Efficacy of biological agents and compost on growth and resistance of tomatoes to late blight.

MAIN CONCLUSION: This study identified biocontrol measures for improving plant quality and resistance under biotic stress caused by the most devastating pathogen in tomato production. The management of plant diseases are dependent on a variety of factors. Two important variables are the soil quality and its bacterial/fungal community. However, the interaction of these factors is not well understood and remains problematic in producing healthy crops. Here, the effect of oak-bark compost, Bacillus subtilis subsp. subtilis, Trichoderma harzianum and two commercial products (FZB24 and FZB42) were investigated on tomato growth, production of metabolites and resistance under biotic stress condition (infection with Phytophthora infestans). Oak-bark compost, B. subtilis subsp. subtilis, and T. harzianum significantly enhanced plant growth and immunity when exposed to P. infestans. However, the commercial products were not as effective in promoting growth, with FZB42 having the weakest protection. Furthermore, elevated levels of anthocyanins did not correlate with enhanced plant resistance. Overall, the most effective and consistent plant protection was obtained when B. subtilis subsp. subtilis was combined with oak-bark compost. In contrast, the combination of T. harzianum and oak-bark compost resulted in increased disease severity. The use of compost in combination with bio-agents should, therefore, be evaluated carefully for a reliable and consistent tomato protection.

Pythium spp. Associated with Rooibos Seedlings, and Their Pathogenicity Toward Rooibos, Lupin, and Oat.

Rooibos (Aspalathus linearis) is an important indigenous crop in South Africa. Oomycetes are a common problem in rooibos nurseries, causing serious losses, but limited information is available on the species involved. Molecular and morphological analyses of 117 oomycete isolates from 19 rooibos nurseries and 33 isolates from 11 native rooibos sites revealed the presence of several Pythium spp., including Pythium acanthicum, P. irregulare, P. mamillatum, P. myriotylum, P. pyrilobum, P. cederbergense, and Pythium RB II, and Phytophthora cinnamomi (native site). Most of the species were identified in nurseries and native rooibos, with Pythium irregulare being the most common species occurring in all nurseries and 46% of the native sites. Phylogenetic analyses of the internal transcribed spacer region of the P. irregulare isolates showed that isolates within this species complex fit into three subclades, of which only two have previously been reported. On rooibos, all species except P. acanthicum and the previously characterized P. cederbergense and Pythium RB II were pathogenic and highly virulent. On lupin and oat, rotation crops in nurseries, the three aforementioned species were also nonpathogenic. All the other oomycete species were pathogenic on lupin but less so than on rooibos. On oat, only P. irregulare, P. myriotylum, and P. pyrilobum were pathogenic. This is the first report of P. mamillatum, P. pyrilobum, and P. myriotylum as pathogens of lupin, and P. irregulare and P. pyrilobum as pathogens of oat. The three nonpathogenic Pythium spp. were able to significantly reduce disease caused by pathogenic species in the less susceptible lupin and oat but not on rooibos. On lupin, the nonpathogenic species enhanced the virulence of Phytophthora cinnamomi.

Pythium cederbergense sp. nov. and related taxa from Pythium clade G associated with the South African indigenous plant Aspalathus linearis (rooibos).

The genus Pythium consists of more than 120 species and is subdivided into 11 phylogenetic clades (A-K) based on internal transcribed spacer (ITS) region sequence data. Pythium clade G contains only seven known species, with most not being well described. Our study characterized 12 Pythium isolates from Aspalathus linearis (rooibos) that fit into clade G. Phylogenetic analyses of the ITS region and a combined phylogeny of four gene regions (ITS, ß-tubulin, COX1 and COX2 [cytochrome c oxidase subunits I, II]) identified five clade G subclades. The rooibos isolates formed two groups, Pythium Rooibos I (RB I) and II (RB II), that clustered into two separate clades within subclade 1. The nine Pythium RB I isolates formed a distinct clade from P. iwayamai and is described here as a new species, Pythium cederbergense sp. nov. The three Pythium RB II isolates had P. canariense and P. violae as their closest relatives and were genetically diverse, suggesting the presence of several new species or a species complex that cannot be resolved with the current data, thus precluding a species description of this group. Morphological analyses showed that P. cederbergense and Pythium RB II were indistinguishable from each other but distinct from known clade G species. Clade G studies are being hampered by imprecise morphological descriptions of P. violae, P. canariense and P. iwayamai and each species being represented by only one isolate. The P. cederbergense and Pythium RB II isolates all were nonpathogenic toward rooibos, lupin and oats seedlings. One oligonucleotide was developed for each of P. cederbergense and Pythium RB II, which was able to differentiate the isolates with DNA macro-array analyses.

Suppression of Pythium and Phytophthora Damping-Off of Rooibos by Compost and a Combination of Compost and Nonpathogenic Pythium Taxa.

Pathogenic oomycetes, including Phytophthora cinnamomi and several Pythium spp. (Pythium irregulare, P. mamillatum, P. myriotylum, and P. pyrilobum), cause serious damping-off problems in rooibos (Aspalathus linearis) nurseries. The management of these pathogens in organic nurseries is problematic, because phenylamide fungicides may not be used. Compost, or compost in combination with Pythium taxa that are nonpathogenic to rooibos (P. acanthicum, P. cederbergense, and Pythium RB II), were investigated as alternative management options. Compost was able to suppress damping-off caused by several oomycete isolates but there was within- and between-species variation among the 30 evaluated isolates. This phenomenon was observed using two compost batches (A and B) sourced from independent suppliers. Compost B significantly reduced damping-off caused by 60% of the isolates, whereas compost A controlled only 37% of the isolates. The pathogens that were more readily controlled by both composts included P. mamillatum and P. pyrilobum, whereas the composts were ineffective at suppressing damping-off caused by >62% of P. irregulare and >50% of P. myriotylum isolates. Based on the evaluation of one Phytophthora cinnamomi isolate, this pathogen may also be controlled by compost. Neither of the composts as a stand-alone treatment could suppress damping-off caused by a combination of pathogenic species (P. cinnamomi, Pythium irregulare, P. mamillatum, P. myriotylum, and P. pyrilobum). However, damping-off was significantly reduced when nonpathogenic Pythium taxa (P. acanthicum, P. cederbergense, and Pythium RB II) were combined with the composts. Similarly, damping-off caused by a P. irregulare isolate that was not suppressed by either of the composts alone was significantly suppressed when the two composts were inoculated with the nonpathogenic Pythium taxa.