Factors Affecting Alfalfa Stand Longevity in Montana

A. Statewide Monitoring: A large amount of data were compiled from the 12 producer field sites. Arthropod populations: Significant numbers of alfalfa weevil larvae, pea aphid and other insect pests were found at some sites during the growing season, however rarely above established economic thresholds for control (Table 2, Appendix Table 2). Producer awareness about the losses of first-harvest alfalfa due to alfalfa weevil larvae has increased in Montana during the past decade. Interestingly, two growers applied insecticide in all three years and one grower in 2006 and 2007, despite all being below the threshold for alfalfa weevil larvae.

Among beneficial arthropods, there were significantly higher (P< 0.10) ladybird beetle populations in irrigated fields and monoculture alfalfa stands than in dryland or mixed stands, respectively (Table 2). There were higher (P< 0.10) pea aphid numbers in irrigated fields than in dryland stands, which could explain the differences in ladybird beetle populations in irrigated fields. Populations of plant bugs and alfalfa caterpillar were higher P< 0.10) in monoculture alfalfa stands than in alfalfa-grass mixes (Table 2). Across years, three alfalfa fields that were sprayed with insecticides (2005 – 2007 or 2006 – 2007) for alfalfa weevil larvae control subsequently had reduced (P< 0.10) numbers of CRC adults and pea aphid compared to untreated fields (Table 2). Insecticide application tended to reduce CRC adult numbers each year (NS), and across years (Table 3). Interestingly in the insecticide-treated fields, there were no significant reductions in the number of alfalfa weevil larvae and adults (target pest) or other pest or beneficial arthropods as compared to the untreated fields. The temporal and spatial variability of insect populations among these 12 sites preclude many global conclusions, however the trends noted will be of interest for producers of dryland or mixed stands of alfalfa-grass. The CRC was detected at all sites, though not in all years at each site (Table 3). Fairly high numbers were noted in Phillips County (site 10) in all years. This is the first report of widespread distribution of the CRC in Montana. All cooperators in this project manage large acreages of alfalfa, so infestations of new stands by the CRC would appear to be fairly common across these counties. All of the notable foliar and root diseases in Montana were observed in the statewide monitoring project (Table 4). Alfalfa mosaic virus (AMV) has not been widely monitored in the state, but was confirmed by the DAS-ELISA tests in all fields except site 2 (Rosebud2). Average infection by AMV approached 20% by the end of 2005 (Table 4). By the completion of the trial in 2007, AMV infection was confirmed in 56% and 10% of the plants in irrigated and dryland fields, respectively (P< 0.10). The pea aphid is a vector for AMV, so these differences likely reflect the higher aphid numbers in irrigated alfalfa fields. The foliar symptoms are quite classic, however there was not much agreement in visual observations and results from the DAS-ELISA test (Table 5). For plants with visual AMV symptoms, DAS-ELISA confirmed that 20 to 65% of the plants were AMV-infected. Conversely, 14 to 30% of visually asymptomatic plants were infected with AMV according to DAS-ELISA. These results differ from those of Martin et al. (1997), who reported that 100% of all symptomatic red clover plants infected with AMV under laboratory conditions tested positive for AMV, and all symptomless plants tested negative. These data are the first statewide confirmations of AMV incidence across Montana. The virus is seed-borne and the majority of fields likely were initially contaminated with AMV through the planting of infected seeds. Once in the field, the virus spreads from infected plants to healthy plants by sap transmission or by plant fertilization, either through insect feeding or harvesting machinery. The insect most commonly associated with transmitting the disease is the pea aphid. Crop losses due to AMV are not well described, but forage yields in alfalfa are generally reduced due to reductions in nitrogen fixation and, and increased energy consumption through increased respiration. Spring blackstem SBS was found at all sites, ranging from 34 to 93% incidence in the spring and 12 to 40% in the fall (Table 4). Spring incidence of SBS was generally higher in irrigated stands than in dryland stands, but this difference was only significant (P< 0.10) in 2005. The disease was present at similar levels in both monoculture alfalfa and mixed stands. This disease is widespread across Montana in the early summer, though losses to this disease are not clear. Crown rot symptoms of alfalfa increased from 9% in Fall 2005 to 73% affected plants in Fall 2007, which is similar to previous evaluations in Montana. Average