Sustainable and efficient control of sunflower downy mildew by means of genetic resistance: a review

The breeding of sunflower (Helianthus annuus L.) for resistance to downy mildew (caused by the oomycete Plasmopara halstedii Farl. Berl. & de Toni) is reviewed in this work under the scope of its sustainability and efficiency. When sunflower turned into an oilseed crop, resistance to the disease was included in its initial breeding strategies. Subsequent development of genomic tools allowed a significant expansion of the knowledge on the diversity of its genetic resistance and its application to the genetic control of the disease. Simultaneously to genetic improvements, and as a consequence of the close interaction between the pathogen and its host plant, an enormous variety of pathotypes has been described in all the sunflower-growing areas worldwide. Thus, the genetic control of sunflower downy mildew is an active research field subjected to continuous evolution and challenge. In practice, genetic resistance constitutes the base tier of Integrated Pest Management against sunflower downy mildew. The second tier is composed of elements related to crop management: rotation, removal of volunteer plants, sowing date, tillage. Biological control alternatives and resistance inducers could also provide additional restraint. Finally, the top tier includes chemical treatments that should only be used when necessary and if the more basal Integrated Pest Management elements fail to keep pathogen populations under the economic threshold.

Fungal Planet description sheets: 868-950

Novel species of fungi described in this study include those from various countries as follows: Australia, Chaetomella pseudocircinoseta and Coniella pseudodiospyri on Eucalyptus microcorys leaves, Cladophialophora eucalypti, Teratosphaeria dunnii and Vermiculariopsiella dunnii on Eucalyptus dunnii leaves, Cylindrium grande and Hypsotheca eucalyptorum on Eucalyptus grandis leaves, Elsinoe salignae on Eucalyptus saligna leaves, Marasmius lebeliae on litter of regenerating subtropical rainforest, Phialoseptomonium eucalypti (incl. Phialoseptomonium gen. nov.) on Eucalyptus grandis × camaldulensis leaves, Phlogicylindrium pawpawense on Eucalyptus tereticornis leaves, Phyllosticta longicauda as an endophyte from healthy Eustrephus latifolius leaves, Pseudosydowia eucalyptorum on Eucalyptus sp. leaves, Saitozyma wallum on Banksia aemula leaves, Teratosphaeria henryi on Corymbia henryi leaves. Brazil, Aspergillus bezerrae, Backusella azygospora, Mariannaea terricola and Talaromyces pernambucoensis from soil, Calonectria matogrossensis on Eucalyptus urophylla leaves, Calvatia brasiliensis on soil, Carcinomyces nordestinensis on Bromelia antiacantha leaves, Dendryphiella stromaticola on small branches of an unidentified plant, Nigrospora brasiliensis on Nopalea cochenillifera leaves, Penicillium alagoense as a leaf endophyte on a Miconia sp., Podosordaria nigrobrunnea on dung, Spegazzinia bromeliacearum as a leaf endophyte on Tilandsia catimbauensis, Xylobolus brasiliensis on decaying wood. Bulgaria, Kazachstania