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Saturday, September 22, 2018

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1414. Cook, R.J.. 1990. Diseases caused by root-infecting pathogens in dryland agriculture.. Advances in Science 13:215-239.
This is a review paper of plant diseases that infect cereal and pulse crops through their root systems. Root diseases can be grouped as well-defined or chronic, subclinical. Soil fumigation was used in various experiments to observe the effect of crop rotation and the frequency of the host plant in rotation on disease impacts. Fumigation led to the following yield responses in winter wheat when grown in the respective rotation: continuous wheat - 70%; 2-yr rotation - 22%; 3-yr rotation - 7%. Most disease inoculum occurs in the top 20-30 cm of soil, which is also the area of greatest root concentration. Fusarium solani (pea pathogen) survived for over 20 yr in 30-50 cm depth of soil, even when no peas were grown during this time. Burning straw greatly reduced Pythium populations in a wheat field. Pythium are quick to establish and only need a short period of favorable soil moisture (wetter than -0.04 MPa), a situation not frequent in many dryland soils. Pythium is more likely to be lethal to germinating large-seeded plants such as pea and chickpea, in comparison to a stunting effect in wheat and barley. Cephalosporium stripe requires at least a 2-yr break from winter wheat or winter barley, while a one year break is usually adequate for take-all. Since Rhizoctonia solani can survive on many crop hosts, a period of bare ground for several months may be the best control measure. The same may be true for Pythium species. Tillage generally hastens the decline of many pathogens, and can substitute for some rotation effects. Researchers have noted lower yields from stubble mulch fields over the past 50 yr, probably due to the residue favoring many disease organisms. The mulch produces wetter, cooler soils and a food base for pathogens. Field tests have shown that the straw itself is probably not toxic. Volunteer plants also allow carryover of pathogens from one crop to the next. Several strategies are outlined for disease control. First, lower inoculum levels throgh a combination of rotation, tillage, fumigation, burning, and clean seed. Next, prevent infection from carryover inoculum via fungicides or inhibitory baceria. Lastly, maximize the plant's own resistance and defenses. Management for yields higher than the moisture resource can lead to plant water stress which can enhance some diseases, such as Fusarium culmorum.

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