severity (ASI) of crown rot symptoms across the 12 sites was 1.80 in Fall 2007, which indicates low to moderate infection. There were no differences in crown rot incidence or severity among irrigated vs. dryland stands or monoculture vs. alfalfa-grass mixed stands (Table 4). Root rot incidence increased from 21% in Fall 2005 to 85% in Fall 2005 (Table 4). The incidence of root rot symptoms was higher (P< 0.01) in irrigated stands than dryland stands in the Fall of 2005 and 2007, so it appears that irrigation impacts disease development by the root rot organisms. Average root rot severity (ASI = 1.45)) was low in Fall 2007, and there were no differences related to irrigation or stand types. Clover root curculio (CRC) root feeding damage increased more rapidly over time than the crown and root rot symptoms (Fig. 3). Average incidence of CRC damage increased from 7% affected plants in Spring 2005 to 98% in Fall 2007 (Table 4). The incidence and severity of CRC were generally higher in irrigated stands than in dryland stands, however these differences were only significant (P< 0.10) for ASI in Fall 2005. Interestingly, grass-alfalfa stands tended to have higher incidence and severity (ASI) of CRC damage than monoculture grass stands, however these differences were not significant in all years. It is not known if the grasses in a mixed stand act as preferred hosts or beneficial habitat for CRC oviposition, or simply the reduced alfalfa density per unit area concentrates CRC feeding on fewer plants. A further difference was noted in CRC root feeding damage in fields where insecticides had been sprayed to control alfalfa weevils. Insecticide-treated fields had lower (P< 0.10) incidence and severity of CRC feeding in Fall 2005 and lower severity in Spring 2006 than untreated fields, though these differences diminished by 2007 (Table 6). These CRC feeding damage results generally agree with the reduced CRC numbers in sweep samples following insecticide application (Table 3). The timing of CRC seasonal migration and egg laying in Montana are not clearly understood, and the efficacy of controlling adult CRC with insecticides is questionable. However these data indicated that insecticides or other control strategies may have potential for reducing damages from this pest. Interactions of CRC with root diseases: The overall increase in crown rot, root rot and CRC damage indicated a positive correlation among these pests (Fig. 3). Leath and Hower (1993) reported a positive relationship among crown and root rot in alfalfa and the activity of CRC. Kalb et al. (1994) also found highly significant, positive linear correlations between CRC feeding predisposing alfalfa plants to more severe crown and root rot especially to Fusarium spp. In this statewide pest survey, 240 individual plants were scored for the severity of crown rot, root rot and CRC feeding damage in 2007. Regression analyses indicated that there were limited relationships in CRC feeding damage vs. root rot severity (R2 = 0.0135) or crown rot severity
(R2 = 0.000, Fig. 4). Many plants had severe root or crown rot symptoms independent of CRC feeding damage (Fig. 5).

These results contrast with those of previous reports (Leath and Hower, 1993, Kalb et al., 1994). The crown and root rot complexes in Montana have not been fully identified, but Fusarium spp. are routinely present in diagnostic samples received to MSU. Most of the cultivars evaluated in this trial have a high level of resistance to F. oxysporum, however resistance to other Fusarium spp. or other local root and crown rot pathogens is not known. The stands at the 12 Montana sites were monitored in the Spring and Fall through three years of production post-planting. Symptoms of crown rot, rot and CRC feeding damage were mild to moderate at the end of the trial, and perhaps more significant relationships between CRC and root disease symptoms would develop over time with more severe disease symptoms. However, at the termination of this trial there appears to be no evidence of a cause-and effect relationship between CRC feeding sites and increased root or crown rot disease development.

Soil texture and CRC: During this statewide pest survey, soil samples (0 to 15 cm cores) were obtained from the 12 sites and tested for texture, organic matter, pH and salinity. Regression analyses were computed with sand and clay contents compared with the average 2007 severity scores for crown rot, root rot and CRC feeding damage. CRC damage was negatively associated with sand concentration (R2 = 0.2632) and positively associated with clay concentration (R2 = 0.6118, Fig. 6). These data agree with the report by Pacchiolo and Hower (2004), and it appears that clay content is fairly predictive of potential CRC feeding damage. Alfalfa is grown on a large acreage in Montana with heavy-textured soils, so these results will be of interest to many producers.

