Even if you aren’t an expert in plant pathology, you can usually tell if a plant looks “sick”. Yellowing leaves, spots, wilting or even oddly-shaped flowers are all signs the plant isn’t doing well.
Can you diagnose plant failure by yourself? For most situations, the answer is “Yes”.
What to look for. Plant pathologists usually group causes of plant failure into two categories: infectious (caused by a pathogen(s), or noninfectious (caused by environmental stress or improper cultural practices). As a general rule, noninfectious disease symptoms are distributed evenly over a large area or over several different plant species. You can sometimes associate the symptoms with some cultural practice, such as where herbicide applications were prepared or with some environmental events such as a frost or a hail storm. Infectious diseases, conversely, will develop sporadically or be distributed unevenly and are usually restricted to a particular plant species or cultivar.
While plant diseases may be caused by environmental factors, viruses, bacteria or other causes, by far the majority (more than 3/4) of plant diseases are caused by fungi. Fungal diseases are the athlete’s foot of the plant world. You can take action to prevent them from spreading, but at best you are practicing a form of prevention by spraying fungicide as most must be applied before infectious agents arrive.
Fungal diseases appear on leaf surfaces as discolorations such as black, white or rust-colored spots or as powdery or moldy growths. Bacterial diseases can cause similar symptoms, but generally kill plants rather quickly. Fungal diseases tend to cause a slow death. The best defense against fungal diseases is prevention—which we will address farther on in this article.
Viral diseases of plants are sub-microscopic, infectious particles that multiply only inside living host cells. Viruses are for the most part beyond the resolution capabilities of a light microscope and have to be determined primarily by electron microscopy and x-ray defraction. Symptoms along with other criteria are used to identify virus diseases. Symptoms would include abnormal leaf color, abnormal vein patterns of leaves, mottling in leaves, spotting patterns in leaves, and abnormal leaf shape as well as abnormalities of flower color, fruit size, shape and color.
Viruses can be spread from plant to plant by several means, including transmission from the parent plant to an offspring through the genetic structure of the plant or through vegetative propagation, grafting and budding, seed transmission and mechanical spread by insects and man. No effective chemicals will control virus diseases. Sanitation and use of resistant varieties of plants has been the most effective means of controlling plant viruses. Loss from disease can be reduced by removing diseased plants and replacing them with healthy replants. Reducing the population of insect vectors by insecticides or by other means, such as elimination of host plants for the insects has given partial control of virus diseases.
Bacterial diseases are microscopic, single-celled organisms that have a cell wall. While most bacteria in the environment are beneficial, several are able to cause leave spots, stem rots, root rots, galls, wilts, blights, and cankers.
Plant pathogenic bacteria generally survive in infected plants, in debris from infected plants, and, in some cases, in infested soil Most require a wound or natural opening in the plant to gain entry and require warm, moist conditions in order to cause disease. In most bacterial diseases, photosynthesis and respiration are severely altered to the detriment of the plant.
Bacteria reproduce very rapidly. They are splashed easily from the soil to the leaves and from leaf to leaf by overhead irrigation. They are also easily moved from soil or debris when a worker handles such material and then handles the live plant.
The strict sanitation practices required to control bacterial diseases include destruction of infected plants as well as cleaning and disinfecting tools, benches, flats and pots that are used repeatedly. Overhead irrigation should not be used in crops particularly susceptible to bacterial diseases.
Bacteria are normally present on plant surfaces and will only cause problems when conditions are favorable for their growth and multiplication. These conditions include high humidity, crowding and poor air circulation around plants. Too much, too little or irregular watering can put plants under stress and may predispose them to bacterial infection. Other stressors are low light intensity, fluctuating temperatures, poor soil drainage and deficient or excess nutrients. Once bacterial disease begins on the plants, chemical control is not effective.
Prevention is by and far the best defense against plant disease. Plant diseases are best stopped before they start.
