Preceding crops affected soybean iron-deficiency chlorosis and vesicular-arbuscular mycorrhizal fungi in soybean cyst nematode-infested fields

Iron-deficiency chlorosis (IDC) and soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) are important factors limiting soybean [Glycine max (L.) Merr.] yield. Vesicular-arbuscular mycorrhizal fungi (MF) are generally beneficial to crops. The effect of preceding crops with known variation in MF colonization was tested in two SCN-infested fields in Waseca and Lamberton in Minnesota to determine the effect of MF and other soil biotic and abiotic factors on IDC. No crop effect on soybean yield was detected. There was no difference in SCN population density following nonhost crops at Lamberton, but sunn hemp (Crotalaria juncea L.) resulted in lower SCN population density than corn (Zea mays L.) at Waseca. Chlorosis rating and colonization frequency of soybean roots by MF were lowest following canola (Brassica napus L.) and sugar beet (Beta vulgaris L.), the two nonmycorrhizal crops. Soybean had lowest number and weight of N-fixing nodules following soybean, highest following corn, and intermediate following other crops. Iron-deficiency chlorosis was positively correlated with MF and negatively correlated with nodulation and soil Fe at both sites. The correlations of SCN, other soil elements, and soil penetration resistance with IDC were either insignificant at both sites or inconsistent across the sites. No consistent correlation of soil factors with MF was detected. This study suggests that a causal relationship between MF and IDC might exist, but it requires further exploration.