Alfalfa yield and stand dynamics: Stands declined from an average of 10.9 plants per 0.1 m2 (Spring 2006) to 3.7 plants in Fall 2007 (Table 7). Irrigated stands had higher (P< 0.10) plant populations than dryland stands in Spring 2005 and in 2006, but these differences diminished by 2007. These data are similar to previous stand observations in Montana where stands declined from about 200 plants per m2 in the fall of the establishment year to about 50 plants per m2 in the third production year. Under ideal growing conditions and in the absence of pests, there is tremendous interplant competition that leads to significant plant mortality in a young alfalfa stand. Dryland stands are planted at lower seeding rates than irrigated stands because the rainfed environment will support a lower final alfalfa population. Stems per 0.1 m2 declined from an average of 55 in Spring 2005 to 32 in Fall 2007 (Table 7). Irrigated stands had significantly more (P< 0.10) than dryland stands in 2005, Spring 2006 and Fall 2007. Stem numbers per unit area (comparing spring to spring or second cutting to second cutting across years) typically remain fairly constant in the early years of an alfalfa stand. Stems per plant were variable, ranging from 4.8 to 11. The stems per plant and apparent decline in stem densities in this trial could reflect an interaction of sampling dates with environmental or management practices across the 12 sites evaluated. Alfalfa crown diameters increased from an average of 4.8 mm in Spring 2005 to 12.9 in Fall 2007 (Table 7). Crown diameters did not vary between irrigated and dryland sites, but alfalfa in grass-mixed stands had larger (not always significant at P< 0.10) crowns than monoculture alfalfa stands. These data reflect differential alfalfa root development rates in mixed stands. Annual hay yields of irrigated stands were 5.3 to 5.9 tons per acre, compared to 1.4 to 1.9 tons per acre on dryland (P< 0.10, Table 7). In 2005, alfalfa–grass mixed stands (6.0 tons per acre) had higher yields than monoculture alfalfa stands (4.8 tons per acre, P< 0.10), but over time these differences diminished. Percent weed cover varied considerably across sites. Dryland stands had higher weed populations than irrigated stands, but these differences were only significant in Fall 2005 and Spring 2006. The difference in weed populations in irrigated vs. dryland is interesting, however since all stands evaluated were planted in 2004, these results may strictly be related to the year of establishment. Producers planting dryland alfalfa always face considerable risks. Producers in this project used a variety of accepted crop rotation and herbicide strategies prior to alfalfa establishment. Only two producers (one irrigated, one dryland) used herbicides after planting, and clipping or haying during establishment is often fairly successful for weed control. At low production potentials, it is unlikely that dryland alfalfa producers will routinely invest in annual herbicide applications, so alternative weed control strategies are employed. Prediction models for of alfalfa yield: Several predictive models have been developed to estimate alfalfa yield based on plants or stems per unit area. These models are most useful for producers early in the spring to make the decision to terminate a stand that is not economically viable to maintain. Models to predict alfalfa yield based on stand density have not been evaluated in Montana. Data from the statewide monitoring trials were analyzed to determine the relationships between seasonal hay yield and spring densities of plants or stems per 0.1 m2. Prior to regression analyses, the plant data were converted to plants per square foot to compare with existing models, and for potential use by producers. Spring counts of plants per square foot provided poor prediction of seasonal yields, with R2< 0.03 for both dryland and irrigated stands (Fig. 8). The low relationships could be due to the limited numbers of samples (all three years of dryland stand data were included) or the young age of the stands. In previous unpublished research trials, yield predictions based on plants per unit area have not proven to be accurate in Montana, and are not applicable for dryland alfalfa. For example, ‘Ladak 65’ alfalfa has very broad crowns, and in many cases can sustain reliable forage yields with plant populations of one plant per square foot, as in this trial. Stem count in spring has been reported to be a good predictor of subsequent seasonal forage yield in Wisconsin by Cosgrove and Undersander in 1992 (cited in Undersander et al., 1998). In the 2005 through 2007 statewide monitoring sites, the University of Wisconsin model overestimated forage yield at dryland sites (Fig. 8). A new predictive equation from the Montana dryland data was developed (Fig. 8) with fair fit between stem counts per square foot and hay yield (R2 = 0.26). In the irrigated statewide data, only 2006 and 2007 were used due to extremely high stem counts at two locations (sites 5 and 6) in 2005. For irrigated sites, regression of yield on stem counts resulted in very poor fit. This is reflected by four irrigated site-years with seasonal yield of about 5 tons per acre that had spring counts of 12, 28, 57 and 77 stems per square foot (Fig. 8). From these results, it appears that existing models to predict alfalfa forage yield from stand density have limited applicability to Montana stands of less than four years in age. Likely, more research in older stands will be required to develop accurate prediction models. Dryland alfalfa yields were positively related to growing season precipitation in 2005 through 2007 (Fig. 9). The marginal relationship (R2 = 0.1117) could likely be improved by calculating precipitation from the previous autumn, however inadequate reliable data were available. Within a dryland site, annual hay yield is closely related to annual precipitation. These data evaluated across alfalfa stands suggest that other factors such as soil type, pests and grower management also impact alfalfa yield. B. Nurse Crop Trial: Dryland alfalfa stands at the Moccasin, MT site declined from an average of 11 plants per 0.1 m2 in the fall of the establishment year (2004) to 4.2 in 2007 (Fig. 10). Alfalfa seeded alone had significantly (P< 0.05) better stands in 2004 and 2005 than the average of all nurse crop treatment combinations, however these differences diminished for the duration of the trial (Fig. 10). Forage yields in July 2005 were affected by an interaction in companion crops and their seeding rates (Table 8). Alfalfa planted with spring wheat (high seed rate) harvested for hay had significantly higher yield in 2005 than alfalfa planted with hay barley (high rate) or barley for grain (high rate, Table 8a). Interestingly, alfalfa planted with nurse crops at high seeding rates tended to have higher alfalfa yields in 2005 than lower seeding rates, however this difference was not significant. In spring 2006, alfalfa stands planted with a nurse crop planted perpendicular to the alfalfa were superior (P< 0.05) to those planted in-furrow (Table 8b). There was an interaction of orientation with nurse crops which was likely due to differential slopes in the response. Statistical differences among planting orientation (in-furrow vs. perpendicular with alfalfa), nurse crops or monoculture vs. nurse crops (averaged over seed rates and orientation) were not detected for the majority of stand, crop height or weed measurements made for the duration of this study. From these data, it would be premature to conclude that nurse crops have minimal effects on dryland alfalfa establishment and longevity. This type of trial should be repeated over more dryland sites and planting seasons with collection of alfalfa yield data to be confident for grower recommendations. In the meantime, our current recommendations of planting nurse crops that are less competitive (wheat), planting the nurse crop at an angle or perpendicular to the alfalfa, and harvesting the nurse crop as forage would appear to be viable. Reduced seed rates of the nurse crops would also be more economical than high seeding rates. C. Cultivar and Harvest Management Trials: In 2003, replicated plots of 18 alfalfa varieties grown in MAES trials planted near Bozeman and Huntley, MT were evaluated for crown rot and CRC damage. There were significant differences among varieties for crown rot symptoms at Bozeman in 2003 (Table 9). The cultivar ‘4200’ (ASI = 2.29) had significantly lower crown rot ratings than six other cultivars at Bozeman. A majority of the 18 varieties were released since 1995, and are reported to have high levels of resistance to F. oxysporum and numerous root diseases http://www.alfalfa.org/pdf/NAFA%2007-08%20Varieties%20Leaflet.pdf, but there did not seem to be any correlation in this information and the crown rot symptoms we noted. ‘Ladak 65’ is the oldest cultivar tested in these trials. This variety (released in 1965) predates most of the standard tests for disease resistance. Only four cultivars tested had lower crown rot severity ratings than Ladak 65 (ASI = 2.84) at Bozeman. There were no differences among cultivars for CRC feeding damage at both locations or for crown rot at Huntley (Table 9.) Crown rot ratings among locations were correlated across locations (r= 0.428), and fourth-year forage yields were negatively correlated to crown rot severity at Bozeman (r= -0.428) and Huntley (r= -0.302). Across cultivars, average CRC damage at Bozeman (ASI = 4.55) was more severe than at Huntley (ASI = 3.25, P< 0.001). Although there were no differences among cultivars for CRC feeding damage, about 58 plants (of 5827) at Bozeman and 142 plants (of 720) at Huntley at CRC root damage scores of 1. These plants were selected for breeding purposes, and if CRC tolerance is a heritable trait, it should be possible to develop cultivars with increased tolerance. In the harvest management trials at Bozeman, 52 cultivars were evaluated under a regime to evaluate the