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Root Rot in Edible Beans – No Easy Fix

Root rot is  becoming visually evident in many edible bean fields now that beans are entering  a more rapid vegetative growth phase and will have a high demand for water and nutrients. Pounding rains, compaction, tillage pans, poor internal drainage and soil structure are common culprits for inducing root rot. Common visual symptoms include stunting and uneven growth, yellowing of lower leaves and marginal leaf necrosis. Roots will show red-brown discolouration and in severe cases the taproot will brown inside when split open.

No Sure Fix: Where there is no root growth, there is no top growth. Plants try to compensate by intiating new lateral roots above where the root rot occurs. Inter row cultivation to throw loose soil around the base of plant will provide habitat for new roots to grow and open up the soil to improve oxygen exchange. Infected plants try to compensate for the root rot by producing new lateral roots off the hypocotyl above existing root system.  Avoid close cultivation to minimize root pruning.  Be particularly careful not to prune off the shallow newly formed roots on plants with root rot.  In wide row beans that are being pulled and harvested this is good strategy. Scuffling a second time 5-10 days apart may be required to throw enough loose dirt around base of bean plants

Application of Nitrogen can help stimulate new root growth where no nitrogen was applied earlier, or where heavy rains have occured moving nitrogen down in the soil profile where plants cannot currently access it.  If beans are being scuffled, apply a small amount of nitrogen prior to scuffling to place some of the nitrogen closer to the root zone.  There is no research that supports the amount to apply, but generally 20-50 lb N/ac should be sufficient where some nitrogen was applied at planting time.  In direct harvest beans, broadcasting on the nitrogen may encourage new root growth. If nitrogen is to be broadcast, forms other than urea are preferred due to potential volatilization losses with urea. These could include ammonium nitrate, ammonium sulphate, or potassium nitrate the later being helpful where soil potassium levels are low.

No evidence that foliar fertilizer or fungicides may help with combating root rot. A foliar fertilizer is very effective for applying micronutrients where plants only require a very minute amount of a nutrient. However for the major nutrients –  nitrogen, phosphorus, potassium and sulfur plant uptake through the leaf is minuscule for amount plants require. Typically foliar fertilizers that include nitrogen will provide a temporary ‘green-up’ effect without the desired lasting fix necessary. Foliar fungicides have no currative effect for root rot. or indicate a benefit where root rot is present.

Longer term solutions include improving soil structure and internal drainage, longer term rotations and varietial selection. A rotation of 3 to 4 years or longer with non host crops is best Fusarium and Rhizoctonia have a wide range of crop hosts.  Corn, cereals, and forage grasses, alfalfa are good alternative crops that do not host bean root rot.   Plant residue from soybeans, canola, peas, potatoes all host bean root rot organism.  There are no resistant types of beans to root rot, and differences in variety differences is currently underway.

Source: Fieldcropnews