10 common plant disease symptoms

Loss of Turgor Pressure (decrease in water pressure within plant cell walls)
Causes: abiotic factors, bacteria, fungi, nematodes, insects
Examples: Ralstonia solanacearum, Verticillium dahliae, Bursaphelenchus xylophilus

Photo credit: biology4isc.weebly.com


Enations (scaly leaflike structures that lack a vascular system)
Causes: viruses
Examples: Cherry raspberry leaf virus, Pea enation mosaic virus

Photo credit:  fftc.agnet.org


Stunting (dwarfing or loss of vigor)
Causes: viruses, phytoplasma, fungi, nematodes
Examples: Rhizoctonia, Strawberry lethal yellows

Photo credit: ucanr.edu/


Witches’ Broom (dense clustering of abnormally small twigs)
Causes: phytoplasma, fungi, insects, mites
Examples: Ash yellows phytoplasma, Sphaerotheca sp.

Photo credit: economist.com


Galls (abnormal growths)
Causes: fungi, nematodes, mites
Examples: Gymnosporangium sabinae, Meloidogyne spp.

Photo credit: isponature.com


Fruit and Seed Deformation (malformed fruit or seeds)
Causes: abiotic factors, viruses, bacteria, fungi, nematodes, insects
Examples: Catfacing tomatoes, Little cherry disease, Spiroplasma citri, Thrips palmi

Photo credit: eorganic.info


Tumors (aggregates of cells that have multiplied excessively)
Causes: viruses, bacteria
Examples: Agrobacterium tumefaciens, Cryphonectria parasitica

Photo credit: cals.ncsu.edu


Gummosis (oozing of sap from wounds/cankers on fruit trees)
Causes: abiotic factors, bacteria, fungi, insects, injury to the cutting sites
Examples: Pseudomonas syringa, Colletotrichum gloeosporioides

Photo credit: utahpests.usu.edu


Rot (decay)
Causes: bacteria, fungi
Examples: Arruina carotovora, Ralstonia solanacearum, Phomopsis obscurans, Botrytis cinerea

Photo credit: clemson.edu


Necrosis (death of cells or tissues)
Causes: abiotic factors, viruses, bacteria, fungi, nematodes, insects
Examples: Xanthomonas axonopodis, Fusarium oxysporum, Milbe Tetranychus cinnabarinus

Photo credit: srs.fs.usda.gov

For more information about plant diseases read:

A Brief Introduction to Plant Diseases


buffer zones & buffer strips – what they are and why we need them

photo credit: pubs.usgs

Buffer zones and buffer strips are the areas between aquatic and terrestrial zones.  The best-known buffer strips are wetlands and riparian zones.  They can consist of natural or planted vegetation and serve as a place for water and matter storage.  

The two types of limitations that impact buffer zones are internal limitations and external limitations.  Internal limitations are those that have to do with the qualities of the buffer zone itself e.g. the width, the soil qualities, the pH levels, the organic matter content and the soil porosity. The external limitations include outside influencing factors like the size of the basin, the geochemistry of a location, the climate, hydrology, slope and stream morphology.Some buffer zones can also link ephemeral (short-term) and perennial areas with non-point source loads via surface or groundwater paths.  

We need these unique natural treasures because they offer valuable services that man-made options and replacements simply cannot reproduce.  This means that existing buffer zones should be protected in an effort to benefit the majority, rather than the minority.  In areas that have buffer zones, the following benefits are enjoyed:

  • During warm periods, buffer zones cool in the summertime via evapotranspiration and shading
  • Many unique species of plants and animals have a place to live creating havens of biodiversity
  • Water is filtered water slowly through the dead and decomposing organic matter, as well as non-organic components
  • Sedimentation occurs which keeps water cleaner and reduces the likelihood that unwanted particles enter other water sources
  • Embankments are stabilized and coastlines are protected from storm and flood damage 
  • Groundwater recharge takes place which keeps aquifers full
  • Groundwater composition is changed as excess nitrogen and other nutrients/toxins are removed which improves water quality and reduces the need for artificial water filtration efforts
  • Litter and dead wood is allocated, concentrated and distributed to aquatic organisms
  • Carbon is sequestered

With so many positive benefits, it is clear that buffer zone protection is imperative.  This is especially true with the many environmental uncertainties that we are facing in the modern world making it essential to protect and preserve these valuable natural service providers.

photo credit: elibrary.dep.state.pa.gov


question: what is the difference between plant resistance and plant tolerance?

