Factors Affecting Cover Crops for Nematophagous Fungi Enhancement

1) Indigenous Nematophagous Fungi Present in the Soil

Performance of sunn hemp in enhancing nematophagous fungi was consistently promissing in a series of the experiments using pineapple soils in Hawaii (Wang et al., 2001, 2002, 2003). However, when effect of sunn hemp was examined in soils with distinct differences in organic matter content, performance of sunn hemp for enhancement of nematode-trapping fungi (NTF) varied. In Florida, sunn hemp increased NTF population densities in the soil that was rich in organic matter (HYW) but not in a same series of soil with lower organic matter (NYW) (Fig. 5-1). However, in another soil (Expt. II), sunn hemp increased endoparasitic fungal population densities even though this soil had low organic matter (Fig. 5-1). These results indicated that performance of cover crop depends on the species of nematophagous fungi present in the soils.

Soil	Initial organic matter content (%)
HYW	8
NYW	2.5
Expt II	2

Fig. 5-1.Population densities of nematophagous fungi in three soils (HYW, NYW, and Expt II) with different organic matter contents. Columns with different letters indicate difference between sunn hemp treatments according to analysis of variance (P < 0.05). Cj+ and Cj- indicate with and without sunn hemp amendment respectively (Wang and McSorley, unpublished).

2) Time after Nematicides Application

Some nematicides might be suppressive to activity of nematophagous fungi. Population densities of parasitic NTF were reduced in field recently treated with 1,3-dichloropropene (1,3-D) (soil D205, Fig. 5-2) (Wang et al., 20023). Ability of sunn hemp to increase population densities of parasitic nematode-trapping fungi (NTF) measured by an enhancement factor (Fig. 5-2) increased as the time after the last 1,3-D treatment occurred, except on MP282 soil which had a different soil texture than the other soils. A high enhancement factor in the 10-years fallow soil indicates that enhancement of NTF is not due to the planting of pineapple.

Fig. 5-2. Ability of sunn hemp to enhance population densities of parasitic nematode-trapping fungi (NTF) as measured by an enhancement factor in six pineapple field soils with different time periods after the last 1,3-dichloropropene (1,3-D) application. Cj+ and Cj- indicate with and without sunn hemp amendment respectively (Wang et al., 2003).

3) Ecological Habitat

Three soils collected from distinctly different agricultural sites were used to examine the ability of sunn hemp to enhance nematophagous fungi. These soils were collected from Buck Island (BI), a long-term pasture; Immokalee (IM), 10 years of vegetable cultivation amended with compost yearly; and Pine Acres (PA), fallow with native weeds. Although sunn hemp can induce various species of nematophagous fungi in BI, only population densities of Harposporium and Dactylaria brochopaga were stimulated by sunn hemp amendment in IM soil. On the other hand, no nematophagus fungi were enhanced by sunn hemp in PA soil (Fig. 5-3).

Fig. 5-3. Number of propagules of nematophagous fungi per g of soil in soil treated or not treated with sunn hemp amendment (Cj+ or Cj- respectively). In soil collected from Buck Island (BI), Immokalee (IM) and Pine Acres (PA). * signified no propagule detected. No difference were detected between Cj + and Cj- for all the fungi (P > 0.05). pNTF = parasitic nematode-trapping fungi, sNTF = saprophytic nematode-trapping fungi (Wang and McSorley, Unpublished).