Adopt An Integrated Approach To Disease Management In Vegetables

Successful control of vegetable diseases requires the use of multiple tactics to manage disease problems from transplant production to harvest. Growers cannot rely on only one approach (e.g., chemical control) and expect to successfully cope with the many disease problems that occur in Florida. 

A multifaceted approach to disease management is often referred to as integrated pest management or IPM. IPM is a strategy that draws on a range of management tools with the goal of using the least ecologically disruptive techniques to manage diseases within economically acceptable levels. 

IPM is knowledge-intensive. A well-designed IPM program is built on a combination of compatible control tactics: including cultural, biological, chemical, and mechanical methods; and should emphasize preventative practices to reduce the need for control tactics.

The cornerstone of IPM is knowledge of the diseases attacking a crop and an understanding of the biology and environmental conditions conducive to diseases that might cause crop damage.

For disease management, it is important to understand the potential of a pathogen to infest and spread in the crop. The development of disease is dependent upon the interactions of the pathogen, plant host, and the environment. In plant pathology, this set of interactions is known as the disease triangle. Environmental conditions play a critical role in determining the nature of plant disease epidemics. Practices aimed at managing crop diseases usually focus one or more of the legs of the disease triangle. 

Understanding the biology of the pathogen, host-pathogen interactions, and the effect of environmental factors on this dynamic process in time and space (disease epidemiology), is critical for planning and implementing effective and efficient disease management strategies.

The first step in IPM is correct identification of the disease affecting the crop.  Many disease control failures are linked to misdiagnosis of the problem and not to any problem with the control measures implemented.  Growers should be aware of which pathogens are present or are likely to appear in a particular field or season. Descriptive and pictorial manuals are helpful for identification of diseases commonly found in Florida. It is important to know the common diseases of a given crop specific to your area.

University of Florida Extension Faculty as well as professional crop advisors can provide assistance in disease diagnosis. Diagnosis can also be obtained by sending samples to the UF/IFAS Plant Disease Clinic in Gainesville.

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An integrated approach to disease management is dependent upon frequent scouting to detect emerging disease issues. Scouting services are widely available in Florida.  Growers can take advantage of the expertise of professional crop advisors or scouts who bring their knowledge and diagnostic skills to the farm and who systematically inspect fields for emerging disease problems.

Where possible, it is preferable to prevent pest problems rather than manage them after they become established  – as the old saying goes:” an ounce of prevention is better than a pound of cure.”

Growers should select cultivars that are resistant to diseases when they are available. For example, tomato cultivars with resistance to Fusarium wilt, Verticillium wilt, and gray leaf spot are widely available. Likewise tomato varieties with resistance to the potentially devastating Fusarium crown rot are now available. If one is growing in fields with a history of crown rot, these cultivars should be considered. More recently, commercial varieties resistant to Tomato Yellow Leaf Curl Virus (TYLCV) and Tomato Spotted Wilt Virus (TSWV) have been released.

Similarly, pepper cultivars with resistance to one or more races of the bacterial spot pathogen are commercially available and should be used whenever possible particularly in the fall when wet weather conducive to the development of bacteria is present.

Cultural practices, such as crop rotation, sanitation, crop/residue destruction, host-free period and use of bed and row covers, are an important part of an effective disease management strategy.

Growers should strive to start clean and stay clean. Good field sanitation practices are becoming more important a disease management tools due to the loss of pesticides with fewer new products coming in, and the reduction in efficacy due to resistance problems.

Field sanitation practices should be an important aspect of production from field preparation through harvest and beyond. Transplants should be free of disease when set in the field.

Avoid movement of equipment from infested fields to non-infested sites (i.e. Sclerotinia and bacterial wilt). Work clean areas first, then move to diseased areas and then clean and/or disinfect. Decontaminate stakes that were used in disease infested fields prior to reuse.

Choice of land can be very important in management of several diseases, particularly those that are soilborne. Fields with a history of fusarium crown rot of tomato or Phytophthora blight (pepper) will have the same problem consistently from year to year. Avoiding planting susceptible hosts in these fields will help break the cycle of yearly outbreaks.

Much hand labor is used in vegetable production. Such operations include pruning, thinning, and tying. Pathogens, such as bacterial spot and speck, can be readily spread throughout fields when these intensive handling operations are carried out. Mechanical transmission of bacteria and certain viruses has been shown to be reduced by handling plants only when dry and periodically disinfesting workers’ hands and tools with a topical disinfectant.

Weed management aids in plant disease control. Weeds and volunteer crops can harbor diseases like late blight and tomato yellow leaf curl virus.  Destruction of weeds eliminates these inoculum sources. Cover crops can help reduce weed populations. Non-host cover crops will help reduce weed populations that may harbor pathogens between seasons as well as reduce the primary inoculum for soilborne pathogens. Use cover crops that grow fast and provide maximum biomass.

Maintain good sanitation procedures during harvest. Dirty buckets, bins and gondolas should be cleaned to reduce decay cross-contamination and those with rough surfaces, which can cause abrasion injuries, should be replaced. Plastic containers and bins can be readily cleaned and sanitized.

Destroy crops immediately after harvest is complete. Do not wait until all blocks are harvested. Do not rely on “Mother Nature” for effective crop destruction.  Include either oil (2% to 3%) or a contact insecticide with burn-down chemicals to reduce movement of insects such as whitefly which vector certain viruses out of fields.

Make sure that plants are completely killed, especially if double cropping is planned.

Crop rotation is another well-recognized cultural practice for disease management.  Crop rotation helps keep some diseases from building up over time.  A good rotation plan moves crop families around and alternates cover crops with cash crops. 

