AVOIDING ROOT DISEASE
The two most common causes of root diseases of plants and crops are fertilizer toxicity and plant pathogenic fungi. In some cases, fertilizer toxicity predisposes plants to pathogenic fungi. These causers of root diseases can be classified in twos; abiotic causes and biotic causes.
Abiotic or non living causes of root disease include excessive soluble salts, ammonium toxicity, and suffocation. Most commercially available fertilizers that are preferred by farmers exist in the form of salts. When excessive amounts of salts accumulate in the soil solution, they desiccate plant roots and effectively cause root disease. There is also the possibility of ammonium toxicity occurring, when fertilizers containing urea or ammonium sulfate are used in excess amounts.
Excessive levels of ammonium may also occur following steaming of organic soils, especially those containing manure. The conversion of ammonium to nitrate is carried out by soil microorganisms which are non-existent or in low numbers in soil-less media. The conversion can be inhibited by certain pesticides, cool wet soil, low pH, excessive soluble salts and poor aeration.
Roots must have oxygen or suffocation will occur. Soil composed of very fine particles is dense and has few air spaces. Similarly, a waterlogged soil contains little air. Plant pathogenic water molds thrive under such saturated conditions. Earlier symptoms of root disease can be detected by removing the pot and examining the roots. Healthy roots are generally white and firm; decayed roots may be water-soaked in appearance and/or darkened and easily macerated between the fingers.
Root diseases can also be caused by biotic factors such as fungi. Pythium is one of the most common pathogens found in the roots of crops especially greenhouse crops. It is often associated with excessive nutrient levels or ammonium toxicity. Pythium species can also cause damping-off, crown or stem rot. It is favored by high fertility and high moisture and is a natural inhabitant of the soil where it can survive indefinitely. It can also persist in soil and debris in the greenhouse and on greenhouse floors.
Phytophthora, another pathogen associated with root disease, is generally more pathogenic than Pythium , though it is encountered less frequently. The pathogen causes root and crown rot as well as foliar blighting. Phytophthora is favored by excess moisture and excess nitrogen fertility. Unlike Pythium, species of Phytophthora are more aggressive, more likely to be host specific. Phytophthora species are soil-borne as they can survive many years in the soil. Optimum conditions for root disease development are saturated soil and high nitrogen fertility. The pathogen is not likely to be seed-borne in commercial seed, and it does not travel easily through the air for long distances. It is possible that contaminated irrigation water can introduce the fungus to new sites.
Rhizoctonia is also a common cause of root disease and stem canker. It causes damping-off, root rot, crown rot, web blight, and stem canker in numerous greenhouse grown crops. Unlike Pythium and Phytophthora, dry soil is more favorable for root disease development. For this reason, Rhizoctonia is more active in the upper portion of the soil. This pathogen is usually a problem in the cuttings and small transplant stage and the presence of wounds caused by insects or mechanical damage can predispose plants to Rhizoctonia infection. Other root disease causing pathogens occasionally encountered include Thielaviopsis, Fusarium, Sclerotinia and Cylindrocladium.
A laboratory diagnosis is necessary to determine the cause of root disease and establish the most effective way to control it.