Pathogens include fungi, bacteria, nematodes, and viruses, all biological organisms that can cause disease symptoms and significantly reduce the productivity, quality, and even cause the death of plants. Pathogens can also infect agricultural animals, but for this module, we will focus on plant pathogens. Read the following brief overview of plant pathogens, Plant Disease: Pathogens and Cycles.
Pathogens can be introduced and spread to host plants in many ways. Bacteria and fungal spores can be transferred by wind, in rain, and from the soil via rain splashing onto plant tissues. Insects can vector or infect a plant with a pathogen when they feed on an infected host plant, and then move and feed on an uninfected plant. Pathogens can also spread through infected seeds, transplants, or contaminated equipment, irrigation water, and humans.
Figure 8.2.22.: This pumpkin plant is infected with Bacterial wilt (Ralstonia Solanacerarum) that was vectored (introduced) by a cucumber beetle insect. Photo Credit: Beth Gugino, Penn State University, Associate Professor of Vegetable Pathology
Figure 8.2.23.: Center pivot irrigation of crops, such as this canola, could facilitate pathogen infection and disease development via soil-splashing and by creating high humidity in the plant canopy that could favor some pathogens. Photo Credit: Chad Swank, courtesy of USDA Natural Resources Conservation Services, via Wikimedia Commons.
Plant Disease Triangle: Plant pathologists have identified three factors that are needed for a plant disease to develop:
i. a susceptible host Some pathogens have a narrow host plant range, meaning they can infect just a few host species. For instance, the primary host crops of Late blight (Phytophthora infestans) are tomato and potato. For more information see Tomato-Potato Late Blight in the Home Garden. By contrast, pathogens with a wide host plant range can infect many different host species. There are almost 200 plant species that can be infected by Bacterial wilt (Ralstonia solanacearum). For more information, see Bacterial Wilt - Ralstonia solanacearum.
ii. a disease-causing organism (pathogen). Plant pathogens include fungi, bacteria, viruses, and nematodes. For examples, again, see the reading: Introduction to Plant Diseases, A. D. Timmerman, K.A. Korus. 2014. University of Nebraska-Lincoln. Extension. EC 1273.
iii. a favorable environment for the pathogen. Pathogens usually require specific humidity and temperature conditions for pathogen infection and disease symptoms to manifest. For instance, Late Blight disease symptoms are most likely to occur when the weather is cool and wet.
Figure 8.2.24.: Disease Triangle. Credit Heather Karsten
The three disease triangle factors are important for diagnosing the cause of disease symptoms. Pathologists consider the weather, environmental conditions and the host species to diagnosis what pathogen is causing disease symptoms. Pathologists also consider other factors that could favor and help diagnose a disease, such as i. the field history, particularly what crops and pathogens were present in the past, ii. current crop management practices, iii. when disease symptoms were visible, and on what other species. To assist farmers and others with disease diagnosis, many land-grant universities in the US have crop and animal diagnostic disease clinics where one can submit diseased tissue samples with detailed information that can aid in the diagnosis, such as the host species, environmental conditions, the site history, and management.
Although disease control practices could be categorized into the pest control approaches that were discussed earlier for managing insects and weeds (genetic, cultural, chemical, etc.), plant pathologists typically describe pathogen control tactics with more specific language. For instance, Exclusion tactics involve rejecting infected transplants from being introduced to a farm.
Prevention or Avoidance of pathogen introduction and spread tactics include:
- crop rotation, particularly for pathogens with narrow host ranges
- sanitizing equipment for planting, trellising, pruning, and harvesting
- managing for healthy vigorous crops with optimal soil and water nutrient management
- managing to avoid environmental conditions that promote pathogens, such as avoiding very humid conditions due to over-watering, or promoting drying of plant surfaces with wide row spacing to facilitate air flow, or using drip-tape irrigation that waters plants at or below the soil surface versus over the canopy
- using physical barriers such as row covers, mulch to reduce water splashing; and high tunnel/hoop houses or greenhouses to prevent the introduction of rain and wind-borne pathogens
Figure 8.2.25.: Multiple pest control practices on this organic farm help prevent pathogen infection and spread, while also helping to control insect pests and weeds. Wide row spacing allows for inter-row weed cultivation as well as airflow to reduce crop canopy humidity; crop rotation and intercropping plants from different plant families interrupts pathogen and insect spread; and straw mulch that prevents soil-borne pathogens from splashing onto plants also suppresses weeds. Photo Credit: Heather Karsten
Figure 8.2.26.: Drip tape irrigation under plastic mulch avoids splashing soil-borne pathogens onto crops and is also a more efficient use of irrigation water. Plastic mulch also raises soil temperatures, promoting crop growth and helps to suppress weeds. Photo Credit: Elsa Sanchez, Penn State, Professor of Horticultural Systems Management, Dept. of Plant Science
Figure 8.2.27.: High tunnels, plastic mulch, and sanitized stakes all avoid introducing pathogens to these tomato plants, while also promoting crop growth. Photo Credit: Elsa Sanchez, Penn State, Professor of Horticultural Systems Management, Dept. of Plant Science
Genetic resistance to pathogens is a very valuable and important pathogen control tool. Many plant breeding programs select for genetic resistance to pathogens. When available, pathogen resistance traits are included in most crop variety descriptions to help growers select appropriate crop varieties for their farm.
Figure 8.2.28.: This tomato variety trial included tomato varieties that were resistant or susceptible to late blight. Photo Credit: Beth Gugino, Penn State, Associate Professor of Vegetable Pathology.
If disease symptoms develop, infected plants may be Eradicated or destroyed. And materials that may have been contaminated with pathogens, such as the soil and planting containers, can be heated to very high temperatures with pasteurization equipment or through solarization. For instance, soil may be solarized by placing black plastic over the crop bed (planting zone) during the warm season to increase the soil temperature and destroy pathogens prior to planting the crop.
Therapy or Fungicides (chemical control) may be applied to infected plants to terminate pathogens. Particularly when plant pathogen symptoms are identified early and favorable weather conditions for the pathogen are projected to continue, fungicides can prevent disease spread and significant economic losses. In some high-value crop systems, the soil may be fumigated prior to planting crops.
Similarly, in agricultural livestock systems, animals with disease symptoms can be treated with antibiotics. And in some livestock production systems, antibiotics and vaccinations are administered to animals to prevent diseases and pathogen infection.
Activate Your Learning (short answer)
Read Nonchemical Disease Control, from Colorado State University Extension and identify some pathogen control tactics that could also qualify as other types of pest control categories that we have explored in this module (such as genetic, cultural, and chemical control).
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- Cultural control strategies: Prevention and Avoidance through crop rotation and management of the crop environment. Exclusion and Eradication of inoculum through sanitation to the survival of plant pathogens on crop residues and agricultural equipment and managing for healthy vigorous crop growth.
- Genetic: Resistance, Selecting and breeding crop varieties for resistance to plant pathogens is one of the primary means of disease management, particularly in agronomic crops.
- Chemical: Fungicides that are toxic to pathogens
- Physical or mechanical: Protection of crops via barriers such as plant netting or soil mulching. Eradication could also be qualified as a physical control strategy
- Regulatory: Quarantines