by Tamara Scully
Farmers are expected to recognize and moderate causes of plant disease. From plant abiotic stressors such as mineral deficiencies and water availability or poor germination due to soil temperatures to pest and pathogen control, farmers need to monitor and maintain crop health.
Plant phytopathogens – those which cause plant diseases – directly impact plant health. Plant pathogens enter the plant through wounds, stomata, hydathodes or lenticels or through the roots. Some pathogens are able to degrade plant cell walls. Insects can vector disease causing organisms, injecting pathogens into the plant while feeding. Pathogens can also spread by rain, irrigation water, wind or from contaminated materials.
Chemical and biological control mechanisms, as well as cultivation practices such as crop rotation, precision irrigation, selection of resistant cultivars, companion planting and field sanitation practices, can all reduce the spread, prevalence or severity of phytopathogens.
Eating a plant infected with a virus, bacteria, fungus or other phytopathogen doesn’t translate into human disease. Many pathogens are host-specific or thrive in specific environmental conditions, so the disease does not spread to humans who contact or consume the produce.
There are some exceptions to the general rule, however. They include grains infected with various fungi, such as Fusariaum spp. These fungi produce mycotoxins, which are capable of causing illness in livestock and humans. It isn’t the pathogen itself which infects humans, it is a byproduct – the toxin produced by the fungi – that causes sickness. Pseudomonas aeruginosa, a bacteria causing a type of soft rot on some produce such as lettuce or tomato, can also cause severe human disease, particularly in compromised patients.
Food Safety Pathogens
If the diseases that cause a plant to become ill aren’t typically transmittable to humans, then how do the pathogens involved in food recalls – such as the recent romaine scare, or previous melon and spinach recalls – contaminate food?
Farmers are expected to keep the food supply safe and free of contamination from pathogens which cause human illness. Many food recalls are due to contamination concerns during improper processing. But fresh produce recalls are, unfortunately, not uncommon.
Contamination with human illness-causing pathogens can occur anytime from field to fork. Pathogens can be introduced at any step post-harvest, including in wash water, in contaminated storage bins, through contact with humans, through machinery such as packaging lines or as foods are stored and the process of decay occurs.
Recent government reports have implicated irrigation water contaminated with animal feces in several recent illness outbreaks associated with fresh produce. Testing irrigation water before use, or implementing post-harvest intervals after irrigation, can reduce the prevalence of bacterial contamination, such as from E. coli. This is reflected in the Food Safety Modernization Act Final Rule of Produce Safety.
Salmonella spp. can survive in soils for almost a year. Salmonella in soils can cause food safety risks, and studies have connected outbreaks of disease with production practices including the use of contaminated irrigation water and poultry litter fertilizer.
In these studies, tomato fruit not in contact with soil was not found to be contaminated despite the presence of the pathogen in the soil. Tomato leaves were found to be contaminated only in plots without plastic mulch, which served as a barrier between the plant and the contaminated soils.
But recent studies suggest that some pathogens are capable of contaminating produce directly, pre-harvest. Although they do not act as phytotoxins causing plant disease symptoms, they persist in the plant – including edible portions – and can cause human illness from consumption. Unlike those instances where produce is exposed to human pathogens post-harvest, this type of pathogen is present in the plant as it grows.
Food safety protocols such as extending the post-harvest interval don’t help if the edible parts of the plant itself contain human pathogenic bacteria. These human pathogens seem to enter the plant differently than do phytopathogens, as they are not able to degrade the plant cell walls.
A new study from researchers with the Indian Institute of Science and the University of Agricultural Sciences has shown that lateral root emergence causes the plant to become more vulnerable to certain non-phytopathogenic microbes. This type of root growth forms a cavity which some pathogens can use to gain entry into the root tissue in an opportunistic manner.
If human pathogenic bacteria are already present in the soil, and the growing conditions favor lateral root development, the microorganism can enter the plant tissue, spread throughout the plant and eventually cause human illness when the produce is consumed.
The formation of lateral roots is a function of soil conditions. The researchers determined that increased salinity level is a factor which enhances plant susceptibility to Salmonella infection. Osmotic soil stress due to soil salinity promotes lateral root development, which increases the opportunity for this non-phytopathogenic bacteria to infect the plant.
The researchers conclude “that salinity treatment increases the risk of Salmonella colonization on roots and its transmission to the fruits” of brassica family and tomato seedlings used in the study. “The percentage of the fruits infected with Salmonella was higher in the plants grown in saline soil as compared to the control soil.”
Soil salinity can increase due to over-fertilization, the use of irrigation water containing dissolved salts, excessive evapotranspiration or the encroachment of sea water into fields (which is associated with climate change and rising sea levels). Evapotranspiration increases with higher temperatures, as soil moisture decreases, as relative humidity increases and under windy conditions.
It appears that all plant diseases may not be created equal. While phytopathogen control keeps the crop healthy, and post-harvest controls can keep produce free from contamination with many human pathogens, pathogens which infect the plant in the field without causing plant loss but which then can cause a human illness when consumed are another produce safety concern which farmers will need to keep on their radar.