Seed-borne Fusarium pathogens in agricultural crops

Diseases caused by Fusarium spp. occur worldwide throughout all production areas of edible and ornamental plants as well as tree plantations. Fusarium spp. cause yield losses of up to 80% in both field- and greenhouse-grown crops, resulting in serious economic losses. It is known that plant material such as fruits and seeds as well as soil, can be a pathway of disease transmission; therefore, different strategies to control Fusarium have been used e.g., planting new cultivars, grafting, using inorganic and organic substrates, soil solarization, biofumigation and seed treatments. Nonetheless, new Fusarium pathotypes are often detected or reported to be the causal agents of diseases. To determine the pathway of Fusarium transmission, specifically through seed, is challenging and scientific reports in this regard are lacking. To prove that seed is the pathway of pathogen transmission is time-consuming and laborious, since pathogenicity tests with several Fusarium isolates must be conducted during several growth stages of the host. The isolates then have to be re-isolated from infected plant parts to confirm Koch's postulates and seeds from inoculated plants need to be obtained which in turn need to be tested again for the presence of the pathogen. To prevent the introduction and distribution of Fusarium spp., growers (for local and export markets), technical advisors, and seed producers need to use disease-free seed. The potential production of Fusarium mycotoxins during storage is an additional concern when using Fusarium infected seeds for food. Depending on the region, it is important to know which crops (hosts) are susceptible to a particular Fusarium sp., how this pathogen can be avoided, and how to produce and select healthy fruits and seeds. Efforts must be directed towards avoiding seed contamination with pathogens and developing or optimizing seed health testing methods to detect Fusarium.ISHS Acta Horticulturae 1204: VII International Symposium on Seed, Transplant and Stand Establishment of Horticultural Crops - SEST2016.http://www.actahort.org2019-06-01hj2019Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

Screening of plant extracts for antifungal activities against Colletotrichum species of common bean (Phaseolus vulgaris L.) and cowpea (Vigna unguiculata (L.) Walp)

The aim of the present investigation was to evaluate the antifungal activities of plant extracts which can be used to control bean and cowpea anthracnose. Acetone, ethyl acetate and water extracts of Ipomoea batatas, Carica papaya, Allium sativum, Syzygium cordatum, Chlorophytum comosum and Agapanthus caulescens were screened in vitro for their antifungal activities against Colletotrichum lindemuthianum and Colletotrichum dematium of common bean and cowpea using the agar disc infusion and microtitre double-dilution techniques. The same extracts were then tested for antifungal activity in vivo as seed treatments against anthracnose disease. The water extracts of Carica and Syzygium were active against C. lindemuthianum and had minimum inhibitory concentrations (MICs) of 1·56 mg/ml. Syzygium, Allium and Chlorophytum water extracts were active against C. dematium and MICs were 3·13, 6·25 and 12·5 mg/ml, respectively. The MICs of Allium, Syzygium and Agapanthus acetone extracts were 0·78, 3·13 and 6·25 mg/ml, respectively, against C. lindemuthianum and 0·78, 6·25 and 3·13 mg/ml against C. dematium. Agapanthus water extracts and all the acetone extracts tested in vivo effectively reduced the incidence and severity of bean anthracnose disease in the greenhouse. Agapanthus acetone, Allium water, and both acetone and water extracts of Carica and Syzygium performed well in vivo in reducing cowpea anthracnose disease and compared well with reductions due to the application of the synthetic fungicide fludioxonil+mefenoxam (the commercial product Celest® XL) applied at 25 gai/l and also with levels in the non-inoculated control. The Agapanthus, Carica, Syzygium and Allium extracts were active on both Colletotrichum spp. in vitro and also reduced anthracnose disease of bean and cowpea and are potential seed treatments in anthracnose disease control. The easy seed treatment process and the accessibility of plants used in the present study could lead to high adoption of the use of the plant extracts as seed treatments by resource-poor, smallholder farmers.http://journals.cambridge.org/action/displayJournal?jid=AGSam201

