Plant Pathology  

What better way to spend time on a cloudy, greyish October or November day than  reviewing a little  basic plant pathology - well, maybe thinking about  what EPA's latest predicted 7°F  global temperature rise  means for our species competes... Anyway, let's get started.

Bacteria canker. Now present on Long Island.
Photo: Cornell U.

What Indentifies a Sick Plant

There are many  sympthoms that backyard gardeners see; and just to keep us on our toes,  any given symptom can probably also be associtated with multiple  conditions.  Usually the symptons we  see on our plants are  caused by some kind of pathogen - a biological agent that  inteferes with a plant's basic  activities like transpiration, photosynthesis etc. But they can also be caused by environmental (i.e. abiotic) factors such as too much or too little of some type of nutrient, air pollution or just poor growing conditions [heat, cold, lack of sunlight, too little water, too much water etc].

Some of the most common symptoms of   plant diseases include: 

• Abnormal celluar growth - examples include motteled leaf coloration, an over  abundance of root hairs, galls (tissue enlargements on a plant's stems, leaves or roots), malformed petals etc.. 

• Cankers - a plant's response to some type of physical damage. The damage can allow fungal or bacterial infections to enter the plant before healing is completed. A good example of a canker is the smooth rounded bark that a tree grows trying to cover up  damage from a broken branch. 

• Chlorosis - yellow spots on leaves where chlorophyll  has been destroyed by a virus.

• Damping off - a seemingly sudden callapse and death of seedlings and young plants.  High moisture levels trigger various fungal diseases that overwhelm a stressed plant.

• Defoliation (leaf loss) - Innumerical causes: transplanting stress, climatic factors, pests and pathogens.

• Fruit drop - Commonly triggered by environmental conditions, but pests can also cause loss, and sometimes the plant itself maybe just self correcting  from having set too many fruit.

• Necrosis -  dead areas [not little tiny  spots] on leaves, stems etc. Typicaly caused by a bacteria.

Brown discoloration from root rot.
Photo: U Wisconsin Extension.

• Root rot - Commonly associated with house plants that get over loved and over watered.  It can happen out in the garden in very wet weather too. Too much moisture creates the conditions for several different fungi to activate. The plants may appear to wilt, turn yellow or be stunted. If you pull one up, you will probably see that the roots look very, very brown. Just get rid of such plants. Unfortunately, these  fungal spores can stay dormant in soil for a long time waiting for the right environmental conditions to reappear.

Grey mold on strawberry. Photo: OSU

• Fruit rot - Frequently called "molds" these are generally fungal infestations although some are bacterial. A very common one is "grey mold,"  Boytrytis cinerea, that affects many backyard favorites: tomatoes, cucumbers, peppers -- 200 plus in all. If you have seen clusters of grapes that appear  grown together and are covered with a brownish web, blame  grey mold.  To make matters worse, grey mold can affect different parts of a plant and appear different on different parts of it. Symptoms also can be different at different stages of plant growth.  

• Stunting  - A lot of environmental factors can affect plant growth, but sometimes  biotic sources, as will be noted below,  are to blame.

• Wilting - Lack of water, of course, is probably the most common cause, but over watering can also lead to wilt as the plant fails to get both moisture and nutrients because the roots are suffocating.   The drowned roots allow phytopthoria, a fungus, to get established; and the plant dies from root rot.   Just remember that there are  other fungal  and bacterial infections that can  also block water from reaching the rest of the plant causing it to wilt.

And now for a quick review of the nasty (but interesting) culprits responsible to creating the sympthoms...

Common Pathological Agents 

Bacterial leaf spot on peppers

• Bacteria - These are simple, single cell organisms without a recognizable nucleus. Bacteria usually  enter a plant by means of a wound, e.g. an insect piercing or stem breakage.  Different types of bacterial infections can cause galls, cankers, wilts  and rots; but a very common indicator of a very common type of a bacterial infection is the appearance of  leaf spots. Bacteial leaf spots first appear as   discoloration in  leaf tissue between a leaf's veins. Affected areas  appear dark and damp when held up to a light; eventually the tissue dies and  might even become smelly. Unlike viruses that enter and take over plant cells, bacteria stay in the tissue area between cells and absorb nutrients needed by the cells.There is also another class of bacteria, the phytoplasmas and spiroplasmas that infect plants.   Originally these were considered to be viruses, but  they are now viewed as bacterial.  Like regular bacteria, they lack a nucleus and, interestingly, they also lack a distinct cell wall. Because the 'plasmas'  and "spiros" congregate in a plant's phloem, they interfere with  nutrient transport throughout the plant.     Managing the risk posed by  bacteria involves selecting resistant plants, spraying for insect carriers, careful watering etc.  

