By Dr. Thomas T. Yamashita
As we have discussed elsewhere, excessive soil moisture is conducive to the proliferation of various soil-borne diseases. The organisms responsible for many such diseases can multiply on undecomposed debris and plant parts in the soil, growing their numbers between plantings. Most soils have minor population levels of disease organisms which remain, for the most part, inconspicuous under standard horticultural practices. However, as the populations begin to reach or exceed a threshold level, disease incidence can figuratively explode.
This happened with hantavirus, a human virus which drew national attention due to a series of outbreaks in western Navajo, Hopi, Ute, and Zuni tribal reservations in 1993. 12 victims succumbed to the disease. Extensive epidemiological studies revealed that the lethal virus was carried in the feces of field mice. While the mice had always been present in the region in low numbers, low inoculum pressures minimized the incidence of disease outbreaks. However, a drought resulted in large populations of mice migrating closer to tribal towns and villages. Thanks to the food and shelter provided by the towns, the mice population grew, leaving more droppings and producing higher inoculum levels of the virus, ultimately leading to outbreaks of the previously unknown disease.
This same scenario has played out many times with soil-borne plant diseases.
What are the major soil-borne pathogens?
The primary soil-borne disease organisms commonly found in ornamentals and turf are:
- Various nematode parasites
The first four organisms are fungal pathogens which thrive under wet conditions. For example, Pythium and Phytophthora species are considered ‘water molds,’ which are well-adapted to moist soil environments. For example, growers who secure their irrigation from canal or river water oftentimes inherit more water mold pressures, as these organisms can breed in standing water environments.
Many years ago, I ran studies to demonstrate this phenomenon by sprinkle irrigating model Russian olive trees in the early fall with well water and canal water. The trees sprinkle irrigated with canal water developed gumming phytophthora lesions, while those sprinkled with well water remained free of disease. Rhizoctonia and Fusarium can also proliferate rapidly on various types of undecomposed tissues and debris.
There are many species of plant-parasitic nematodes which can attack ornamentals and turf hosts as well. All plant-parasitic nematodes are armed with a mouth spear, which they use to pierce the host tissues and extract cell sap. As such, almost all plant-parasitic nematodes depend on a plant host and are unable to feed on debris or other non-living hosts. Such organisms which depend on a living host are termed ‘obligate parasites.’ The important points to remember, in light of heavy rains and moisture, is that nematodes are aquatic organisms, which thrive under moist conditions. When the soil dries, many nematodes will adjust their physiology and remain quiet until remoistening of the soil. Other species will vertically follow the zone of moist soil downwards, then migrate upwards with remoistening of the top soil.
We conducted a test to demonstrate this latter phenomenon. A field was fumigated to a depth of 1 foot with a well-known product. Following fumigation, soil samples examined from the top 1 foot of soil had undetectable levels of plant-parasitic nematodes. We then used a 10” diameter auger and drilled a hole to a depth of 4 feet. In the hole was placed a 10” diameter PVC pipe. The pipe was screened off at the end, and in the length of pipe we had cut windows at 2.5’ and 3.5’, each covered with 60-mesh screen. The pipe was refilled with clean, sterilized soil, and remoistened, and 3 tomato seedlings were planted on top. Soil cores were pulled from the pipe 7, 14, and 21 days later. On day 14, we detected nematodes, and it was surmised that these populations had migrated from lower depths left unharmed by the previous fumigation treatment.
The takeaway is that moist soil, especially under unusually warm soil conditions, will house active nematodes. If there are roots (such as turf or rose roots) upon which they can feed, they will thrive and multiply.
What should you do for perennials or before the planting of annuals?
Excellent control of soil-borne fungal pathogens can be achieved with a number of fungicides. One of the more readily available effective fungicides is Ortho Daconil. It comes as a thick, grayish colored liquid in a 16-ounce container. The active ingredient is chlorothalonil, which is used in the commercial grower market in a product called Bravo 500.
A good preemptive means of protecting against soil diseases before planting flower beds is to lightly drench the bed on two successive days with a mixture containing two to three tablespoons of Daconil, along with a liquid fertilizer (check the label for the appropriate mix ratio) in 3 gallons of water. A garden watering can works well for application. Drench the flower bed, providing enough product to thoroughly wet the first 6” of soil. Allow this to work for 24 hours before repeating it again. Usually, the second application can be toned down, as the soil will still be quite moist from the previous application and will be easier to drench.
For perennials, merely increase applications to cover a minimum depth of 1.5 feet.
Although we have not directly addressed the nematode issue, we have observed several side-benefits from using fertilizer alongside fungicide treatments in the form of both increased tolerance and indirect enhancement of control.
If you have objections to the use of fungicides, you might consider the use of biological agents that are known to be antagonistic to a broad range of pathogens. We have examined some of these products and find them to be quite effective when used properly. Many of these products come as a powder or liquid suspension containing bacterial or fungal species selected for their ability to combat various diseases. To use such a product, simply mix the product at the rate recommended on the packaging, and use it as described above as a replacement for fungicide.
If you wish, biological products containing antagonistic bacteria can be combined with the previously outlined Daconil program. The fungicidal properties of this product will not affect the beneficial bacteria. We have found such a program to be quite effective and able to sustain the longevity of protection that initially comes from the fungicide, and is thereafter supplemented for a long period of time through the antagonistic activities of the beneficial microbial agents.
Again, for perennials, merely increase the volume of application to wet to a deeper depth of about 1.5 to 2 feet.