Fungi associated with Aizoaceae seed in the Succulent Karoo

The Aizoaceae, commonly known as mesembs or “ice plants”, is a plant family endemic to Namaqualand, an area inside the Succulent Karoo biodiversity hotspot in South Africa. The more than 1800 unique mesembs are in part characterised by their hygrochastic seed capsules dispersing their seed by jet action. Six capsule types: Mesembryanthemum-, Delosperma-, Drosanthemum-, Lampranthus-, Ruschia- and Leipoldtia-type are distinguished by differences in funicles, covering membranes and closing bodies. With the existence of microbial endemism, now widely recognised, research into community ecology in the Succulent Karoo is needed to enable proper conservation. Sadly the microbial life associated with Aizoaceae has received little attention, Alternaria, Colletotrichum and Fusarium, have however been isolated from the halophyte Sesuvium portulacastrum, one of the few species from Aizoaceae but not endemic to South Africa. Fungi are known to aid in germination, seedling establishment, growth, water relations and nutrition in the Cactaceae (a family closely related to Aizoaceae). We believe that fungi may play a role in the ability of Aizoaceae to thrive in Namaqualand due to their specialization in the form of thick-walled spores that remain viable and have the ability to grow slow even at extreme aridity. We have isolated species of Alternaria, Aspergillus, Bipolaris, Cladosporium, Fusarium and Talaromyces from seeds of common mesemb species from Namaqualand. This is the first report of fungi associating with the specialised propagative material of Aizoaceae.ISHS Acta Horticulturae 1204: VII International Symposium on Seed, Transplant and Stand Establishment of Horticultural Crops - SEST2016.http://www.actahort.org2019-06-01hj2019Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

Phenolic content as an indicator of tolerance of cowpea seedlings to Sclerotium rolfsii

The role of phenolics in plant tolerance to pathogen infection is well documented. The objective of the present preliminary investigation was to study phenolic metabolites involved in the tolerance or susceptibility of cowpea (Vigna unguiculata Walp.) cultivars to Sclerotium rolfsii Sacc. and to use their presence as a possible screening tool. Total, free acid, ester-bound and cell wall-bound phenolics of 10 cowpea cultivars were quantified. In healthy seedlings, the tolerant cultivars displayed the higher phenol content than the susceptible cultivars. In S. rolfsii infected seedlings, the highest increase was found from 48 h after inoculation. The net effect of inoculation was a 630 % increase in total phenolics (soluble and insoluble) in the stem of tolerant cultivars while the total phenolic content increased only by 212 % in the stems of susceptible cultivars. Two phytoalexins were detected. Although, no significant difference (P = 0.05) was detected among cultivars, in terms of free acid phenolics, the amount of ester-bound and cell wall-bound phenolics significantly increased, therefore demonstrating a similar trend to the one observed for the total phenolic content. These preliminary results showed that the presence of phenolics before and after S. rolfsii infection may be used as a rapid screening method for detection of tolerance to S. rolfsii damping-off and stem rot of cowpea.The National Research Foundation, South Africahttp://www.elsevier.com/locate/ejpe2018-10-13hb2017Plant Scienc

A survey of brassica vegetable smallholder farmers in the Gauteng and Limpopo provinces of South Africa

A study was taken to investigate the types of brassica vegetables mostly grown by smallholder farmers in two provinces of South Africa. Thirty-one smallholder vegetable farmers in the Gauteng province and Waterberg district in the Limpopo province were surveyed. In addition, the study also sought to establish the common diseases, the management strategies used and problems encountered by the farmers. Farmers were interviewed using a questionnaire with closed and open–ended questions. The results indicated that the smallholder farmers mostly grew cabbage (93.6%) as their main brassica crop followed by rape (41.2 %). Thirty percent of farmers could not identify or name the predominant disease/s encountered in their fields. Major diseases encountered by farmers surveyed were an unknown disease/s (33.3 %), black rot (26.7 %), Alternaria leaf spot (6.7%) and white rust (6.7 %). Smallholder farmers have inadequate technical information available especially relating to crop diseases, their identification and control. Farmers encountered challenges with black rot disease especially on cabbage, rape and kale and the disease was a problem during winter and summer. Generally, the smallholder farmers used crop rotation (74.2%) as a major practice to manage the diseases experienced. They rotated their brassica vegetables with other crops/vegetables like tomatoes, onions, beetroots and maize. Most of the farmers interviewed (61.3%) did not use chemicals to control diseases, whereas 38.7% of them used chemicals. This was mostly because they lacked information and knowledge, high costs associated with use of chemical fungicides and some were shifting towards organic farming. From the study it was noted that there was a need for technical support to improve farmers’ knowledge on disease identification and control within the surveyed areas.European Union – TESTA Project number FP7-KBBE-2012-6-311875 and the Department of Science and Technology, South Africa.http://www.jarts.infoam2016Forestry and Agricultural Biotechnology Institute (FABI)Plant Scienc

