Plant diseases can result from a combination of many factors. Under certain conditions, viruses, nematodes, bacteria, fungi, heat, cold, chemicals and air pollution can all promote plant disease. By creating conditions that encourage plant vigor, losses due to disease can be minimized.
- When diseases begin early in the season, yield and quality are more likely to be reduced. Always use disease-free seed, transplants, or cuttings.
- Use hot water and/or fungicide-treated seed where appropriate. Hot water treatment may prime the seeds to sprout; treatment should be done as close to planting as possible.
- Pasteurize seedbed soil and potting soils with steam or chemicals. Unopened bags of soilless media do not have to be treated.
- Disinfect pots, flats, tools and greenhouse surfaces before planting.
- Avoid the reintroduction of disease-causing organisms to disinfected soil.
- Do not let pathogens get a head start by practicing good sanitation. Destroy plant debris, trash piles, and weeds that harbor pathogens. Remove and destroy infected plants and plant parts when feasible. Practice crop rotation.
- Promote plant vigor. Do not overwater or plant in poorly drained soils. Use fertilizers, growth regulators, herbicides, insecticides, and fungicides only as directed. Measure them accurately and apply them properly to avoid toxicity to the plants and to yourself.
- Some healthy plants can succumb to disease. Select varieties that are resistant to the diseases that you know are a problem in your area.
- Purchase plants from a reputable source and protect plants from pathogens by following a suitable spray program. Knowing the diseases of your crop and the most effective time to begin spraying can save a crop and your money.
- Air-blast sprayers can increase the spread of bacterial diseases.
- Effective control of plant diseases depends upon the accurate identification of the disease. Contact your regional or state Extension specialist or plant disease clinic (see Specimens for Disease Diagnosis below).
Ideally, seed should be custom treated by request. If this is not possible, seed can be hot-water treated at home. Some lots of seed can be vulnerable to heat treatment. Always treat a small amount of seed (50-100) of each lot before treating the remainder of the lot. After the test treatment, air dry completely and then moisten a sample for a germination test. Include untreated seed of the same lot for comparison. Treated seed should be used in the current season. Small seeded crops such as Brassicas, carrot, pepper, etc. are the most appropriate for hot water treatment. Each seed type has a corresponding temperature and length of time for treatment. Full instructions for treating seed on your own may be found here: https://ag.umass.edu/vegetable/fact-sheets/hot-water-seed-treatment. Always check with the seed source to make sure it has not already been treated.
Specimens for Disease Diagnosis
Effective control of plant disease depends upon the accurate identification of the cause. Accurate and rapid diagnosis of a plant disease by diagnostic labs requires examination of specimens that are representative of the disease, plus a review of information concerning the growing of the crop. The diagnosis and recommendations reported to growers are based on this information. Plants in advanced stages of decay or desiccation, or those that arrive with no case history information, cannot be diagnosed properly. Before sending the specimen, contact your regional or state Extension specialist or plant disease clinic. Deliver the specimen by overnight mail. Some states may charge a fee for diagnostic services. Supply as much of the following information as possible:
- Your name, address, zip code, phone number and e-mail address.
- Crop, cultivar, and date collected.
- Planting size (acres) or number of plants.
- Percentage of plants diseased.
- Distribution of disease (general or all over field, scattered here and there, or localized in a small area of field).
- Symptoms you are concerned about (blight, wilt, stunting, death, yellowing, leaf mottle, stem rot, root rot, fruit rot, leaf spot, die back, etc.).
- Chemicals applied, including dates and rates of fertilizers, fungicides, nematicides, herbicides, and growth regulators.
- Any information that you believe may be important about the circumstances leading to the disease.
Many different viruses can infect vegetable crops. Some, like Papaya Ringspot Virus-W, have a narrow host range, while others, like Cucumber Mosaic Virus and Tomato Spotted Wilt Virus, infect a wide variety of vegetable crops as well as ornamentals and weeds. Symptoms of viral infection are most evident on foliage and fruit. However, the symptoms are not always unique to viruses and may closely resemble nutritional disorders, herbicide injury, or insect feeding. A subtle but common symptom of viral infection is overall stunting and reduction in yield.
Viruses are spread in a variety of ways. Mechanical transmission through handling of plants or use of contaminated tools is an efficient means of spreading Tobacco Mosaic Virus and Potato Virus X. Most viruses, however, are not spread in this manner. Insects such as aphids, thrips, mites, leafhoppers, and beetles provide the most important means for viruses to move from infected to healthy plants. Some viruses, such as Tomato Ringspot Virus and Squash Mosaic Virus, can be transmitted through infected seed. Perennial weeds and ornamental hosts provide an important reservoir for viruses to survive from one season to the next.
