Plant Disease Basics - The Disease Triangle

Disease management begins with the fundamental understanding the three factors must be present and interact for disease to develop.
Plant Disease Basics - The Disease Triangle - Articles


Tomato late blight. Photo credit Beth Gugino, Penn State Plant Pathology.

These include:

  1. a susceptible host crop
  2. the pathogen
  3. an environment that is favorable for the pathogen.

This concept is called the disease triangle and all disease management practices can be related back to breaking/disrupting one or more of these interactions.

Many of the concepts/tools important for organic disease management are the same as for growers who use conventional IPM tools however; there is an even greater emphasis on prevention and being proactive rather than reactive since in-season management tools are more limited. Keep in mind that disease management requires the integration of multiple strategies in order to be successful and that knowledge is the key to success.

  • Know what diseases can occur on your crops and how they impact yield (directly affect fruit or reduce photosynthetic area which impacts fruit quality)
  • Know the symptoms and signs
  • Know the biology of the pathogen (sources of inoculum, how it spreads, conditions that favor development)
  • Know the available management practices/strategies both in the short-term and long-term

Below is a general list of concepts important to the organic management of plant diseases.

Resistant cultivars

Resistant cultivars are still one of the most important disease management tools. For late blight, tomato cultivars with specific late blight resistance genes now include Defiant, Mountain Magic, Mountain Merit, Plum Regal and Iron Lady (also with tolerance to early blight and resistance to Septoria leaf spot). Other cultivars with observed resistance to late blight include Jasper and Matt's Wild Cherry. Pumpkins with homozygous resistance to powdery mildew are more effective than those with heterozygous resistance (from only one parent). Peppers with resistance to Phytophthora blight include Paladin, Aristotle, Alliance, and Revolution. Aristotle and Revolution also have resistance to bacterial leaf spot (race 1 -3). For a list of recommended cultivars for our production region and there corresponding disease resistance see the recently updated tables at the beginning crop section in the 2016 Mid-Atlantic Commercial Vegetable Production Recommendations. It is important to understand not only what diseases are more prevent in the crops that your growing but also what races or pathovars are predominant because that can affect the effectiveness of the host resistance.


Sanitation reduces potential sources of pathogen inoculum. Use new or cleaned and disinfected planting materials (flats for transplants, tomato stakes, etc.). Clean greenhouse surfaces. Hose down equipment between fields (especially important for soil-borne pathogens like white mold or Phytophthora blight that can easily be spread through movement of the soil). Chlorine-based materials, hydrogen peroxide and peracetic acid are allowed by the NOP. If you have successive plantings, work the newest plantings first to reduce potential movement of pathogens and plow down crop residue as soon as possible after harvest to minimize pathogen spread. Also locate subsequent plantings upwind or as geographically separated as possible.

Clean pathogen-free seed

Purchase clean pathogen-free seed whenever possible from a reputable source. If saving seed, only save seed from healthy plants. Many pathogens can be harbored in the seed and spread to the crop next season. Consider hot-water seed treatment if seed-borne pathogens especially bacteria are a concern. Several hot-water seed treatment equipment stations have been purchased with funds from the Pennsylvania Vegetable Marketing and Research Program and PA Vegetable Growers Association that are available for use (check with your local extension educator). Chlorine-based products can also be used to surface disinfest the seed. Plant only visibly healthy transplants.

Crop rotation

Crop rotation is important to reduce the potential build-up of soil-borne pathogens and to facilitate the degradation of crop residue which can harbor many pathogens. The pathogens that cause many common diseases like early blight on tomato and black rot on brassicas cannot not survive in the soil on their own once the crop residue is decomposed. However, keep in mind that crop rotation is not effective for soil-borne pathogens that have a wide host range, can survive saprophytically on organic matter or do not overwinter in this region (eg. cucurbit powdery and downy mildews). Crop rotation is most effective when used proactively.

Promote soil health

Promote soil health through use of organic amendments, green manure crops, cover crops, reduced tillage, etc.

Create conditions that are unfavorable for disease development

Most pathogens require either a period of free moisture on the leaf or high relative humidity to infect so in general, the longer you can keep leaves dry, the less likely diseases are to develop. Leaf wetness can be minimized through use of drip irrigation, weed management (weeds in the same plant family as the crop can often harbor pathogens), trellising plants, maximizing air circulation through plant spacing and row orientation, etc. For soil-borne pathogens, improving soil drainage or planting on raised beds can help. Use plastic or straw mulch to reduce soil splashing that can spread pathogens and reduces direct contact of the fruit with the soil.

Disease forecasting

Use disease forecasting to help determine disease risk and be aware of current disease outbreaks in the region. Forecasting for cucurbit downy mildew is based on

  1. regional disease occurrence
  2. when conditions are favorable for disease development
  3. the forecasted weather in the eastern US.

This information is used to determine the relative risk of downy mildew development in a given region. During the season, forecasts are updated every Mon, Wed and Fri and are available at the Cucurbit Downy Mildew ipmPIPE website. Other forecasting systems assume that the pathogen is present and use in-field weather station data or interpolated weather data to determine how favorable conditions are for disease development and recommend when fungicides are needed (eg. TomCast for early blight and BLITECAST for late blight). Information on late blight outbreaks can be found at the USABlight website and information regarding general disease outbreaks in PA can be found on the Penn State Extension website.

Scout regularly

Scout your crops regularly, thoroughly and by cultivar if they differ in disease susceptibility. Fungicides will be most effective at slowing disease development when applied when the very first symptoms develop or preventatively when disease outbreaks are reported in the region. Keep disease maps for future reference and to help inform crop rotations. For help with disease diagnosis, contact your local extension office or submit a sample to the Penn State Plant Disease Clinic.

Use fungicides when cultural practices are not adequate and the disease is in an early stage of development

Fungicides can be an important tool during the season when intervention or prevent crop loss is required (eg. Cucurbit downy mildew, late blight on tomato/potato, powdery mildew, etc.). Products that act by affecting the plants' natural defense mechanisms (eg. Regalia, Companion) are most effective when applied prior to the onset of disease. Other OMRI-approved products are most effective when applied preventatively or at the first sign of disease. Products for the management of soil-borne pathogens are best applied before planting at the transplant stage. For a list of recommended products geared towards conventionally production see the 2016 Mid-Atlantic Commercial Vegetable Production Recommendations.

Rogue and destroy symptomatic plants

Rogue and destroy symptomatic plants to reduce disease spread and incorporate crop residue at the end of the season. Removing symptomatic plants when symptoms are first observed can help reduce pathogen spread to the rest of the field and successive plantings. Incorporation of crop residue immediately after harvest will facilitate faster break down by soil microbes thus reducing pathogen survival in the soil.

More information

Funded by USDA Specialty Crop Block Grant Project ME#44166076 – “Sustainable Production and Pest Management Innovations for Next Generation Young and Hispanic/Latino Specialty Crop Growers”