Identification and etiology of Fusarium spp. associated with asparagus crown disease in southern California and northern Mexico

Abstract

Considering the economic importance of asparagus as a crop and the historical association of Fusarium spp. as a principal cause of stand decline of this crop, a study was conducted from 1995-1997 in Southern California and Northern Mexico. The main objectives were to determine the causal agent of asparagus crown rot and the study the etiology of the causal agents which affects asparagus spears in these two important growing regions. Asparagus crowns exhibiting symptoms of crown decay were selected from each of the above production regions and processed in the laboratory. Based on morphological characteristics, F. proliferatum and F. oxysporum were the dominant species isolated from crowns. F. proliferatum produced mono and polyphialides and conidia in long chains. F. oxysporum was distinguished by the production of chlamydospore and conidia not produced in chains. Both species were recovered from marketable spears with an incidence ranged from 20-90%. Pathogenicity test on asparagus seedlings with isolates of F. proliferatum and F. oxysporum obtained from spears were positive. To determine the source of spear infection in commercial asparagus plantings, crowns and spears were collected from two fields in the Imperial Valley of California. Both F. proliferatum and F. oxysporum were recovered from crown tissues and from spears. However, F. proliferatum was the most prevalent species of Fusarium isolated from both spears and crowns. Evaluation of the influence of Fusarium species on quality characters of marketable asparagus was also studied. The quality of marketable spears infected with F. proliferatum and F. oxysporum was found to decrease significantly as the length of storage increased from five to ten days and as temperature of storage was increased from 5 C to 26 C. Since some species of Fusarium are known to produce fumonisins (a mycotoxin), and investigation of the possible presence of fumonisins in commercial asparagus spears was conducted. Spears were obtained from two different geographic regions of Mexico and two in California. Spear samples naturally colonized by Fusarium spp. were analyzed for fumonisin B1, B2 and B3. No detectable levels of fumonisin, regardless of geographic location of samples, were founded.