Plant tolerance is the characteristic of a plant that allows a plant to avoid, tolerate or recover from attacks from insects, among other things, under conditions that would typically cause a greater amount of injury to other plants of the same species. These inheritable characteristics are what influence the ultimate degree of damage caused by a pest. Tolerance in terms of agricultural production means that despite stress from a pest or disease, the production levels will remain above the economic threshold.

Resistance means that a plant completely immunizes itself from a particular stress.  This is typically a biotrophic pathogen infection. The host has a resistance gene which prevents the proliferation of the pathogen. The pathogen typically contains an avirulence gene which triggers plant immunity.

Resistance and tolerance are the best defense mechanisms of plants against pests. 

developing new plants that are resistant to freezing  photo credit: phys.org

There are two main types of resistance: ecological resistance/pseudo-resistance and genetic resistance. Ecological resistance is resistance related to favorable environmental conditions at a given location at a particular time. This type of resistance can be broken down into 3 forms:

  1. Host evasion – this phenomenon occurs when a host passes through the susceptible stages very quickly or during a period when pests are fewer.  This type of resistance applies to an entire species population.
  2. Induced resistance – this type of resistance is the result of some type of changed condition for the plant, such as an increase in available nutrients or water
  3. Escape – this is more or less luck as there is an absence of infestation or injury to a host plant as a result of incomplete infestation.

Genetic resistance is resistance related to (you guessed it) plant genetics.  There are several types of genetic resistance:

Resistance based on the number of genes

  • Monogenic – controlled by a single gene which makes it easy to both incorporate and exclude in plant breeding programs
  • Oligogenic – controlled by a few genes
  • Polygenic – controlled by several genes
  • Major Gene Resistance – controlled by one or a few major genes (vertical resistance) 
  • Minor Gene Resistance – controlled by many minor genes; the cumulative effect of minor genes is called adult/mature/field/horizontal resistance

Resistance based on biotype reaction

  • Vertical Resistance – specific resistance against specific biotypes
  • Horizontal Resistance – effective against all known biotypes; nonspecific resistance

Resistance based on miscellaneous factors

  • Cross-Resistance – when resistance against a primary pest results in resistance to a secondary pest
  • Multiple-Resistance – different environmental stresses (e.g. insects, diseases, nematodes, heat, drought, etc.) results in a new resistance

Resistance based on evolution

  • Sympatric Resistance – resistance that is acquired through the coevolution of a plant and insect (which is why it is important to protect our native pollinators!); governed by major genes
  • Allopatric Resistance – not governed by a co-evolution; governed by many genes

There are three mechanisms of resistance in plants.

Antixenosis (non-preference) resistance mechanisms are those by which host plant has characteristics that result in non-preference for insects in terms of shelter, oviposition, feeding, etc. There are morphological or chemical factors that influence insect behavior and results in the poor establishment of insects. Plant shape and color can also be an important influencing factor.

Antibiosis is the negative effect of a host plant on the biology of an insect. This can include decreased rates of survival, development and/or reproduction and is a result of biochemical and biophysical factors. Antibiosis may be a result of the presence of toxic substances, the absence of essential nutrients or a nutrient imbalance. Physical factors, such as thick cuticles, glandular hairs and silica deposits, also contribute to antibiosis.

Tolerance is a plant’s ability to grow and produce an acceptable yield despite a pest attack. Tolerance is typically attributed to plant vigor, regrowth of damaged tissue and a plant’s ability to produce additional stems/branches.

Both plant tolerance and plant resistance are extremely important to IPM efforts as it helps to reduce costs and amount of other control tactics that need to be implemented. This includes improving the efficacy of insecticides and promoting non-chemical benefits like shifts in predator-prey relationships and reduced pest populations.

The benefits of the use of resistant cultivars are numerous as this method has a high level of specificity, is eco-friendly, lower cost and adaptable to a given situation. The benefits are limited only by the amount of time required to develop new cultivars in breeding programs and by a lack of known resistance genes that can be used. To identify and capture relevant genes, crop wild relatives are often used.

a brief introduction to plant diseases

A disease, as it relates to plants, is a disturbance from plant pathogens or environmental factors that interfere with plant physiology.  When a disease is present, plants will express symptoms. Symptoms are the detectable expression of a disease, pest or environmental factor.  These symptoms are usually the result of complex physiological disturbances.  They result in changes to plant appearance, yield and/or quality.  Knowledge of plant diseases is essential because plant diseases:

  • Affect food quality in terms of safety to eat.  
  • Impact landscape value.
  • Alter the quantity, quality and diversity of both domesticated and wild crops.

