Incidence of Fusarium Root Rot in Asparagus Seedlings Infected with Arbuscular Mycorrhizal Fungus as Affected by Several Soil Amendments.

Seedlings of asparagus (Asparagus officinalis L., cv. Mary Washington 500 W) were inoculated with arbuscular mycorrhizal (AM) fungus (Glomus sp. R10) and grown in the presence of different soil amendments : carbonized chaff (CC), coconut charcoal (CO) and manure of coffee residue (MC) mixed with the soil in the ratio of 1 : 9 (v/v, 10% plot) or 3 : 7 (30% plot). Nine weeks later, the seedlings were exposed to fusarium root rot, caused by Fusarium oxysporum f. sp. asparagi (Foa) to test their tolerance. When asparagus seedlings were examined 13 weeks after Foa inoculation, incidence of fusarium root rot reached 90-100% in all the AM fungus-noninoculated plants, irrespective of the soil amendments. However, the incidences of root rot were 14.3% in the 30% CO and 20% in the 10% MC plots among the AM fungus-inoculated plants. In the non-AM inoculated plants, the disease indices reached 92.5 in the soil only plot and 48.9 in the 30% CC plot. As for AM-treated plants, the indices attained remarkably low values of 2.9 in the 30% CO and 4.0 in the 10% MC plots. More storage roots developed in the AM-treated plants than in the control, whether they were diseased or healthy. AM fungal infection levels were higher in the root systems of 10% and 30% CO and 10% MC plots than in the other plots ; little difference occurred among 10% and 30% CO and 10% MC plots. Furthermore, the infection area of the AM fungus in a root system was most extensive in the CO plots. These findings suggest that the addition of coconut charcoal or manure of coffee residue to bed soil is effective for increasing the tolerance to fusarium root rot in AM fungus-infected asparagus plants, though the effect differed with the quantity of the soil amendments.