Better Landscaping Today Newsletter

Volume 2, Issue 12                                                                                                                               December 1999

LOSING PLANTS TO SOIL-BORNE PATHOGENS IS AVOIDABLE

Successful cultivation of horticultural plants depends in part upon the microbial world of the soil.  The fungal flora of soil contains both beneficial and parasitic fungi.  Beneficial saprophytic fungi and bacteria decompose dead plant residue and form semi-stable soil organic matter.  Other microbes are parasitic and live on hosts.  Mycorrhizal fungi are symbiotic with plants.  These fungi help to solubilize unavailable nutrients in the soil.  In addition they colonize plant roots.  This is also beneficial to the plants because parasitic invasion is reduced.

Thousands of microbial species exist in the soil.  Competition keeps the abundance of any one species low.  When disease causing organisms are not present in the soil in large numbers the infections are less, and the plant is capable of warding off attack.  When they are present in large numbers, healthy roots encased in a sheaf of beneficial fungi are able to avoid disease.

Root-rot, damping-off and crown rot are caused by parasitic fungi.  These microbes are opportunists and are nearly always present.  In many cases, they are unable to infect plants.  But, when the conditions are favorable, they will readily attack plants.

Influence of Soil and Soil Conditions upon Fungal Root Diseases

Soil Moisture - Some common pathogenic fungi such as Phythium, Phytophthora and Fusarium are phycomyeetous, organisms which contact the host by means of free-swimming zoospores; when soil moisture content falls below a certain value, infection is usually completely inhibited.  Some of these fungi can attack the host at moisture contents of 60% saturation and above, but not at those of 45% saturation or below; the severity of infection increases with rise in moisture content.  A few of the Fusarium wilts are increased by frequency of irrigation.  Partial drying between irrigation is usually worthwhile.

In certain situations, high soil moisture content may increase the prevalence of soil-borne diseases in another way, i. e., by flood dispersal.  Very high soil moisture content may also, by reducing soil aeration, injure the roots of the host plants and thus afford ingress to parasites.

Soil Types - It is not accurate to conclude that because the activity of a root-infecting fungus is favored by poor soil aeration, the disease will be less severe on sandy soils.  The fungi also require aeration for growth.  More importantly it is necessary to remember that the sandy soils are usually naturally poorer in plant nutrients than clayey soils.  Well nourished plants can usually outgrow diseases.

Soil Organic Matter - Some diseases are reported to be encouraged by the application of organic materials to the soil.  Possible stimulating effects may be due to increased carbon dioxide levels in the soil atmosphere and trace amounts of volatile substances formed by decomposition of organic matter.  Woody materials can serve as "food-bases" from which fungi can infect plant roots.

Attention needs to be made to the possibility of "biological control" of soil-borne diseases by application of decomposable organic matter. it is assumed that increased activities of beneficial saprophytes growing on readily decomposable organic materials must occur at the expense of, and to the detriment, of plant parasites.  Parasites must compete with saprophytes.  The development of a high population of micro-organisms may greatly limit the predominance of any one species.  Soil tilth is enhanced by organic matter after its microbial metabolic conversion into mucilages and gums.  The cellulose-utilizing fungi do not generally form mucilages which are mainly formed by non-wood decomposing fungi and bacteria.  Some composts, especially when fortified with appropriate bacteria have disease suppression ability.

Better Plants Through PAM (polyacrylamide) Soil Conditioners

Plant root diseases can be minimized with soil conditioning polymers due to better soil aeration, to less flooding because of good drainage and to less frequent irrigation.  Soil moisture content is more optimum for plant growth and less optimum for parasitic fungi when soil is conditioned.  Plants which are less stressed but are more vigorous are less prone to be infected.  The Soil Drain PAM technique of amending soils works well with reduced amounts of organic amendments.  Readily decomposable ones are very effective with the polymer and also help to limit the inoculum potential of parasitic fungi.  The best method to control soil-borne diseases is to maintain a good balance of plant nutrients and to have excellent physical soil properties which impart good soil porosity.  Soil Drain is a water-soluble polyacylamide that will greatly enhance good soil structure and limit plant root diseases.  No one should plant without it.

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