molopis from the gut of the beetle Molops piceus. Croatia, Mollisia endocrystallina from a fallen decorticated Picea abies tree trunk. Ecuador, Hygrocybe rodomaculata on soil. Hungary, Alfoldia vorosii (incl. Alfoldia gen. nov.) from Juniperus communis roots, Kiskunsagia ubrizsyi (incl. Kiskunsagia gen. nov.) from Fumana procumbens roots. India, Aureobasidium tremulum as laboratory contaminant, Leucosporidium himalayensis and Naganishia indica from windblown dust on glaciers. Italy, Neodevriesia cycadicola on Cycas sp. leaves, Pseudocercospora pseudomyrticola on Myrtus communis leaves, Ramularia pistaciae on Pistacia lentiscus leaves, Neognomoniopsis quercina (incl. Neognomoniopsis gen. nov.) on Quercus ilex leaves. Japan, Diaporthe fructicola on Passiflora edulis × P. edulis f. flavicarpa fruit, Entoloma nipponicum on leaf litter in a mixed Cryptomeria japonica and Acer spp. forest. Macedonia, Astraeus macedonicus on soil. Malaysia, Fusicladium eucalyptigenum on Eucalyptus sp. twigs, Neoacrodontiella eucalypti (incl. Neoacrodontiella gen. nov.) on Eucalyptus urophylla leaves. Mozambique, Meliola gorongosensis on dead Philenoptera violacea leaflets. Nepal, Coniochaeta dendrobiicola from Dendriobium lognicornu roots. New Zealand, Neodevriesia sexualis and Thozetella neonivea on Archontophoenix cunninghamiana leaves. Norway, Calophoma sandfjordenica from a piece of board on a rocky shoreline, Clavaria parvispora on soil, Didymella finnmarkica from a piece of Pinus sylvestris driftwood. Poland, Sugiyamaella trypani from soil. Portugal, Colletotrichum feijoicola from Acca sellowiana. Russia, Crepidotus tobolensis on Populus tremula debris, Entoloma ekaterinae, Entoloma erhardii and Suillus gastroflavus on soil, Nakazawaea ambrosiae from the galleries of Ips typographus under the bark of Picea abies. Slovenia, Pluteus ludwigii on twigs of broadleaved trees. South Africa, Anungitiomyces stellenboschiensis (incl. Anungitiomyces gen. nov.) and Niesslia stellenboschiana on Eucalyptus sp. leaves, Beltraniella pseudoportoricensis on Podocarpus falcatus leaf litter, Corynespora encephalarti on Encephalartos sp. leaves, Cytospora pavettae on Pavetta revoluta leaves, Helminthosporium erythrinicola on Erythrina humeana leaves, Helminthosporium syzygii on a Syzygium sp. bark canker, Libertasomyces aloeticus on Aloe sp. leaves, Penicillium lunae from Musa sp. fruit, Phyllosticta lauridiae on Lauridia tetragona leaves, Pseudotruncatella bolusanthi (incl. Pseudotruncatellaceae fam. nov.) and Dactylella bolusanthi on Bolusanthus speciosus leaves. Spain, Apenidiella foetida on submerged plant debris, Inocybe grammatoides on Quercus ilex subsp. ilex forest humus, Ossicaulis salomii on soil, Phialemonium guarroi from soil. Thailand, Pantospora chromolaenae on Chromolaena odorata leaves. Ukraine, Cadophora helianthi from Helianthus annuus stems. USA, Boletus pseudopinophilus on soil under slash pine, Botryotrichum foricae, Penicillium americanum and Penicillium minnesotense from air. Vietnam, Lycoperdon vietnamense on soil. Morphological and culture characteristics are supported by DNA barcodes.

