African Journals Online
African Plant Protection

Volume 7, Issue 1, February 2001

The role of host resistance in the evolution of plant pathogens
Crill, J.P. 1-19

Abstract: Exact terminology and definitions are presented and used to describe specific host-pathogen interactions that are used to illustrate examples of pathogen evolution. The factors most influencing the evolution of pathogen races are identified and described. Two types of host resistance are recognised and these are described with examples and definitions. The effect of both types of resistance on the evolution of pathogen races is discussed with examples. Previously published data are reviewed and new information is presented relative to the evolution of pathogen races. Manipulating the evolution of pathogen races with resistant varieties is presented as an alternative method of plant disease control with certain specific host-pathogen interactions.

Powdery mildew resistance in selections from Moroccan barley landraces
Czembor, J.H.Czembor, H.J. 21-31

Abstract: Eleven barley (Hordeum vulgare) landraces were tested for resistance to powdery mildew caused by Erysiphe graminis f. sp.hordei. Landraces were collected in 1985 in northern Morocco and originated from the collection of the International Centre for Agricultural Research in the Dry Areas (ICARDA), Aleppo, Syria. In 1998 all 11 land-races showed resistance to natural infection by powdery mildew in IHAR Radzików, Poland. From these, 26 single-plant lines were selected and tested at the seedling stage with 23 differential isolates of E. graminis f. sp. hordei collectively having virulences to all major resistance genes used in Europe. Line 176-3-2 showed resistance to all isolates, exhibiting resistance reaction type 2 after inoculation with 17 (74 %) of the isolates. A further seven lines (191-3-3, 264-1-2, 265-2-3, 265-3-3, 270-2-3, 270-3-3 and 271-1-1) were susceptible to 1-3 (4.3-13 %) of the isolates. Fourteen lines (55.8 %) expressed infection type 2 with more than 50 % of the isolates. Two lines (264-1-2 and 265-3-3) exhibited infection type 0 with most of the isolates. In total, 74.3 % of all infection types observed among the lines could be classified as powdery mildew resistance. The most frequent resistant infection type was 2 (54.2 %). Infection type 1 (hypersensivity) occurred at a frequency of 0.4 % and infection type 0 (immunity) at 19.7 %. In 16 of the lines it was impossible to determine which particular allele (or alleles) for resistance was present. However, expression of infection types 0, 1 and 2 by these lines indicates that they may have more than one allele for resistance. Allele Mlat was postulated to be present in 10 lines, either alone or in combination with other unknown genes.

Plant nematodes in South Africa. 3. Douglas area, Northern Cape Province
Marais, M.Swart, A. 33-38

Abstract: During a survey of plant nematodes in the Douglas area in the Northern Cape Province, 135 localities were sampled. The genera Pratylenchus, Helicotylenchus and Mesocriconema were most frequently encountered, whereas Aphelenchoides, Criconema, Hemicriconemoides, Hoplolaimus, Longidorus, Meloidogyne, Meloinema, Paratrichodorus, Paratylenchus, Rotylenchulus, Rotylenchus, Scutellonema and Xiphinema were found at fewer localities. The genera Ditylenchus, Hemicycliophora and Zygotylenchus were each identified from a single locality. Various nematode-plant associations are reported in South Africa for the first time.

Spiders in macadamia orchards in the Mpumalanga Lowveld of South Africa : species diversity and abundance (Arachnida : Araneae)
Dippenaar-Schoeman, A.S.Van den Berg, M.A.Van den Berg, A.M.Van den Berg, A. 39-46

Abstract: Arboreal spiders were collected over a 12-month period (July 1997 - June 1998) from three macadamia orchards in the Mpumalanga Lowveld of South Africa. The spiders were sampled every 2-3 weeks from 10 trees per orchard using dichlorvos as a knock-down spray. In total, 2778 specimens representing 21 families, 57 genera and 80 species were recorded. The Salticidae represented 72.7 % of all spiders collected, followed by the Sparassidae (6.9 %), Hersiliidae (3.9 %) and Araneidae (3.3 %). The families richest in species numbers were the Salticidae (17), followed by the Araneidae (16) and the Thomisidae (11). Wandering spiders dominated the fauna, representing 95.8 % of the total number of specimens collected compared to 4.2 % that were web-builders.

Salticid spiders in macadamia orchards in the Mpumalanga Lowveld of South Africa (Arachnida : Araneae)
Dippenaar-Schoeman, A.S.Van den Berg, M.A.Van den Berg, A.M. 47-51

Abstract: Spiders were collected over a 12-month period (July 1997 - June 1998) from three macadamia orchards in the Mpumalanga Lowveld of South Africa using dichlorvos as a knock-down spray. Of the 2778 spiders collected, 2020 (73 %) belonged to the Salticidae. The salticids were also the richest in species, with 17 species recorded of which four represented 61 % of all spiders collected. Thyene coccineovittata was the most abundant and represented 30 % of all the spiders collected, followed by T. natalii with 14 %, Viciria alba with 9 % and Tusitala guineensis with 8 %. These four species were present in all three orchards throughout the year.

Pest status of two blackfly species on citrus in South Africa and Swaziland
Van den Berg, M.A.Greenland, J. 53-57

Abstract: The spiny blackfly, Aleurocanthus spiniferus, and the citrus blackfly, Aleurocanthus woglumi, are introduced pests of citrus in South Africa. Leaves, fruit and branches of infested trees are usually covered with sooty mould and tree vigour and production can be impaired. Past and present outbreaks of the two blackfly species have been documented. A. woglumi is effectively controlled by the parasitoid Eretmocerus serius in parts of KwaZulu-Natal. Within two years after A. woglumi was first noticed at Nkwalini, KwaZulu-Natal, Er. serius had spread to these orchards. Nkwalini is 51 km from Gingindlovu where Er. serius had occurred in the early 1960s. Effective biological control prevailed in the orchards at Nkwalini within a year. Encarsia cf. smithi provided classical biological control of A. spiniferus in areas where it was released and also parasitised A. woglumi. However, under conditions that pervailed at Tambuti Estates, Big Bend, Swaziland, from 1996 to 1999, En.cf. smithi could not control A. woglumi or A. spiniferus. After the establishment of Er. serius at Tambuti Estates, both A. woglumi and A. spiniferus were controlled effectively by Er. serius and En.cf. smithi. A. spiniferus and A. woglumi still remain potential pests in many parts of southern Africa. En.cf. smithi and Er. serius have recently been introduced to areas where the two blackfly species became pests. The numbers of both parasitoids increased within a few months and established effective biological control.