Seed health status and germination of Eucalyptus spp.

The presence of disease causing microorganisms on seeds raises serious quarantine and economic concerns to nurserymen, foresters and seed traders. The agar plate method was used to examine seed-borne mycoflora associated with Eucalyptus seed lots and their effect on seed germination was determined. A total of 35 fungal species from 29 genera were identified from 12 different Eucalyptus species. The Eucalyptus nitens seed lot was the most infested, whereas the lowest incidence of fungi was from the E. dorrigoensis seed lot. Penicillium was the most abundant fungus. Colletotrichum, Aureobasidium and Disculoides were recorded for the first time associated with Eucalyptus seeds. There was a significant reduction in seed germination of seed lots inoculated with selected seed-borne fungi compared to non-inoculated controls. Fusarium oxysporum and F. solani reduced seed germination the most on E. badjensis, E. dorrigoensis, E. nitens, E. pellita, E. teritecomis and E. urophylla seed lots with percentage germination of 31.3 and 33.5; 30.5 and 30.0; 38.8 and 37.0; 30.5 and 32.3; 25.0 and 26.8; 33.3 and 31.8; 31.3 and 33.5%, respectively. Similarly, seed germination was lowest on the E. benthamii seed lot (29.8%) inoculated with C. gloeosporioides, whilst germination of E. grandis, E. smithii and E. viminalis seed lots inoculated with Botrytis sp. and F. solani were 37.0 and 37.5%; 35.8 and 36.3%; 28.3 and 30.0%, respectively. This study has shown that commercial Eucalyptus seed lots carry a wide diversity of fungi and suggests that infested seeds may be a primary reason for poor seed germination.The South African Forestry Company Limited (SAFCOL), the University of Pretoria for the Postgraduate Research Support Bursary and forestry seed companies.https://link.springer.com/journal/10658hj2022Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

Response of radicle cells of fungicide treated and untreated maize seed subjected to stress conditions

Ultrastructural changes within cells are influenced by stress such as increased temperature due to improper storage, lack of oxygen and blockage in pathways responsible for water uptake. The objective of this study was to assess the effect, if any, of fungicide treatments on the ultrastructure of radicle cells of maize (Zea mays) after seeds had been subjected to stress conditions. Maize seeds were treated with Celest® XL (fludioxonil + mefenoxam) and Apron® XL (metalaxyl-M). The control consisted of untreated seeds. Following treatment, seeds were subjected to 2 d accelerated aging and 48 h rapid imbibition and thereafter prepared for transmission electron microscopy. Initial percentage germination (81%) of the seed lot was reduced after 2 d accelerated aging, with the untreated control having the lowest percentage germination (61%) followed by Apron® XL (65%) and Celest® XL (69%). The most obvious ultrastructural difference between the untreated control and the two fungicide treatments was the position of the lipid bodies. These formed a layer in close association with the cell wall in fungicide treated seeds, but in the untreated control they appeared more concentrated in the cytoplasm. Treated and untreated seeds may use different mechanisms, namely numerous vauoles and/or the movement of lipid bodies from the cell wall, to tolerate the stress conditions during rehydration of the seed after accelerated ageing and rapid imbibition.ISHS Acta Horticulturae 1204: VII International Symposium on Seed, Transplant and Stand Establishment of Horticultural Crops - SEST2016.The National Research Foundation, South Africa and the University of Almería, Almería, Spain for funding collaborative research visits by Profs Aveling and Blanco, respectively and Syngenta South Africa (Pty) Ltd for fungicides.http://www.actahort.org2019-06-01hj2019Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