effects of a poorly timed third seasonal harvest. Across the three trials, fourth-year and total alfalfa yields were significantly reduced by the September 1 harvest date (Table 10). The fourth-year forage yields (as a measure of cumulative treatment effects) of the intensive treatment were 39, 15 and 21% lower than resulted from the normal third harvest date in the 2000, 2001 and 2002 trials, respectively (Table 10). The 2000 trial received the intensive cutting treatment in the year of establishment (2000) and the two subsequent years, whereas the 2001 and 2002 trials had normal harvest management in their first year. The “intensive” harvest schedule imposed in the 2000 trial matches the protocol published to evaluate space-plants of cultivars for winter survival (http://www.naaic.org/stdtests/wintersurvivalnew.htm). Interestingly in these 2000 – 2002 trials, there were no significant differences among cultivars of known winter survival for differential yield effects; i.e. the poorly-timed harvest resulted in similar (NS) percent yield losses in all cultivars. Alfalfa stand density and root/crown biomass were reduced, and weed biomass increased as a result of the intensive late summer harvest treatment, although these effects were not always significant (Table 10). Many of the weeds were winter annuals that likely invaded the stands during the fall when there was adequate soil moisture and limited crop canopy. Interestingly, there were significant (P< 0.05) increases in the severity of crown rot and CRC damage in the intensively-harvested plots in the 2000 trial (Table 10). It is possible that the increased stress of alfalfa plants related to the harvest regime was responsible for the increased pest symptoms. This trial should be conducted again to determine if these results are repeatable, as there are no previously published reports of alfalfa cutting schedules impacting CRC feeding damage. It is clear that late summer harvest schedule impacts yield and longevity of irrigated alfalfa stands. Recommending a critical “no harvest” period is difficult because the decision is made well ahead of freezing conditions, which vary widely among years and locations in Montana. It is assumed that poorly-timed grazing by livestock and wildlife (deer, elk, antelope) would have similar effects. No data are available for dryland alfalfa, however if adequate summer rains resulted in significant regrowth for a second cutting, it is likely that the same impacts would be noted. Of the 24 third harvests in the statewide monitoring trials (eight irrigated sites x three years), 12 (six of eight growers) were harvested during the critical late summer period. It is apparent that a significant level of education for growers is needed on this topic. Aside from the severe economic losses in hay production from poorly-timed harvest, these data indicate that harvest management has critical implications on stand longevity and crop rotation. Nitrogen credits from alfalfa stand termination and weed problems should be considered by alfalfa producers in their farming plans for long-term sustainability. D. Insecticide Trials: Mixed results were obtained in the single-application insecticide trials at two locations. In The Amsterdam 1 site, average CRC root feeding damage following Furadan application (ASI = 1.13) was significantly lower than the control (ASI = 1.79, Table 11). At the Amsterdam 2 site, there was no statistical difference in CRC feeding damage between the control (ASI = 1.38) and Furadan-treated (ASI = 1.35) plots (Table 12). In the Amsterdam 2 trial, the average incidence of CRC root feeding damage increased from 10% in the spring to 88% on 3 October 2005 when the trial was terminated. In the same period, CRC adults collected were 0.3 per 10 sweeps and the spring and 11.5 per sweep in the autumn. Based on the rapid CRC symptom development at the Amsterdam 2 site in 2005 and the different results in these two trials, it appears that the timing of insecticide application is critical. Further, the migration of CRC adults after insecticide application from untreated areas (the majority of the field and control plots) and the short duration of Furadan activity may confound the results of these plot trials. Trials of multiple-applications of insecticide were conducted at two locations. In the Huntley trial, the initial Furadan application was sprayed on the Fall and Spring/Fall treatments during the year of establishment. Very few CRC adults were collected throughout the trial (Table 13). The incidence of CRC root damage increased from 7% in May 2004 to 100% in May 2005. All Furadan treatments had lower (P< 0.05) CRC feeding damage severity compared to the control in May 2005, however these differences diminished by September 2005 (Table 13). The severity of CRC feeding damage declined in Spring 2006 (Fig. 11), but this is likely due to variability in scoring technique. In May 2006, CRC root feeding severity in Spring and Spring/Fall insecticide treatments were significantly