The relationship between environmental, pathogen and host is a complicated one.  There are varying components of this relationship are explained by a “disease triangle”.  The original concept of the disease triangle was amended by Mazz who incorporated the influence of humans and time.

photo credit: finegardening.com

Environmental factors include soil and air temperature, soil type, pH-value, nutrient availability, precipitation humidity and moisture levels.  Factors relating to the environment can be influenced by the effects of human cultural practices, such as monoculture agriculture, rotation practices, the introduction of foreign pathogens via trade, the quality of seeds used and the amount of chemical ingredients greatly impacts the growth environment

Hosts are susceptible crops and cultivars.  All plants are considered hosts.  The age of a plant (developmental phase) affects disease development.  Each plant has a different level of susceptibility to different types of disease

Pathogens are viruses, viroids, bacteria, phytoplasma, fungi, pests (insects, nematodes, mammalian, etc.).  Their impact is largely dependent on the amount of inoculum, the genetics of the pathogen, the virulence of the pathogen and how the pathogen reproduces (monocyclic or polycyclic).  The ecology and mode of distribution (air, soil, seed or vector dependent) are also major influencing factors

Each infectious disease requires a series of sequential events for its development:

→ Dispersal of the pathogen to the host

→ Penetration and infection of the host

→ Invasion and colonization of the host

→ Reproduction of the pathogen

→ Pathogen survival between growing season and/or the continuous presence of a host

Viruses are not technically living organisms because they cannot reproduce without a host.  They are not fully understood, although efforts are (of course) being made to expand knowledge surrounding their biology.  It is possible for them to exist in surface water, soil, sewage sludge, ancient glaciers clouds, sea water, plants, etc.  In most cases, they are very stable and can infect a wide range of hosts, although some are host specific.  Upon infection, viruses change the mechanical production process of cells which turns the cells into production sources for new viruses.  The cell then bursts, releasing copies of the virus which then infect new cells.

tomato mosaic virus, photo credit: gardenaction.co.uk

Fungi are the largest group of plant pathogens with approximately 8,000 species.  They are characterized by their reproduction via spores and the production of threading hyphae (mycelium).  They can be transferred by wind and water, rhizomorphs and sclerotia.  Fungi infect via natural openings, wounds, natural openings (stomata, lenticels, hydathodes), intact surfaces (enzymatic) and during pollination.  

One of the most important fungi plant pathogens is the Banana Panama Disease that is caused by Fusarium oxyprorum f. Sp. cubense which affects >40% of all cavendish and 21% of all plantain varieties.

banana panama disease, photo credit: media.padil.gov.au/

Bacteria are prokaryotic microscopic organisms that are free-living cells that produce filamentous colonies.  The reproduce via binary fission.  The daughters are identical to the mothers and they require a host or growth medium to survive.  Bacteria infect via natural openings, non-cutinized parts of the plant (root hairs, nectaria) and through cell walls.

soybean bacterial blight, photo credit: agdev.anr.udel.edu/

Nematodes are the most numerous multi-cellular organisms on the planet.  They are, however, poorly understood.  They feed on plants, fungi, bacteria and even other nematodes.  Signs and symptoms of nematode infection are stunting and slow growth, wilting, yield reduction, a lack of response to various treatments (ex. fertilizer) and damage to roots systems e.g. lesions on the roots, galls at root tips or along roots and root tip stunting.  These symptoms are often mistaken for nutrient deficiencies.  As a result, treatment is often attempted when it is too late.  Plant feeding nematodes also cause mechanical injury to cells and tissues, result in cell death, modify cell function, disrupt the uptake of water and nutrients, alter photosynthesis partitioning, create new avenues of ingress and predispose plants to other diseases.  

Nematodes can be transferred via wind or water.  Infections from nematodes are very difficult to address.  The best options are crop rotation, strip cropping and solarization.  Areas that have higher moisture levels are more susceptible to infections.

damage from a root knot nematode, photo credit: infonet-biovision.org/

To reduce disease transmission and engage in effective management techniques, the following guidelines should be followed:

  • Choose resistant cultivars.
  • Engage is thoughtful cultural practices by avoiding activities that stimulate rapid tree growth which causes weak plants by avoiding excessive nitrogen application.
  • Prune infected tissue and prune during dormancy in order to reduce the possibility of disease transmission. Pruning should be completed at 30 – 40 cm below the infected area and should never be done when the plant is wet.  After pruning, burn the infected tissue.