A New Race of Verticillium dahliae Causing Leaf Mottle of Sunflower in Europe

Sunflower (Helianthus annuus L.) plants with symptoms of interveinal chlorosis were observed in the summer of 2013 in one field in Cadiz (Spain) where the performance of 30 hybrids was assessed. Symptoms affected 80% of the hybrids with incidence as high as 90%. Chlorosis and yellowing near the leaf margin were visible at floral initiation, and they progressed from the lower to upper leaves. Mottled leaves were observed near the top of the plants. On severely affected leaves, chlorotic patches enlarged, coalesced, and large areas of the leaves became necrotic and dried. Cross sections of the lower stem showed a brown discoloration of the vascular system. The fungus that was consistently isolated from stem and petiole tissues of sunflower plants was morphologically identified as Verticillium dahliae Kleb. (Vd) (5) and molecularly confirmed by PCR amplification of the 526-bp band (4). The race of the isolates was determined in a greenhouse experiment at 18 to 28°C from February to April 2014. Isolates 1-13 and 2-13 of Vd, obtained from two of the hybrids in 2013, one of them being Transol, were independently inoculated to 1-month-old plants of each of three sunflower genotypes: the susceptible hybrid Transol and the inbred lines HA89 (carrying the V₁ gene for resistance to Vd) (2) and HAR5 (resistant to other diseases but with unknown reaction to Vd). Plants were inoculated by immersing roots in a suspension of 10⁶ conidia per ml for 30 min. Inoculated plants were individually transplanted to 1-liter pots filled with sand/silt. Roots of the control treatments were immersed in water. Six replications (pots) were established for each isolate × genotype combination, according to a complete randomized 3 × 3 factorial design. Five weeks after inoculation, symptoms developed in 100% of the plants in the three sunflower genotypes. Severity of symptoms (SS) in each plant was assessed as percentage of foliar tissue affected. Significantly higher SS occurred on inoculated plants as compared to non-inoculated plants, which did not develop symptoms. Mean disease severity on inoculated plants was 80% (averaged across isolates and genotypes). A significant effect of genotypes was obtained. Mean SS averaged across isolates were 98, 73, and 69% for HAR5, HA89, and Transol, respectively. When stem tissues from the three sunflower genotypes were sampled and incubated on potato dextrose agar at 25°C, the mycelial growth of Vd was confirmed for the inoculated plants but not for the control plants. Isolates of Vd infecting the resistant inbred line HA89 have only been identified in Argentina (1) and the United States (3). To our knowledge, this is the first report of a race overcoming the V₁ gene in HA89 in Europe. This poses a risk to commercial sunflower breeding programs in European countries. References: (1) A. B. Bertero de Romano and A. Vázquez. Page 177 in: Proc. 10th Int. Sunf. Conf., Surfers Paradise, Australia, 1982. (2) G. N. Fick and D. E. Zimmer. Crop Sci. 14:895, 1974. (3) T. Gulya. Helia 30:115, 2007. (4) J. Mercado-Blanco et al. Plant Dis. 87:1487, 2003. (5) W. E. Sackston. Plant Dis. Rep. 41:885, 1957.

Aggressiveness of Cephalosporium maydis causing late wilt of maize in Spain

Late wilt of maize, caused by the vascular and soilborne pathogen Cephalosporium maydis, was identified in the Iberian Peninsula in 2008. During the last years the incidence and economical impact of the disease has importantly increased both in Portugal and Spain. Varieties of maize displaying tolerance to the pathogen are available, but the effectiveness can be dependent on the virulence of the fungus (i.e. ability to cause disease on a specific genotype). On the other hand, strains of crop pathogens from different geographic origins can differ with regard to the degree of disease caused on a specific genotype (i.e. aggressiveness). Our working hypothesis was that isolates of C. maydis from different maize growing areas may differ in aggressiveness towards maize plants. Seven fungal strains were isolated in 2009 from diseased plants collected in the most important maize growing regions of Spain and used to inoculate two susceptible maize varieties grown in shadehouse from March to July 2010. The experimental unit consisted of two 4-day-old seedlings planted in an 8-liter pot filled with sand/silt previously infested with 200 g of wheat grains colonized by the fungi. Non colonized wheat grains were used for the control treatments. Six replications (pots) were established for each variety/isolate combination according to a complete randomized 2 x 8 factorial design. The percentage of necrotic and dry aboveground tissues was recorded 14 weeks after inoculation and thereafter weekly until physiological senescence of the control plants. At the end of the experiment, weights of roots and aboveground parts of the plants were recorded. Initial occurrence of symptoms in the plants was significantly dependent on the isolate of C. maydis and on the maize variety. However, final severity of aboveground symptoms (leaf necroses and drying up) was only dependent on the fungal isolate. All the isolates significantly reduced the root weight of both varieties of maize. The highest root weight reductions were also associated to a significant low weight of above-ground parts. Considering all the symptoms analysed and their progression in the maize plants, our results reveal that a diversity of aggressiveness exists among isolates of C. maydis. The need for a characterization of maize genotypes by their reaction against highly aggressive isolates of the fungus in the Iberian Peninsula is suggested. This study is a first step towards a recommendation of crop varieties that are tolerant to C. maydis in different areas of the Iberian Peninsula. Future research aims at studying the relationship between aggressiveness levels, molecular characteristics and geographical origin whithin C. maydis.