Numerical simulation of Richards equation in unsaturated soil is known to be difficult because of the highly non-linear material properties involved. An extensive study was conducted to clarify the role of mass balance and under-relaxation on the rate in which the correct solution is approached. Both finite element and finite difference techniques were considered. For the former, the hbased formulation was compared with a massconservative mixed form. The conductivity function (K) was under-relaxed in two ways while the capacity function (C) is computed following the standard mass or non-mass conservative schemes recommended in literature. For fairly coarse discretisation, it was found that large errors were produced when K was under-relaxed by evaluating it at the average of heads from current and previous time step (UR1), regardless of the numerical scheme used. Maintaining global mass balance is found to have little impact on the accuracy. All numerical schemes that underrelaxed K by computing it at the average of two most recent iterations in the current time step (UR2) converged quicker to the correct solution with increasing discretisation, although more iterations per time step than UR1 is needed to achieve a stable solution. An important practical ramification is that it appears to be possible to achieve reasonably accurate and oscillation-free results using fairly coarse discretisation by making only minor modifications (namely, using UR2 for conductivity function) to the h-based finite element formulation and applying some minimum time step criteria.

Zucchini yellow mosaic virus (ZYMV) was first found in eucumber in Singapore in 1989. This virus was propagated m Cacurhita pepo cv. First Taste and mechanically transmitted to 12 speeies of six famtiies. It induced milder symptoms than the Connecticut and Florida strains of ZYMV in infected leaves of C. pepo ev. Zucchini Elite. ZYMV-S is neither seed nor aphid transmissible. Immunoelectron microscopy revealed that ZYMV-S is distantly related to WMV-2, Moroccan "WMV, and TelMV, but not related to PRSV or ZYFV. Cytopiasmie pinwheels and scrolls were observed in ultrathin sections of infected leaf cells hy light, eonfocal laser scanning, and transmission electron microscopy. The molecular weights of the viral coat protein and eytoplasmic inclusion protein, RNA and dsRNA were estimated to he 3.2 x 10-' , 6.

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