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Sunn hemp (Crotalaria juncea L.) is a warm‐season legume often used as a cover crop; however, there is limited information about nutrient cycling of sunn hemp residue. The objective of this study was to evaluate the effect of sunn hemp genotype and residue management on plant characteristics, decomposition, and suppression of Fusarium, Pythium, Sclerotinia, and Sclerotium microbe species. Two field experiments were conducted in Ona, FL. In Experiment 1, treatments were the split‐plot arrangement of three sunn hemp genotypes (Ubon, Blue Leaf, and 'Tropic Sun'; main plot) and two biomass managements (removal or leaving the biomass on the field; subplot), distributed in a randomized complete block design. There was a greater biomass accumulation in 2019, but with no differences among genotypes (mean = 4,431 kg DM ha−1). Blue Leaf had greater N concentration than Ubon (23 vs. 19 g kg−1) and greater suppression of the Phytium populations than Ubon and Tropic sun in 2019 (0.9 vs. 2.0 colony forming unit g soil−1). In Experiment 2, the litter bag technique was used to estimate biomass and N decomposition. Treatments were the factorial arrangement of three genotypes and two residue management strategies (incorporated into the soil or placed on the soil surface). Sunn hemp biomass incorporation enhanced organic matter disappearance and N mineralization rate but leaving sunn hemp on the soil surface may result in a steadier N supply for the subsequent crop. Core Ideas: Sunn hemp genotypes differed in chemical composition.The genotype Blue Leaf most effectively suppressed Pythium populations in the soil.Sunn hemp residue management (surface vs. incorporated) affected decomposition.Genotypes did not differ in residue organic matter and N decomposition. [ABSTRACT FROM AUTHOR]