example:

  • Early blight of potato and tomato – Caused by species of fungi in the genus Alternaria
  • Late blight of potato – Caused by the water mold Phytophthora infestans
  • Chestnut blight – Caused by the fungus Cryphonectria parasitica
  • Fire Blight of pome fruits – Occurs in pears, apples, raspberries, and others and is caused by the bacterium Erwinia amylovora
  • Bur oak blight – Caused by the fungus Tubakia iowensis

Cankers

Cankers are primarily areas of dead bark and wood caused by fungal or bacterial pathogens entering the plant through natural or mand-made wounds, where the pathogens attack the cortical and water conducting tissues just below the bark.  While cankers occur primarily in woody plants, they can also occur in herbaceous plants.  They often occur when the plant is stressed from cold or drought, insect injury, nematodes, root rot, etc.

Once infected, the plant reacts by forming callus tissue or wound wood to seal off the infected area.  As the area of callus tissue dries out, the bark often wrinkles, sinks, swells, cracks, becomes discolored, or eventually dies.  The canker will often girdle or spread around the infected stem, branch, or trunk thus killing all the water conducting tissue causing the upper branches and terminal growth to die.

Cankers can be controlled by removing them during dry weather late in the dormant season, quickly treating injuries to bark and wood areas, keeping plants vigorous by using fertilizers, controlling insects and rodents that carry diseases, and by avoiding overwatering and overcrowding.

Wilts

Wilt is caused when fungal and bacterial pathogens infect the vascular system (xylem and phloem) disrupting the flow of water and nutrients to the various parts of the plant.  Fungal wilt diseases occur in both woody and herbaceous plants, while bacterial wilt diseases are most common in herbaceous plants.  Because the pathogens embed themselves in the host’s vascular tissue, external control measures are essentially eliminated as an option.  These types of disorders are some of the most serious, as herbaceous plants will often die by the end of the growing season and infected trees can sometimes die within a year.

Early symptoms of such disease include yellowing, wilting, or defoliation of a few small branches.  As it gets worse, symptoms spread into larger branches and more of them.

Galls

Galls are abnormal growths or swollen plant tissue on leaves, twigs, flowers, or roots.  These are typically cause by parasites like fungi, bacteria, viruses, and nematodes.  They can also be caused by irritation from insects or mites.  There is typically nothing you can do to cure galls other than remove the galled plant.

Rusts

Rusts are plant diseases caused by highly specialized pathogenic fungi from the order Pucciniales that usually appear on leaves, young shoots, and fruits as yellow, orange, red, rust, brown, or black powdery pustules.  Rusts number around 7000-8000 species, make up the largest group of Phytopathogens, and are obligate parasites (i.e. they require a living host).  They reduce plant growth and productivity, can cause stems and flowers part to swell and become distorted, and can cause leaves to wither and die earlier than expected.

Rusts have the most complex life cycles among fungi.  To complete their life cycle, many rusts require five different spore stages and often alternation between two different host plants that are taxonomically unrelated to one another.  Those that parasitize and complete their life cycle on a single plant host are known as autoecious rusts or monoecious rusts.  Those that require two distinct species of plant hosts to complete their life cycle are known as heteroecious rusts.

Rusts can be controlled with appropriate fungicides, by destroying alternate host plants (for heteroecious rusts), and by growing resistant varieties.

Smuts

Smuts are plant diseases caused by about 1,000 species of fungi related to rust fungi in the order Ustilaginales.  These fungal pathogens parasitize flowering plants (primarily grasses, sedges, and grain crops) where they produce galls or masses of black, powdery spores on the host plant called sori (singular is sorum) in place of seeds or grain. 

Smuts often enter embryos or seedling plants where they develop systemically and only appear externally when the plants near maturity.  Other smuts infect only areas of the plant with actively growing tissues.

Smuts can be controlled by growing resistant varieties and treating plant seeds with a fungicide.  Once detected, infected plants and plant parts should be destroyed prior to spores being released.

