ETIOLOGY OF ASPARAGUS REPLANT-BOUND EARLY DECLINE

Asparagus replant-bound early decline (ARED) was characterized and its etiology was elucidated in experiments under greenhouse and field con ditions. Selective soil treatments were used to differentiate between autotoxic compounds and soil-borne pathogens as causal agents. In gree nhouse experiments, there were symptoms of ARED within 12-15 weeks. As paragus plants grown in soil formerly used for asparagus (asparagus so il) showed brown lesions on primary and secondary roots, and many seco ndary roots had rotted. Root weights of plants grown in asparagus soil were lower than those of plants grown in fresh soil. Fusarium oxyspor um f. sp. asparagi (Foa) was by far the most common species among the fungi isolated from roots with lesions. Under greenhouse and field con ditions, there were similar symptoms, which indicates that the results obtained under greenhouse conditions are similar to those in the fiel d. The vertical distribution of the ARED-causing factor(s) was studied in a greenhouse experiment in which plants were grown in soil from th ree layers: 0-30, 30-60, and 60-90 cm. For all four asparagus soils te sted, there were ARED symptoms and similar disease severity in samples from all three depths. The causal factor persisted at least 11 years after soil was no longer used for asparagus. When asparagus soil was d iluted with fresh soil to give mixtures with 100%, 80%, 50%, 20% and 0 % asparagus soil, disease severity did not decrease with increasing di lution of the asparagus soil from 100% to 20%. Disease severity of all mixtures with asparagus soil was significantly higher than that for f resh soil. The results imply that ARED is caused by a pathogen coloniz ing the soil rather than inhibition by autotoxins released from residu es of the preceding asparagus crop. This conclusion is supported by th e results of greenhouse and outdoor experiments with heat and fungicid e treatments of soil. ARED was nullified by heat treatments of 30 min at 55 or 60 degrees C but not 45 and 50 degrees C, eliminating autotox ins as an important cause of ARED because they are heat-stable. Foa is eliminated by a 30-min soil treatment at 55-60 degrees C but not 50 d egrees C. Prochloraz, known for its toxicity to F. oxysporum, also nul lified ARED. Disease severity level was related to the density of Foa in soil. The results provide conclusive evidence that F. oxysporum f. sp. asparagi is the main cause of ARED in the Netherlands, which large ly removes the need to discriminate between early decline and replant- bound early decline, because Foa is the main cause of both diseases.