It can be tricky distinguishing between plant diseases. Many diseases have similar symptoms. And some of those symptoms can occur in both abiotic or biotic diseases. Abiotic diseases are caused by things other than living organisms. Nutrient deficiencies, too much or too little water, air pollution or chemical injury can all cause sick plants. Pathogens — bacteria, fungi, viruses — also cause disease. Parasitic nematodes, which are a type of roundworm, are also plant pathogens.
Insects and other pests play a role in plant disease, too. They can vector the disease, passing it along when they chew on plants, or the damage they do to plants can serve as an entry point for disease pathogens, weakening the plant making it more susceptible to abiotic issues.
The National Plant Diagnostic Network (NPDN), formed in 2002, has been training “first detectors” to help spot and identify plant diseases and to communicate disease information accurately and quickly. Over 11,000 first detectors, including farmers, extension agents, foresters, food processors and more, have been trained. There are five NPDN regions, with diagnostic laboratories in each state.
NPDN first detector training information, compiled by Richard Bostock, Carla Thomas and Richard Hoenisch, University of California-Davis, offers guidelines for plant disease diagnosis.
Abiotic disease does not typically progress over time, although nutritional deficiencies will show some advancing symptoms, but do so slowly, as the plants mature. If all plants in an area show symptoms which were noticed simultaneously, an abiotic cause is likely, according to NPDN information.
Environmental conditions, cultural practices and the growing environment all impact disease susceptibility. If plants of many types are affected, rather than just one or two cultivars of a given vegetable, abiotic concerns should be addressed. Many biotic diseases are host-specific and have specific environmental requirements, so widespread symptoms across crops might indicate abiotic concerns, rather than pathogen presence.
Biotic disease will progress within the plant, and will also spread to other plants in if conditions are favorable for the pathogen to thrive. If symptoms appear on some plants but not others, or appear at differing times, a biotic cause is more likely the disease agent.
Water is often a conduit for plant disease. Too much or too little water is obviously an abiotic cause of plant disease. Water may also contain excessive minerals or even chemical contamination which would cause abiotic disease.
Overly wet conditions often lead to pathogenic infections, too. The water source itself could be contaminated with disease-causing pathogens, or the water could be carrying pathogens from a contaminated field into an otherwise clean field, spreading biotic diseases in runoff.
Signs and symptoms of biotic disease
Signs of disease are actual pathogen or pathogen by-products found on or within the plants. Fungal spores or mycelia, rust, powdery mildew and white mold are signs of fungal pathogen presence. Bacterial ooze or water-soaked lesions are signs of pathogenic activity. Viruses do not leave behind visible signs of their presence.
Symptoms arise from the plant itself in response to disease-causing factors, and indicate that the plant is in decline. Each type of pathogen typically causes certain types of plant symptoms, although there is overlap, and distinguishing between diseases with similar symptoms can be difficult. And abiotic disease factors can cause symptoms similar to biotic disease, including wilting, yellowing of leaves or stunted growth.
Leaf spots, chlorosis or damping off are typical symptoms of fungal disease. But leaf spots can also indicate bacterial disease. Crinkled leaves and stunted plants are indicative of viral pathogens, and viruses can also cause chlorosis. Identifying the correct pathogen and treating the disease you have, not a look-alike, is critical.
Some diseases typically cause initial symptoms to appear in one part of a plant. As infected plants mature, existing symptoms can become exacerbated, or new symptoms can occur. If environmental conditions no longer favor pathogen survival or reproduction, disease symptoms may be slowed or halted. And unhealthy plants are susceptible to secondary infections, making diagnosis more difficult.
Typical symptoms of plant disease
Identifying the symptom, as well as the part or parts of the plant in which it occurs, is a first step in disease diagnosis. Root deformity, vascular system browning or leaf symptoms at certain stages of maturity can give clues to the pathogen responsible. NPDN training teaches typical plant symptoms, classifying them into categories of: Over- or under-development; necrosis or death; alternation of normal appearance; and wilting.
Overdevelopment can be characterized by galls or blisters, or abundant tissue growth, all indicative of excess growth not normal to the plant. Underdevelopment includes stunting, shortening between nodes and a decrease in the production of chlorophyll. Tissue necrosis includes blight symptoms, cankers, dieback, leaf spots or fruit rots. Wilting of the plant, causing all or part of the plant to collapse, can indicate root or tissue damage, damage to the stem or crown from injury, or even weather-related issues. The alternation of appearance of a plant can be seen as mosaic patterns on leaves, yellowing of leaves or stems, curling or rolling of leaves, or discoloration of fruit.
But none of the symptoms points directly to any specific causal agent. More than one pathogen or abiotic issue can cause common symptoms. Signs of pathogenic activity are obvious clues to the culprit, but often aren’t present or readily discernible. To complicate matters, host plants — say one cultivar over another — can vary in disease tolerance and expression, so some pepper plants, for example, might be impacted, but not another cultivar in the adjoining rows.
Laboratory testing, to identify pathogens, may be necessary. Culturing, electron microscopy and molecular, enzymatic activity and serological testing are often used to diagnose disease and identify the causative agent.
When testing for abiotic concerns, pH and nutrient analysis, soluble salts and chemical analysis can be detected in soil samples. Tissue samples can be tested for chemical and nutrient analysis as well.
Practicing best management practices to prevent the spread of disease — crop rotation, cleaning up crop residue, sanitizing equipment, testing irrigation water and planting clean seeds or transplants — is the first step in disease prevention. Knowing which diseases commonly affect different crops, and using resistant cultivars as needed, are other steps in disease protection.
Accurate and early identification of crop diseases can prevent further loss when signs and symptoms of disease do appear. Knowing what to look for can help pinpoint the causative agents of plant disease as rapidly as possible.
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