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Complete three-dimensional X-ray crystal structure analyses have been carried out on both compounds by identical experimental and computational methods. Observations were collected a t 1 20" f 5OK and the intensities corrected for absorption. Hydrogen atoms were not located. After anisotropic refinement by least-squares, R is 0-091 for the cis-isomer and 0.077 for the trans-isomer. The bond lengths, corrected for rotational oscillation are Pt-CI = 2.33 rt 0.01 A and Pt-N = 2.01 f 0.04 A in the cis-compound, and Pt-CI = 2.32 f 0.01 A and Pt-N = 2.05 * 0.04 A in the trans-compound. The only angle significantly different from 90' is CI-Pt-CI = 91 -9" f 0.4' in the cis-co m p o u n d .The crystals 0f.ci.s-dichlorodiammineplatinum are triclinic with two molecules in a unit cell having a = 6.75, b = 6.55, c =6.23 A, u = 92.2". p = 84.6", and y = 11 0.7", and space group PT. The molecules lie approximately parallel to (100) a t x/a -& and -$, with platinum-platinum separations alternately 3.372 and 3.409 A perpendicular to the molecular planes. At low temperature the structure of the trans-isomer is essentially the same as that a t room temperature reported by Porai-Koshits.WE have undertaken the crystal structure analyses of cis-and trans-dichlorodiammineplatinum(II) , to obtain accurate and strictly comparable bond lengths. The same experimental procedures were used with both compounds; the crystals were cooled to 1 2 0 "~, and the observed intensities were corrected for absorption. Both were refined ultimately by full matrix leastsquares, including anisotropic vibration parameters, and the final co-ordinates were corrected for librational errors.The structure of the cis-isomer had not previously been determined, probably because of the difficulty in obtaining a suitable single crystal. It is triclinic with two molecules, related by a centre of symmetry, in the 1 M. Atoji, J. W. Richardson, and R. E. Rundle, J . Amer.Chem. SOL, 1957, 79, 3017. unit cell. As shown in Figures 1 and 2, the planar molecules are parallel to one another and stacked along the a-axis. Many of the crystals examined showed twinning or disorder in the bc-plane. The Pt Pt distances along the stack are alternately 3.372 and 3.409 &0.002 A, which may be short enough to indicate an interaction. The crystals are not pleochroic. In Magnus Green Salt, [Pt(NH3)4]2+[PtC14]2-, there are stacks of parallel alternate anions and cations with a Pt-Pt separation of 3.25 and its dichroism2has been attributed to platinum-platinum interaction.

The omnipresent fungal genus Alternaria was recently divided into 24 sections based on molecular and morphological data. Alternaria sect. Porri is the largest section, containing almost all Alternaria species with medium to large conidia and long beaks, some of which are important plant pathogens (e.g. Alternaria porri, A. solani and A. tomatophila). We constructed a multi-gene phylogeny on parts of the ITS, GAPDH, RPB2, TEF1 and Alt a 1 gene regions, which, supplemented with morphological and cultural studies, forms the basis for species recognition in sect. Porri. Our data reveal 63 species, of which 10 are newly described in sect. Porri, and 27 species names are synonymised. The three known Alternaria pathogens causing early blight on tomato all cluster in one clade, and are synonymised under the older name, A. linariae. Alternaria protenta, a species formerly only known as pathogen on Helianthus annuus, is also reported to cause early blight of potato, together with A. solani and A. grandis. Two clades with isolates causing purple blotch of onion are confirmed as A. allii and A. porri, but the two species cannot adequately be distinguished based on the number of beaks and branches as suggested previously. This is also found among the pathogens of Passifloraceae, which are reduced from four to three species. In addition to the known pathogen of sweet potato, A. bataticola, three more species are delineated of which two are newly described. A new Alternaria section is also described, comprising two large-spored Alternaria species with concatenate conidia.

A survey of anastomosis groups (AG) of Rhizoctonia spp. associated with potato diseases was conducted in South Africa. In total, 112 Rhizoctonia solani and 19 binucleate Rhizoctonia (BNR) isolates were recovered from diseased potato plants, characterized for AG and pathogenicity. The AG identity of the isolates was confirmed using phylogenetic analysis of the internal transcribed spacer region of ribosomal DNA. R. solani isolates recovered belonged to AG 3-PT, AG 2-2IIIB, AG 4HG-I, AG 4HG-III, and AG 5, while BNR isolates belonged to AG A and AG R, with frequencies of 74, 6.1, 2.3, 2.3, 0.8, 12.2, and 2.3%, respectively. R. solani AG 3-PT was the most predominant AG and occurred in all the potato-growing regions sampled, whereas the other AG occurred in distinct locations. Different AG grouped into distinct clades, with high maximum parsimony and maximum-likelihood bootstrap support for both R. solani and BNR. An experiment under greenhouse conditions with representative isolates from different AG showed differences in aggressiveness between and within AG. Isolates of AG 2-2IIIB, AG 4HG-III, and AG R were the most aggressive in causing stem canker while AG 3-PT, AG 5, and AG R caused black scurf. This is the first comprehensive survey of R. solani and BNR on potato in South Africa using a molecular-based approach. This is the first report of R. solani AG 2-2IIIB and AG 4 HG-I causing stem and stolon canker and BNR AG A and AG R causing stem canker and black scurf on potato in South Africa.

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