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While crop rotation may seem straight forward it may be complicated by market demands and the efficient use of equipment and labor.  In addition, it is not always clear exactly how long a period of time is necessary to suppress a disease.

Plant nutrition and soil pH can impact some diseases. Fertilizers with a higher proportion of nitrate nitrogen (NO3) than ammoniacal nitrogen (NH4) will help to reduce the incidence of Fusarium wilt on tomato. Increasing soil pH by liming is a good management strategy to reduce Fusarium wilt incidence as well as Botryis gray mold severity. Optimum calcium nutrition and higher soil pH may reduce the incidence of bacterial wilt in the field.  Adequate calcium is necessary to minimize blossom end rot and to provide for overall healthy growth. Avoiding excessive nitrogen leads to less dense canopies, thus improving air movement in the canopy and reducing the incidence of diseases such as bacterial spot of tomato and pepper and target spot of tomato.

Water management is also critical to overall disease management. High soil moisture enhances the development of soilborne pathogens including Phytophthora, Pythium and bacterial wilt. Excess water damages roots by depriving them of oxygen and creates conditions that favor infection by certain soilborne pathogens. For example, Phytophthora blight of pepper can quickly decimate fields that become flooded.  It is imperative that excess water be pumped off farms as quickly as possible, especially following tropical storm events.

Irrigation management, based on plant needs, will help to create an environment unfavorable for pathogen survival and disease development.   Overhead irrigation can contribute to increases in bacterial spot. UF/IFAS research has shown that drip irrigation is an excellent method for reducing bacterial spot compared to overhead irrigation.

Biocontrol agents for vegetable disease management are increasing in use. These products are considered safer for the environment and the applicator, than conventional chemicals and are mainly used against soilborne diseases as well as some foliar diseases. Examples of commercially available biocontrol agents include the fungi Trichoderma harzianum and Gliocladium virens , an actinomycete Streptomyces griseoviridis, and a bacterium Bacillus subtilis. Bacteriophages (phages) have proven effective biocontrol agents for the management of bacterial spot on tomato. Phages are viruses that infect bacteria. Growers should evaluate the applicability of biocontrol to their particular farming operation when incorporating this technique into a disease management program.

Chemical control involving the application of fungicides and bactericides are an important component of many disease management programs on traditional commercial farms.  It is important to remember that chemical use should be integrated with all the other disease management tactics mentioned in this article.

Information regarding physical mode of action of a fungicide will help producers improve timing of fungicide applications. Physical modes of action of fungicides can be classified into four categories: protective, after infection, pre-symptom, and anti-sporulant (post-symptom).

Protectant fungicides include the bulk of the foliar spray materials available to producers. In order to be effective, protectant fungicides, such as copper compounds, mancozeb etc., need to be on the leaf (or plant) surface prior to arrival of the pathogen.

IPM Strategies For Vegetable Disease Control ctd.

Systemic fungicides display some level of therapeutic activity based on their level of systemicity.  They can range from a true systemic which moves throughout the plant  -i.e. Aliette,  a translaminar which can move across the leaf – i.e. Quadris, and a meso-systemic which can penetrate the cuticle and is active inside of the leaf -i.e. Flint.

Systemic fungicides may stop an infection after it starts and prevent further disease development.

Some of the newer fungicides, e.g., the strobilurins, can have some “kickback” action, meaning they can help alleviate disease when applied after infection occurs.

Many of these new fungicides are prone to development of resistance in pathogen populations and must be used in combination and/or rotation with more broad-spectrum fungicides.

A new strategy to chemically manage plant diseases without direct interference with the pathogen is the triggering of plant defense reaction. Acibenzolar-S-methyl (Actigard), a chemical in this category, is registered for the control of bacterial spot and speck on tomatoes.  Such products “switch-on” a plants natural defenses.  This is often referred to as systemic acquired resistance.

Chemicals must be used at recommended rates and application frequencies. Besides selection of the most efficacious material, equipment must be properly calibrated and attention paid to the appropriate application technique. As always, the key to effective disease management is correct diagnosis of the problem.

Follow the latest fungicide recommendations provided in the UF/IFAS Extension Service Vegetable Production Handbook for Florida.  http://edis.ifas.ufl.edu/features/handbooks/vegetableguide.html.  Always read the pesticide labels and follow the instructions carefully. Remember, the label is the law.

Fumigants can be used to manage soilborne pathogens. Before applying, it is important to review the disease history of the specific site when choosing fumigant materials.

Management of viral diseases requires effective management of whiteflies, thrips, and aphids to reduce the incidence and secondary infections of viral diseases vectored by these insects. Follow UF/IFAS recommendations for insect management. http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide

While Florida vegetable growers can face a formidable array of disease issues adopting an integrated disease control program can help ensure a successful crop.

Resources:

• UF/IFAS Grower’s IPM Guide for Florida Tomato and Pepper Production   http://ipm.ifas.ufl.edu/resources/success_stories/T&PGuide/index.shtml

• Integrated Disease Management for Vegetable Crops in Florida, Fact Sheet PP-193, Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised December 2005. http://edis.ifas.ufl.edu/pp111

• Vegetable Production Handbook for Florida.  Publication HS710, a publication of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised January 2012. http://edis.ifas.ufl.edu/features/handbooks/vegetableguide.html

• Management of Whiteflies, Whitefly-Vectored Plant Virus, and Insecticide Resistance for Vegetable Production in Southern Florida.  ENY-735 (IN695), a publication of the Entomology and Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. March 2007.  http://edis.ifas.ufl.edu/in695