Some interesting factoids:

  • The likelihood of infection during the embryo phase is almost 100% while infection risk during the seed phase is only approximately 2%.
  • Aphids are one of the number one transmitter of plant pathogens.  Ironically, they can be both host and vector.  
  • In 1 tsp of soil there are 100 million – 1 trillion bacteria, 6 – 9 ft of fungal strands and several thousand flagellates and amoebae.


the greenhouse effect

The term greenhouse effect has some pretty negative connotations.  On the one side, there are those who hate all the tree-hugging hippies who are against progress and technology, and think that global warming is a giant scam developed by Al Gore in order to ruin the United States’ economy and turn the country into a communist paradise.  On the other side, there are those that think that the evil meat-eating fascists are trying to destroy the planet and the only way to stop them is by buying all the coolest, newest, greenest, most environmentally-friendly items.

Photo Credit: wunderground.com

However, truth be told, if it were not for greenhouse gases trapping heat in the atmosphere, the world would be a very cold place.  In fact, greenhouse gases are what keeps the earth warm through a process called the greenhouse effect.

The earth gets energy from the sun in the form of sunlight.  The earth’s surface absorbs some of this energy and heats up.  This is why the surface of a road can feel hot even after the sun has gone down.  The earth cools down by giving off a different form of energy called infrared radiation.  Before this radiation can escape to outer space, greenhouse gases in the atmosphere absorb some of it, which in turn makes the atmosphere warmer.

However, it does not have to be quite so warm here on planet earth and it is best to mitigate the environmental effects of human activities when possible.  In order to do this, the discussion needs to shift to other types of greenhouse gases because CO2 is relatively harmless when compared with many others.  Furthermore, CO2 gases can be dramatically reduced simply by planting a whole lot more plants and not destroying the ones that already exist and/or ending the world’s love affair with fossil fuels.

Greenhouse Gas Production by Sector             Photo Credit: epa.gov

A few of the other types of much more concerning greenhouse gases include:

  1. Methane: produced via:
  • livestock production – sheep and cows produce methane as a byproduct of their digestion process and methane as released as manure decomposes
  • trash decomposition in landfills
  • sourcing and transport of natural gas – natural gas is mostly methane and can easily leak through pipes
  • coal mining – pockets of methane are released as the earth is mined

Methane stays in the atmosphere for 12 years and traps 20 times more heat than CO2.

2. Nitrous Oxide: produced via:

  • farming – the introduction of synthetic nitrogen which is oxidised by plants
  • the burning of fossil fuels
  • some manufacturing and industrial processes

Nitrous oxide stays in the atmosphere for 114 years and traps 298 times more heat than CO2.

3. Fluorinated gases: produced via:

  • leaking coolants – produced by certain devices, such as refrigerators and air conditioners
  • manufacturing and industry – computer chip production is a major contributor

The length of time that these gases stay in the atmosphere varies, but ranges from several to thousands of years.  The heat-trapping properties also vary but range from a few hundred to 23,000 times that of CO2.  It is expected that fluorinated gas production will increase dramatically faster than any other greenhouse gases.

So, the greenhouse effect is a cause for concern, but it also a fundamental component of our existence.  As such, it is best when we stop focusing on how to stop this process and start focusing on how we as humans can make more conscientious decisions in order to preserve this special place we live and let mother nature do her job.

8 essential questions to ask before installing an irrigation system

Photo credit: nrcs.usda.gov

Irrigation systems offer a variety of benefits.  They allow for the growth of a wider variety of crops.  They can be timed so that the hands-on portion of crop production is a little less cumbersome.  They protect crops from irregular and dry weather conditions.  They support leaching, which can remove harmful, crop-damaging salts within in the soil.  Crops that are supported by irrigation tend to be much more productive.

To reap said various benefits, several considerations must be made and an assortment of essential questions must be answere

1. Why do you want an irrigation system?

2. What type of soil do you have?

      Do you have sandy, loamy or clay?  It could also be (and often is) a combination of the           three.

