Long-Term Challenges Of Soilborne Diseases

Soilborne Pathogen Structures

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The challenge: Farmers in the west, like growers everywhere, face a daunting number of production, pest, and economic worries in growing vegetable and other row crops. Prominent among these many issues is the challenge of diseases caused by soilborne pathogens. These pathogens lurk unseen beneath the soil surface, lying in wait until a susceptible crop is planted.

These agents may have strange Greek names but are very familiar to experienced growers and include the following: Aphanomyces, Fusarium, Macrophomina, Phytophthora, Plasmodiophora, Pythium, Rhizoctonia, Sclerotinia, Sclerotium, Thielaviopsis, and Verticillium.

Long-Term Management Issues

Why the concern? Soilborne pathogens share certain features that make them so difficult to manage. First, these organisms are long-lived and persistent. They produce structures, most of them microscopic, that resist drying and weathering and account for their long staying power.

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Secondly, these pathogen propagules are very mobile and are a key to management difficulties. Their tiny size and large numbers enable them to be readily moved around. Equipment, vehicles, and tires pick up infested mud and dirt and move it around to other parts of that field. In addition, when contaminated equipment is moved to different lots or ranches, the pathogens hitch rides and end up in previously clean fields.

Another feature that makes these pathogens a problem is that, with a few exceptions, these organisms are able to infect a broad number of crops. For example, if Rhizoctonia or Sclerotinia or Verticillium attacks one crop, the fungal populations can increase and cause significant concerns for a subsequent, different crop.

Finally, these pathogens have multiple attack strategies. Inoculum in the dirt is a significant means of attack, as already mentioned. In addition, these pathogens also find their way into vegetatively propagated materials. Strawberry transplants, artichoke divisions, and asparagus crowns can all harbor these pathogens. Even vegetable seed can be invaded by some of these pathogens. Contaminated seed thereby becomes the vehicle to spread the disease, and the fungi become established wherever such infested seed is planted.

Diagnostic Difficulties

A distinctive difficulty facing growers is the accurate diagnosis of soilborne diseases. Soilborne diseases cause a wide range of symptoms, including root rots, crown rots, vascular discoloration, plant stunting, foliage wilting and collapse, plant death, and other symptoms. However, with few exceptions, most of these symptoms cannot be readily and reliably attributed to a particular soilborne pathogen without laboratory testing. Experienced growers in the west are quite adept at recognizing what the problems could be. Precise identification, however, generally relies on clinical tests.

Another diagnostic difficulty involves testing the soil before crops are planted. Farmers would like to know beforehand the nature of their risk when it comes to these diseases. Unfortunately, very few accurate, practical tests exist for measuring such risks. For the most part, tests for these pathogens are either unavailable, not feasible to do, or lacking in precision. Even for a good soil test, such as exists for Verticillium, the task of deriving an accurate measurement of how much Verticillium is in the field is difficult. Such testing relies on representative sampling of the field and uses mere grams of soil. An estimate of infestation is then made based on extrapolation and assumption. Verticillium testing therefore can often only give growers an indication or hint of disease risk.

Limited Management Tools

Soilborne pathogens can be managed. Planting the crop in locations void of such pathogens is a great management decision. Sanitation and cleaning of equipment can reduce the spread of these organisms to uninfested areas. Careful scheduling of irrigations can limit damage from water molds that are soilborne. Crop rotations with non-host plants are a must.

However, other aspects of an integrated disease management program are missing. Most of these soilborne problems are not controlled sufficiently with fungicides, which is in contrast to the management of foliar diseases that benefits from the use of fungicides. Resistant cultivars are only available for a few of these issues.

For example, excellent resistance is available in tomato against Fusarium and Verticillium pathogens. Pre-plant, soil-applied fumigants have historically been a foundational tool for California growers. Future changes in fumigant availability, use restrictions, and lack of suitable replacement fumigants opens a door for soilborne pathogens to remain serious challenges for row crop farming.

Grower prospects for keeping up with and surmounting soilborne issues will depend on research. Plant breeders will develop new cultivars that are resistant to these pathogens. Pathologists will study these organisms and increase our understanding of their biology and weaknesses. Researchers will develop new methods of control based on this greater understanding of the underground foe.

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