Biotic Diseases and What to do about them

Biotic refers to “living” as opposed to non-living which is referred to as abiotic.  Biotic diseases are pathogenic, infectious and can spread from plant-to-plant, generally of the same species.  They are caused by organisms such as fungi, bacteria, viruses, nematodes and mycoplasma-like organisms (MLOs).  (Please don’t ask me to explain MLOs.)   Fungal diseases are the most common but Iwant to stress that not all fungi cause diseases.  Most organisms are necessary and vital constituents in our environment. 

When clients call about a problem with their trees or other plants, they often do not know whether it is an insect or disease issue and many times do not consider abiotic issues at all.  However, many diseases actually stem from abiotic disorders as pathogens take advantage of a weakened or damaged plant. 

Diseases require three circumstances in order to develop:  1) A pathogen has to be present, 2) a susceptible host has to be available and 3) the suitable environment has to exist.  If any one of those criteria is not present, the disease will not develop. This is called the disease triangle. 

Many organisms exist in a non-pathogenic state performing necessary functions in the environment, only becoming pathogenic when a susceptible host and suitable environment coincide.  Trying to manage diseases by eradicating the pathogen is seldom (if ever) feasible or even desirable.  Instead, reduce the potential of a susceptible host or disrupt the suitable environment.

A plant with susceptibility to a disease becomes the host when the pathogen and suitable environment are present.  A number of plants have been specifically developed to resist common diseases.   Determine what are common and damaging diseases in your area and search out resistant cultivars of plants. 

An example would be many apple and crabapple trees are susceptible to four primary diseases:  fire blight, apple scab, powdery mildew and rust diseases.  There have been many cultivars of these popular plants developed with varying degrees of resistance to one or all of these common diseases.  Instead of fighting a likely problem, minimize the potential by selecting one of these resistant cultivars.

  

A suitable environment is when the temperature and moisture are conducive for disease development.  And it varies from disease-to-disease.  We are somewhat at the mercy of Mother Nature here.  Cool, wet springs are going to be very suitable for certain diseases such as anthracnose. 

Many diseases are spread by wind and/or rain.  However, when you consider that water, not just in the form of rain, but irrigation practices as well, can create a suitable environment, then managing irrigation practices may go a long way to minimizing the potential for diseases being able to dock.

Knowing a disease’s life cycle may give a clue as to whether you should overhead water or not.  Some diseases are dispersed by rain or splashing water and, therefore, you would want to avoid overhead sprinklers.  However, some, for example powdery mildew, do not dock on wet surfaces. In that case, overhead watering susceptible plants, such as bee balm, Monarda spp, and crabapple trees, Malus spp, may be a good idea.  (Note: there are actually a tremendous number of plants susceptible to powdery mildew).   You need to know when the appropriate time of day to overhead water….and that may vary area to area.  Here in Western Montana, I have been successful at controlling powdery mildew by overhead watering in the early afternoon.  This timing may not work for you.   Note:  if a plant susceptible to powdery mildew is surrounded by plants susceptible to other fungal diseases that are enhanced by overhead watering, well, you have to pick your battles.

Some fungal pathogens thrive in a crowded environment which blocks air flow.  Judicious pruning that opens up the canopy may reduce the likelihood of the pathogen developing.

Pruning out diseased portions of a plant is often very effective at controlling certain diseases.  It is recommended to dispose of the debris off site.  It is also often recommended to prune fungal or bacterial diseased tissue from a plant during a dry period.  

Determining the most likely pathogens in your area, the environmental conditions under which they thrive and their life cycle combined with an inventory of your species listing their individual susceptibilities will go a long way in helping you devise a plan allowing you to minimize potential problems.  Nipping the problem in the bud….so to speak.