Photo credit: salinitymanagement.org

3.  What is the amount and distribution of the precipitation?

  • When does the growing area receive the most rain?   
  • Is the rain equally distributed throughout the year?
  • Are there rainy and dry seasons?
  • How much precipitation does the area receive annually?
    • Arid: less than 200mm/year
    • Semi-Arid: less than 400mm/year
    • Humid: more than 1000mm/year
    • Semi-humid: more than 800mm/year

4.  What are the temperature ranges and averages?

Are there hot and cold seasons?
Are there temperature extremes?

5.  What is the climatic water balance?

How does water flow in and out of your environmental system?
Is there a renewable source of water that can support the irrigation system?  


6.  What are the soil conditions?

  • Water storage capacity/Available water capacity (AWC)
    • This is the range of available water that can be stored in soil and be available for
      growing crops

      • formula: (water content at field capacity) – (permanent wilting point)
  • Field capacity (FC)  
    • This is how much soil moisture or water content is retained in the water after the excess water has drained and the rate of downward movement (free drainage) has decreased.  This typically takes place 2 – 3 days following rain or irrigation (assuming the soil is pervious and uniform in structure and texture).
  • Soil depth
  • Humus content
    • Humus is composed of decayed organic matter (plant and animal).  As it decomposes, it provides plants with many nutrients required for their growth.
  • The slope of the growing area
Photo credit: agry.purdue.edu

7. What crops do you want to grow?

  • What are the water demands of the desired crops?
    • Are the crops perennial (trees, vineyards) or annual (tomatoes, beans)?
    • How do the chosen crops react to water stress and irrigation?
    • Will cover crops be used?
    • How and when will crops be rotated?

8. What water will be used for the irrigation system?

  • Is there renewable water (not old water)?
  • What type of water will be used?
    • Recycled, waste or fresh
  • In what condition is the water?
    • Salt levels
    • PH levels
    • Presence of heavy metals
    • Are there hygiene issues (disease, effluence)

Once these questions have been answered, the next step is deciding the type of irrigation system that will be used.  

See also:

agricultural irrigation

soil sealing: what it is and why it’s important

soil sealing2

Soil. A living, breathing fundamental component of the world.  Without it, we would not be able to grow food. There would be no trees and plants growing to provide us with the delicious oxygen so necessary to our existence. Our water would not be purified. We would more or less just be screwed. Yet, we continue to cover this valuable resource with impermeable materials like asphalt and concrete in an effort to build housing, roads, factories and parking lots.

Sure, we need those things too. There is no denying it. But, there are alternatives to impervious materials.They may cost a bit more, but how much exactly is clean water worth? To those without it, it is invaluable or at least much more valuable than a new parking lot. What is the value of a house that has not been destroyed by flooding? Certainly much less than one that is floating down a river towards the ocean in pieces because it was destroyed by flash flooding.

The main perpetrators of this crime against nature are suburban sprawl, a rapidly growing population, and increases in transportation demands. The rapid migration to urban areas is exacerbating this issue.

surface sealing

Some soils are naturally prone to sealing – like those in Southwest USA. However, this is often the result of  poor soil quality (issues with aggregation). There are also issues with soil sealing as a result of poor agricultural practices, such as driving large farm equipment over wet soils and leaving large tracts of land bare of vegetation which would typically improve the structure of the soil and mitigate issues created by rainfall.

Regardless of the source of sealing (although it is usually the fault of humans), the consequences of soil sealing are many including, but certainly not limited to:

  • Increased flood risks
  • Reduced groundwater recharge
  • Increased water pollution (caused by runoff)
  • Loss of biodiversity as a result of habitat fragmentation
  • Disrupted gas, water and energy fluxes

To deal with the issue of soil sealing, many steps can be taken.

The best option is to stop engaging in practices that lead to soil sealing. This means using land more efficiently and intelligently, as well as using existing infrastructure. However, we as humans sometimes have a difficult time changing our habits. This may require that alternatives to impervious concrete and asphalt be more widely used (as alternatives already exist.  see: http://www.perviouspavement.org).

In regards to agricultural causes, crop rotations should be employed, heavy machinery should not be used on wetlands, and cover crops should be planted to encourage aggregate formation and water absorption.

Should we as a species work to address the issue of soil sealing (or even take preventative measures), a myriad of negative consequences could be mitigated. Then maybe one less family will lose everything because of a flood, one less crop will be destroyed taking us one step closer to food security, and mother nature can continue to do her thing – something we can all appreciate (even if we don’t know it).