Aphids are the most important primary and secondary vectors of viral diseases. Depending on their relationship with the aphid, viruses are classified as either persistent or non-persistent. Intermediate forms also exist. Aphids that acquire persistent viruses do so after a minimum feeding time of 10-60 minutes. Following acquisition, a latency period of at least 12 hours must pass before the aphid can transmit the virus. The aphid remains infectious for at least a week, or in some cases, for its entire life. With non-persistent viruses, the aphid can pick up the virus rapidly (within seconds or minutes) while probing the host tissue and can transmit the virus immediately to another plant. However, the ability of the aphid to successfully transmit the virus is quickly lost (within minutes). Insecticides increase the spread of non-persistent viruses by stimulating probing activity of the aphid. Contact insecticides are generally less useful than systemics for controlling insect-vectored viruses.
In general, the spread of viruses is best controlled by cultural practices such as cultivar selection, planting date and location, weed reduction, and roguing of diseased plants. Row covers may prevent aphids from probing and feeding on plants early in the season, preventing the spread of viruses. Occasionally, seed or transplants are infected and the problem is not apparent until well into the growing season. There are no chemical control measures for virus diseases other than those directed at the vector or weed hosts. For more specific control measures, refer to the crop.
Fungicides and Bactericides
Fungicides and bactericides are used to prevent, not cure diseases. Applications initiated before disease appears or at the first sign of disease, are the most effective way to use these pesticides. Knowledge of the cause of the disease is required to select the proper material. Your regional and state specialist can assist you in determining the cause (see Disease Diagnosis above). Note that the continuous use of certain bactericides or fungicides can result in pathogens becoming resistant to these chemicals.
Pathogens that survive an application of a fungicide are likely to pass the trait that enabled them to survive on to their offspring. A single genetic change in the pathogen can render single-site mode of action (systemic) fungicides ineffective. Repeated applications of the same type of fungicide exert selection pressure on the pathogen population and eventually eliminate almost all the susceptible individuals from a pest population. Resistance can develop in a very short time.
It is necessary to practice resistance management to preserve the useful life of a fungicide. The most effective way to extend the useful life of a product is to use it once and then alternate with a fungicide with a different mode of action. Fungicides are grouped according to chemical class or site of their activity and assigned a group number by the Fungicide Resistance Action Committee (FRAC Group). To select fungicides with different modes of action, see resistance group (FRAC) in Table 25 and in the following section Fungicides and Bactericides Alphabetical Listing by Trade Name. Fungicides that are systemic (penetrant) have a single site of action upon the target organisms' physiology and are particularly prone to developing resistance. Use systemic fungicides with a single site of action once per season. Use the most effective chemical against a particular pest first. Do not apply fungicides with a high risk of resistance development (systemic, single mode of action such as Group 11 strobilurins) when disease is severe as this situation results in high selection pressure upon the pathogen.
There are many other techniques that can help delay the onset of resistance. Many resistance management techniques help minimize the use of pesticides so a lower proportion of each pest generation is exposed to the toxin.
- Integrate chemical controls with effective cultural, mechanical, physical, and genetic management options.
- Scout, monitor, and use action thresholds to determine if fungicide applications are necessary.
- Good spray coverage helps do the job right the first time and avoids unnecessary repeat applications: use the proper size nozzles at the correct angle or orientation and an adequate amount of water per acre.
- Tank mixes of systemic materials with contact fungicides help delay the onset of resistance to the systemic fungicides. Most contact fungicides have a multi-site mode of action (FRAC Group M plus a number).
NOTE: There is no relationship between insecticide groups, herbicide groups, and fungicide groups. For example, there is no problem using a Group 1 herbicide and a Group 1 insecticide or fungicide.
Toxicity of Fungicides
All pesticides are poisonous. However, some are more toxic than others. The toxicity of the pesticide is usually stated in the precaution on the label. For example, a skull and crossbones figure and the signal word "Danger" are always found on the label of highly toxic (Toxicity Class I) materials. Those of medium toxicity (Toxicity Class II) carry the signal word "Warning." The least toxic materials (Toxicity Class III) have the signal word "Caution." The toxicity of a pesticide is expressed in terms of oral and dermal LD50. LD50 is the dosage of poison that kills 50% of test animals (usually rats or rabbits) with a single application of the pure pesticide for a given weight of the animal (mg/kg of body weight). The lower the LD50 value, the more toxic the material. Oral LD50 is the measure of the toxicity of pure pesticide when administered internally to test animals. Dermal LD50 is the measure of the toxicity of pure pesticide applied to the skin of test animals. Generally, an oral application is more toxic than a dermal one.