lower than the control and Fall treatments (Table 13). There were no differences among treatments for stand density or the numbers of other insect pests in this trial (Table 13). The results from the Huntley site suggest that early (year of establishment) and repeated insecticide applications may slow the progression of CRC damage in an infested site. However since the insecticide treatment dates were not continuous across years (Furadan treatments did not occur in Fall 2005), it is unclear if the effect of the effect of the insecticide diminished or there are continual migrations and re-infestations by CRC. In the Bozeman trial, the Furadan treatments were initiated in October 2005 during the planting year, and the insecticide treatments were continuous throughout the trial (Table 14). The incidence of CRC root feeding damage increased rapidly from 5% in May 2006 to 85% in October 2006. The severity of CRC root feeding damage in the control (ASI = 3.00) was significantly higher than all Furadan treatments (ASI mean = 1.90) in May 2007 (Table 14). There were no statistical differences among treatments for stand density or numbers of CRC adults and other insect pests sampled during the trial. Previous literature includes reports of varying effectiveness of insecticide treatments in reducing root feeding damage by CRC. Dickason et al. (1968) found no significant differences in alfalfa yield or plant mortality following insecticide treatments to control CRC. In contrast, Neal and Ratcliffe (1975) reported that one early season application of granular carbofuran (Furadan) reduced CRC larval feeding and resulted in significantly higher levels of regrowth as compared to the control. The results from the Amsterdam 1 and Amsterdam trials had varying results similar to published single-application insecticide trials. Very little research has been published evaluating CRC dynamics over several years with repeated insecticide applications. In the multiple-application trials (Huntley and Bozeman), Furadan treatments were initiated in the year of alfalfa planting. The results from these insecticide trials indicate that alfalfa root damage by the CRC can be delayed but likely not eliminated. Evaluation of these trials over a longer period would be beneficial. While the results from these insecticide trials are promising, much more research is needed in Montana. Fairly low adult CRC numbers were present in sweep samples taken in the spring and fall at the time insecticides were applied. Without a thorough understanding of the CRC life cycle and biology, insecticide application is very haphazard. Specifically, simple enumeration methods for CRC larvae in field soil and models for peak CRC oviposition and larval activity based on heat units or date would be very useful. Data from the insecticide trials indicated that the timing of Furadan application is critical. Likely, in some instances the carbofuran was ingested by larvae which may have reduced immediate feeding damage. However, in all cases the positive effects of insecticide treatments appeared to diminish over time. The efficacy, economics and sustainability of multiple applications of insecticides on alfalfa hay are also in question. Furadan was clearly a useful research tool in this project, however it is highly unlikely that multiple insecticide applications over multiple years of an alfalfa stand would be a desired practice. Literature Cited: Bright, D.E. 1994. Revision of the Genus Sitona (Coleoptera: curculionidae) of North America. Annals Entomol. Soc. Am. 87(3): 277-306. Dintenfass, L.P. and G.C. Brown. 1988. Influence of larval clover root curculio (Coleoptera: Curculeonidae) injury on carbohydrate root reserves and yield of alfalfa. J. Econ. Entomol. 81: 1803-1809. Dixon, P., S.D. Cash, J. Kincheloe and J.P. Tanner. 2005. Establishing a successful alfalfa crop. Mont. St. Univ. Ext. Montguide 200504. http://www.animalrangeextension.montana.edu/articles/forage/alfalfa/alfalfa_est.htm Godfrey, L.D. and K.V. Yeargan. 1989. Effects of clover root curculio, alfalfa weevil (Coleoptera: Curculeonidae), and soil-bourne fungi on alfalfa stand density and longevity in Kentucky. J. Econ. Entomol. 82: 1749-1756. 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Phillips, W.G. and L.P. Ditman. 1962. Studies on biology and economic importance of the clover root curculio Sitona hispidulus (Fab.) in Maryland. MD Agric. Exp. Stn. Bull. A-121. 21 pp. Tan, Y. and A.A. Hower. 1991. Development and feeding behavior of clover root curculio (Coleoptera: Curculeonidae) larvae on alfalfa. Environ. Entomol. 20: 1013-1018. Undersander, D., C. Grau, D. Cosgrove, J. Doll and N. Martin. 1998. Alfalfa stand assessment: is this stand good enough to keep? University of Wisconsin-Extension, Cooperative Extension. Publication#A3620. Wunsch, M.J. and G.C. Bergstrom. 2006. Brown root rot of alfalfa caused by the fungal pathogen Phoma sclerotioides. Report of survey results. Dept. Plant Pathology, Cornell University, Ithaca, NY.