Abstract. Effects of two concentrations (200 and 2000 l*g g-t soil) of two heavy metals (copper and zinc as sulphates) applied to clay or sandy soil for 12 weeks on the total counts of fungi, bateria and actinomycetes were studied. Activities of three soil enzymes (urease, nitrate reductase and amidase) were also investigated. Application of heavy metals to the clay soil reduced the microbial populations. However, although neither heavy metal showed any significant increasing effect on microorganisms populations in clay soil samples, some stimulatory effects were noted in sandy soil. Activities of urease and nitrate reductase were inhibited by heavy metal application in both soils. Amidase activity was inhibited only with the higher application rate after some experimental periods.

Among 39 fungal species and one variety belonging to 16 genera isolated from 116 healthy tomato fruits collected from markets in Assiut, Egypt, during 1994, Aspergillus niger was found to be the most prevalent, being isolated from 84.6% of the samples. Of 11 species recovered from 156 diseased tomatoes, Alternaria alternata, Rhizopus stolonifer and A. niger were the most common and isolated from 52.7%, 35.9% and 25.0% of the samples, respectively. Experiments for comparison of the effects of medium containing tomato juice with synthetic medium on the mycelial growth of nine fungal species indicated that, the tomato juice medium was more suitable for growth of all fungal species. The effect of five commercial fungicides and sodium hypochlorite when applied as post-harvest dips after inoculation was studied in laboratory against A. alternata, A. niger and R. stolonifer. At 10 and 100 micrograms ml-1, neither of the fungicide caused a noticeable inhibition of Alternaria rot. At 1000 micrograms ml-1 benlate, rovral and sumisclex completely prevented Alternaria and Aspergillus rot, whereas cuprosan and ridomil were ineffective against rotting caused by A. niger. Rhizopus rot development was inhibited by over 50% with one treatment only (rovral at 1000 micrograms ml-1). Sodium hypochlorite has good curative properties against fruit rots especially those caused by A. alternata and A. niger.

Particle preparations of parsnip yellow fleck virus (PYFV) isolates A-421 and P-121, representing the two major serotypes, were made by clarifying leal extracts with ether o r butan-1-01 and concentrating the virus particles by precipitation with polyethylene glycol and differential centrifugation. The preparations contained c. 3 I nm-diameter particles comprising two sedimenting components. Top component (T) consisted of stain-penetrable protein shells with A2,,,/Azs,, = 0.8-0.9, sedimentation coefficient (L,,) = 56 S (A-421) or 60 S (P-121), and buoyant density = 1.297 g i c n i ' . Bottom component ( B ) consisted of nucleoprotein particles, not penetrable by negative stain, with A,,,,iA2S0 = I .9, sedimentation coefficient (s!,,,,,) = 148 S (A-421) or 153 S (P-121), and buoyant density = 1.520 g/cm3 (A-421) or 1.490 g/cm.' (P-121). Yields of B component particles were up to c. 1 mg/100 g leaf tissue (both isolates); yields of T component particles were up to c. 0.6 mg (A-421) or 5.5 mg (P-121) per 100 g leaf tissue. PYFV particles were found to contain a single RNA species (rnol. wt c. 3.4 x lofi, c.9800 nucleotides), constituting 40% of the particle weight, and three polypeptide species, of mol. wt ( x 10 j) 30, 26 and 24 (A-421) or 31, 26 and 23 (P-121).

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S 11 m in aryAnthriscus yellows virus (AYV), ;I phloem-limited virus transmitted in the semipersistent inanner by the aphid C'civc/rie//u cregopodii, \% a\ purified h y treatment of leaf extracts with cellulasc, follo\ved by d rential and sucrose density gradient centrifugation. 'The preparations cotilaincd isometric particle5 c. 29 nm in diameter which were iin$table unless stored in buffer at pH 8.0 containing 1 mki CaCI,.The part i c I cs sed i men t ed as t \Y o coin po 11 ent s , ' f u I I ' n iic I eop r o t ei ti part i c les w i tti A2h,1/.42,11 = I .83 containing about -12(b' (i nucleic acid, and 'empty' protein shells w i t h A2(,,,/A:h,, = 0.73; their buoyant densities in CsCl solutions were 1.52 and 1.27 g/cni'. respectively. Yields of ihe nircleoprotein particlei were c.

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