Farm406

Pythium root rot is caused by the oomycete (water mold) Pythium spp. It is characterized by poor root system development, and can be difficult to diagnose. In general, roots will be brown in color and the outer cortex will peel off of the inner core of the root (Figure 3). Pythium is very common in the soil and for this reason it is recommended that all seed treatments contain metalaxyl or mefanoxam. Fusarium root rots are caused by a number of different Fusarium species. Disease is favored by cool, wet springs followed by drought, fluctuating water conditions, and high soil nitrogen levels. Belowground symptoms include brown to reddish-brown discoloration and lack of secondary (small) roots. Management recommendations include fungicide seed treatments and good weed control. e severity of the infection oen depends on the previous crop history and species of Fusarium present in the soil. For example, F. graminearum, which causes root and crown rot in wheat, does not infect pea very well. F. redolens, which is decimating to peas, can be found on wheat at low levels causing root rot, but is not a major pathogen of wheat. Rhizoctonia root rot is caused by the fungus Rhizoctonia solani. It is first diagnosed by poor or declining stands. Root development is poor and roots are generally black and so (cover photo, top). Rhizoctonia root rot is very common where volunteer grain and weeds were sprayed with herbicide, particularly glyphosate, within days of planting. e fungus grows quickly in the dying plants and reaches a high population which then aacks the new seedlings. is is called the 'green bridge.' e green bridge can be prevented by delaying seeding two to three weeks aer herbicide application to allow the plant tissue of treated plants to decompose. Other management practices include the use of a fungicide seed treatment for good plant establishment. Aphanomyces root rot symptoms are similar to those of Pythium root rot. Oospores of Aphanomyces euteiches can survive in the soil up to 20 years. e pathogen can cause pre- and post-emergence damping off, and cause root rot in seedlings or older plants as long as soil conditions remain cool and moist. Genetic resistance is limited. Phosphorous acid has been shown to be effective in greenhouse and some field trials to manage this disease on peas in the Pacific Northwest when applied as a foliar application soon aer emergence (Porter et al. 2009). FIGURE 3. Pythium root rot causes brown, reddish or black discoloration of the roots and reduces the number of secondary roots. DISEASE PROBLEMS DURING THE GROWING SEASON Fusarium wilt is caused by Fusarium oxysporum. Subspecies are specific to the crop (ie. F. oxysporum f. sp. pisi on pea, F. oxysporum f. sp. lentis on lentil, and F. oxysporum f. sp. ciceris on chickpea). ere can be several different races of the pathogen within each subspecies, which makes breeding for resistance challenging. However, there is resistance to race 1 and race 2 of Fusarium wilt in many pea varieties. F. oxysporum can infect the plant during any stage of growth. e pathogen is very long lived in the soil (chlamydospores) and can increase in a field each time you replant the suscep- tible crop. For that reason, it is important that wilt-affected fields are entered last, and equipment is cleaned aer each field operation. is is especially important for operations where there is soil contact, such as at planting. e pathogen can also be carried on plant materials such as stubble, and in wind and water. ey may also be introduced on seed. e only econom- ical control is to plant resistant varieties, which are available for lentil and chickpea. Early planting allows the crop to grow when soil temperatures are below those optimal for disease development. Seed treatments to encourage establishment and plant vigor are recommended. A minimum of four years or more between crops is advised to curtail progress of this and other soilborne root diseases. VASCULAR WILTS Photo: Mary Burrows, Montana State University 51 farm406

• Cover