The effects of treatments with selected pesticides on viability and vigour of maize (Zea mays) seeds and seedling emergence in the presence of Fusarium graminearum

The quality of seed is dependent on two very broad aspects: how healthy (disease-free) a seed is and its field performance (germination and vigour). The objective of the present study was to evaluate the effect of pesticidal seed treatments of maize (Zea mays L.) on seed germination and vigour, and on greenhouse emergence in the presence of Fusarium graminearum Schw. Maize seeds were treated with four pesticides: Apron® XL (metalaxyl), Thiram (thiram), Celest® XL (fludioxonil, metalaxyl) and Apron® Star 42 WS (thiamethoxam, metalaxyl, difenoconazole). Viability and vigour of the treated seeds were determined. Thereafter, seeds were planted under greenhouse conditions. The control consisted of water-treated seeds. None of the pesticides reduced the standard germination under laboratory conditions and none had any effect on the quantity of leachate (measured as conductivity) or moisture content of the seeds. The different treatments also had no effect on germination or on seedling weight increase among treatments after rapid imbibition and there was no difference in germination among treatments following the cold test. The proportion of diseased plants harvested from F. graminearum inoculated soil was significantly reduced by Apron® Star 42 WS and Celest® XL. The vigour tests indicated that none of the pesticides tested affected the seeds negatively and that plant biomass in the presence of the pathogen, F. graminearum, was increased after the application of the pesticides to the seeds, with the exception of seeds treated with Apron® XL.We thank the National Research Foundation, South Africa and Syngenta South Africa (Pty) Ltd for funding.http://journals.cambridge.org/action/displayJournal?jid=AGSam201

Alternaria alternata, the causal agent of leaf blight of sunflower in South Africa

Sunflower (Helianthus annuus L.) is an important oilseed crop in South Africa, and is grown in rotation with maize in some parts of North West, Limpopo, Free State, Mpumalanga and Gauteng provinces. Alternaria leaf blight is currently one of the major potential disease threats of sunflower and is capable of causing yield losses in all production regions. Alternaria helianthi was reported as the main cause of Alternaria leaf blight of sunflower in South Africa; however small-spored Alternaria species have been consistently isolated from leaf blight symptoms during recent surveys. The aim of this study was to use morphological and molecular techniques to identify the causal agent(s) of Alternaria blight isolated from South African sunflower production areas. Alternaria helianthi was not recovered from any of the sunflower lesions or seeds, with only Alternaria alternata retrieved from the symptomatic tissue. Molecular identification based on a combined phylogenetic dataset using the partial internal transcribed spacer regions, RNA polymerase second largest subunit, glyceraldehyde-3-phosphate dehydrogenase, translation elongation factor and Alternaria allergen gene regions was done to support the morphological identification based on the three-dimensional sporulation patterns of Alternaria. Furthermore, this study aimed at evaluating the pathogenicity of the recovered Alternaria isolates and their potential as causal agents of Alternaria leaf blight of sunflower. Pathogenicity tests showed that all the Alternaria alternata isolates tested were capable of causing Alternaria leaf blight of sunflower as seen in the field. This is the first report of A. alternata causing leaf blight of sunflower in South Africa.http://link.springer.com/journal/106582019-07-01hj2018Forestry and Agricultural Biotechnology Institute (FABI)Plant Production and Soil Scienc

Effects of soil drenching of water-soluble potassium silicate on commercial avocado (Persea americana Mill.) orchard trees infected with Phytophthora cinnamomi Rands on root density, canopy health, induction and concentration of phenolic compounds

Please read abstract in the article.The National Research Foundation of South Africa and by the South African Avocado Growers Association.http://www.tandfonline.com/loi/tjps202015-07-31hb201