Damping Off

Damping off is a disease of seeds and seedlings caused by soil-borne fungi including Rhizoctonia, Pythium, and Fusarium though species of the soil fungus Pythium are often the cause.  Damping off leads to the rotting of stem and root tissues at or below the soil surface.  This often occurs when old seeds are planted in damp, cold soil, often with poor soil drainage.  High humidity and planting seeds too deep can also lead to damping off.

Pre-emergence damping off occurs when the seed or seedling begins to rot before it can reach the soil surface.  Post-emergence damping off occurs when young seedlings that have already emerged suddenly wilt, collapse and die because of rot that girdles the stem at the soil surface.  Older stems develop secondary stem tissue creating a protective barrier and limiting fungal penetration.

There is no cure for plants with damping off.  But much can be done to prevent it.  Plants should have god air circulation. Seeds can be treated with the fungicide Mycostop.  Potting soil can be sterilized in an oven, but a better solution is to use a light soilless growing mixture of peat, perlite, and vermiculite that provides fast drainage.  Seeds should be planted thinly to avoid overcrowding which often causes humid, moist conditions.  Avoid overwatering.

Root and Crown Rots

Root rot is a disease that infects roots of plants growing in wet soil that, if left untreated, will lead to rotting and decaying of roots and eventually death of the plant.  This may occur due to overwatering, because the soil is too dense to allow proper drainage, or, in the case of container plants, because the containers lack the appropriate number of drainage holes.

In addition to prolonged exposure to excess moisture, root rot can be caused by several species of fungal pathogens including Fusarium, Phytophthora, Pythium, and Rhizoctonia.  Fungal spores from these species are not only spread through the air but can also be spread by insects and other organisms in the soil.

It is often hard to detect until it is too late since the roots lie beneath the soil.  Instead of firm white roots, infected plants will have brown or black roots that are mushy and will break off when the plants are pulled from the soil.  Once this happens, the plant will no longer be able to absorb needed nutrients.  The lack of nutrients will then become apparent in the foliage which will begin to wilt, yellow, or fall off.  Growth of the plant is often stunted, and blooming may be delayed. Stems that make up the crown of the plant where the roots meet the stem may also start to rot.

Once root rot and crown rot have been detected, it is typically too late to effectively treat it.  Therefore, focus should be placed on prevention of the disease.

Viral Diseases

Viral diseases in plants are the result of intracellular pathogenic particles that live inside the cells of the plant.  These viral pathogens are obligate parasites lacking the molecular machinery to replicate themselves without a host.

Plant viruses are transmitted naturally from one plant to another primarily via vectors, usually in the form of insects but also via soil-borne nematodes or through infected seeds or pollen of the host plant.  They can also be transmitted through cuttings, since infected plants typically have the virus inside of most of its cells except in the areas of new growth.  In tissue culturing, healthy cells from the area of new growth are removed and grown into new, virus-free plants.

Some viruses are only viable for transmission by insects for a few hours after being acquired by an insect vector and are called nonpersistent viruses.  Other are viable for transmission for days after being acquired by an insect vector and are called persistent viruses.

Viral infections of plants are often hard to identify.  Their symptoms differ depending on the pathogen, the species of plant being infected, as well as conditions under which they are grown.  They can often be confused with symptoms caused by extreme heat, fungal infections, insect infestation, herbicides, and deficiencies and excesses of certain minerals.

Viral diseases do not exhibit any signs because they exist inside the cells of plants and require extreme magnification to see them.  Several common viral disease symptoms include stunted growth, light and dark green or yellow and green mosaic leaf patterns, crinkled leaves, yellowing of entire leaves, and yellow streaking or spotting on leaves.  It is obvious that these are all symptoms of many diseases, and for this reason it is impossible to diagnose viral diseases based on symptoms alone.  Laboratory analysis and testing is required to confirm such diagnoses.