• Fungi - Constituting a kingdom separate from those of animals or plants,  fungi derive their nutrients from other biotic sources and experience most of their [limited] mobility  by just growing in the environment they happen to occupy as well as occasional adventurous dispersals by sporing. Like other biotic entities, fungi have both beneficial and harmful actors; but fungal agents account for about 85% of all plant diseases according to MSU's Extension Service.  Most fungi are cylindrical in shape and grow at their tips by developing hyphae - thread-like structures that  can branch.  Hyphae have great penetrating ability, and it is through them that fungi  exude digestive chemicals whose resulting products are absorbed by the fungi.  For backyard gardeners, powdery mildew  and downy mildew along with rusts on apples and quinces (to say nothing of black spot on roses) might be the most commonly encountered  problems. Anthracnose is the another biggy - triggered by cool wet weather.  It shows up on  trees, shrubs and veggies by stunting young plants and leaves in the spring or spoiling fruit  and spotting leaves in the fall.  On the other hand, homebrewers and bakers are grateful for  the beneficial fungi that cause fermentation  and make bread rise. Control of harmful fungi requires selection of resistant plant varieties along with good gardening practices to remove infected plants or plant parts.  Sulphur can be employed as an effective fungicide; and, as a natural product, it  is permited in organic practices.

Tobacco Mosaic Virus on Tomato.
Source: American Phytopathological Society

• Viruses -  These are insidious, rogue pieces of DNA or RNA with only a surrounding protein coat that  only reproduce by injecting parts of themselves into other cells and using those cells to duplicate themselves. They are usually about l00x smaller than bacteria, and to complete the grim picture there are even viral agents called viroids that are smaller than your standard virus. Viroids lack the protein coat of their bigger cousins, and they seem to only exist on plants. All home gardeners have probably encountered the most common  viral infection, tobacco mosaic virus, that can infect many different plants.  Symptoms are variable -varying with the plant variety,  its age, the growing conditions and the viral strain. Leaves can be yellow, growth can be stunted, fruit can be miscolored -- to name just a few.  Control mainly consists trying to select resistant varieties combined with the removal and destruction of infected plants. In controlled environments such as greenhouses, sometimes contaminated soil is removed, and occasionally  young plants are even innoculated with known weak viral strains to spur natural resistance.  Giving asprin and making the plants go to bed doesn't work; but watering at soil level instead of spraying with a hose can minimize viral  diseases spreading between plants.

Nemotode cysts on potato roots.
Photo: Xiaohong Wang - USDA ARS

• Nematodes - Most backyard gardeners probably don't think about nematodes at all or just think that they mainly cause  root-knot on their cabbages. Nematodes are small to very small round worms; they are not earthworms. Nematodes usually range in size from .1 mm to 2.5 mm, but a few do reach 5 cm.  They exist in all environments. Half of them are parasitic; and as a life form, nematodes probably comprise about half of all the animals on and in the earth and sea! For humans, trichinosis is perhaps the  most familiar disease that is associated with nematodes.   In addition to root-knot, other notably harmful nematodes affect grapes by introducing a deadly virus, while another bad guy is the golden nemotode [Globodera rostochiensis] that affects  solanaceae (tomatoes, potatoes etc).  But, some nematodes are beneficial such as the ones that prey upon cutworms and corn ear worms.  There are various approaches to controlling nematodal damage.   You can plant marigolds, or you can even try steaming your soil. But, there are also a couple of other natural approaches. One involves chitosan, a type of polysaccharide that is derived from the shells of crustacians.   It  enhances both a plant's growth and resistance to infestations; the other  approach involves a friendly fungus, Clonostachys rosea f. rosea, also known as  Gliocladium roseum, that acts as a natural control.  If you really want to get into the detail of G. roseum, start with  this  journal article  by John C. Sutton in Plant Research.

Source: Missouri Botanical Garden

• Parasitic plants - Ever hear of "dodders?" I know about misteltoe; but, I've not experienced, or at least not recognized, dodders  attacking my vegetables - until now. Dodders are native to North America;   and, unfortunately,  they have become one of our really big unwanted exports to the rest of the world.   Field dodder (Cuscuta campestris)  and swamp dodder (Cuscuta gronovil) are two of the common ones you might encounter. They can seriously affect your carrots, tomatoes etc. Backyard gardeners can mostly control them by pulling, but dodders are much more challenging for farmers.  If a dodder finds your tomato, the vine will wrap itself around the stem, drop its leaves and live off of your plant by inserting haustoria (little root-like growths) into the tomatoe's tissue. Dodders are fast growing and  colored between yellow and orange. More detailed information on these beasts can be found on this data sheet from the Invasive Species Compendium of CABI (Center for Agriculgtural and Biosciences International).

That's all for now folks, but some of these fellas are so interesting that we will probably provide more details about a few of them during the upcoming dreary months...

And remember:

"Don't judge each day by the harvest you reap, but by the seeds that you plant." - Robert Louis Stevenson