Following are some key ways to prevent diseases before they start:
1. Have a good understanding of infectious diseases as outlined above. Diseases occur only when the following three components are present at the same time:
A. A susceptible host plant.
B. A pathogen capable of causing disease.
C. An environment conducive to disease.
Break the links in the triangle at any point and disease will not be as likely to occur. Ways to accomplish this are: plant a disease-resistant cultivar therefore removing a susceptible host and avoiding infection, clean up black-spotted rose leaves in the fall reducing the amount of black spot fungal inoculum, and adopt cultural practices that make the environment less conducive to infection. By eliminating overhead watering or limiting overhead watering to early morning so that plants can dry off quickly, you can discourage infection by foliar pathogens that thrive on damp leaves.
2. Select the right plant for the site you intend to use it in.
A. Read the site. Observe the site at different times to understand the sun and wind exposures.
B. Test the site. Don’t guess—soil test.
C. Know your plants. Find out how the varieties that you want to plant tolerate sun, wind, and various soil conditions.
D. Find out about common problems. Get a handles on the diseases of the plants you want to grow.
3. Use disease-resistant varieties. Selecting plants with genetic disease resistance is the best way to prevent disease. No plant will be completely disease-free, but some plants have fewer major disease problems. Get information from your local garden center or the Extension Service in your state.
4. Keep a clean garden: rouging, rotating crops, and sanitizing tools. Eliminating the pathogen that can cause disease is a time-honored approach to disease prevention. Sanitation for disease control is just as important today. An extreme form of sanitation is called rouging: removing infested plants as soon as they are noted. Rotating crops is another ways to prevent disease. Many soilborne pathogens remain in the ground for a considerable time, often for many years. Sanitize tools with alcohol between pruning cuts. Clean and remove soil from tools periodically.
5. Create a well-balanced soil. Soils with good texture and good organic matter content tend to have a healthy balance of soil organisms, from earthworms to bacterial and fungal microorganisms that cycle nutrients. Developing soils that have good overall texture and drainage characteristics and good organic matter content, which will optimize microbial activity. Adding compost to your soil will help you achieve these goals.
6. Keep plants healthy: proper watering, mulching, pruning and fertilizing.
You can limit the amount of stress your plants suffer and the likelihood that they will develop certain infectious and non-infectious diseases. Proper planting should be one of the first considerations. Once plants are installed, proper watering is crucial. Plants have individual needs that depend on the species, the size of the plant, and its stage of development. Soil type and weather conditions also come into play. Too little water can stress plants and excessive watering results in poor overall root function and greater susceptibility to root rot diseases. Many plants, including your lawn, need about one inch of moisture per week during the growing season.
Mulching is a crucial disease prevention practice. Mulches help cool soils and conserve and moderate moisture over long periods, which can help plants resist stress due to lack of water.
Pruning is another important disease preventive assuring better air movement and sun penetration of the canopy. Most fungi thrive under moist conditions, and fungal spores are more likely to germinate and infect leaf tissue when leaves remain wet.
Fertilizing plants will both prevent non-infectious diseases (disorders), such as nutrient deficiencies, and help plants tolerate the damage resulting from infectious diseases.
7. Don’t overwater, overmulch, etc. . .
You can overdo many good horticultural practices such as mulching. Too much mulch piled against the base of trees and shrubs will keep too much moisture against the trunks, encouraging fungal infections. Excessive mulch can reduce oxygen levels in the soil, leading to poor root metabolism and increased root rot disease. Add only enough to maintain a total depth of 2-3 inches.
Vigorous overwating is a common trigger of plant disease. Waterlogged soils cause root function to suffer because oxygen levels are low.
Overfertilizing is another case of too much of a good thing. Roots can suffer from overly high levels of fertilizer: they will lose water through osmosis when fertilizer salt concentrations are higher in the surrounding soil-causing “fertilizer burn” on roots.
8. Learn to tolerate some diseases
For instance, Lilac are infected by powdery mildew year after year, but it never seems to hurt the plant.
9. Fungicides are preventative not curative
Even when you do decide to use fungicides, prevention is still the key. All but a few fungicides must be applied before the fungus enters plant tissue to prevent infections and subsequent disease development. Timing is crucial: even with fungicides, prevention– not cure– is the name of the game.
