Phytophthora and Phytopythium in walnut plantations of North Patagonia, Argentina

JORGE ARIEL MARFETAN; DIANA FERNANDEZ; MARIA LAURA VELEZ

Santiago de Chile

Congreso; X congreso Latinoamericano de Micologia; 2020

Walnut (Juglans regia L.) is the most widespread tree nut in the world. At present, walnut is cultivated commercially throughout southern Europe, northern Africa, eastern Asia, USA and South America. World production of whole walnut (with shell) was around 1.5 × 106 t in 2008. China is the leading world producer, followed by the USA, Iran, Turkey, Ukraine, Romania, France and India, but production in other countries such as Chile and Argentina has increased rapidly in recent last years. Walnut is a crop of high economic interest to the food industry: the edible part of the fruit (the seed or kernel) is consumed, fresh or toasted, alone or in other edible products. It is globally popular and valued for its nutritional, health and sensory attributes. Argentina has notably increased both the production and the quality of walnut produced, with more than 15.000 hectares involved in walnut production. In Argentina, walnut tree nurseries have focused their supply mainly on rootstocks and grafted plants. Usually, the rootstock is planted and after a year the graft is made in the field. Instead, the grafted plant consists of a 2 or 3-year-old plant composed of a chosen rootstock and a developed graft of the selected variety. Various rootstocks are marketed, being the most sold and used Juglans regia and Juglans hindsii, and to a lesser extent the hybrid Paradox (J. hindsii x J. regia) and Juglans nigra. The grafted varieties are mainly Chandler (98%) followed by Franquette (2%) and in a very low extent Tulare and Serr. In North Patagonia the most widely planted rootstock is J. hindsii, together with the Chandler variety. However, J. regia and Paradox are less frequently used, and J. nigra is rarely used nowadays. In Argentinean North Patagonia a decline of J. hindsii rootstock has been recently observed. Symptoms range from chlorosis and withering to sudden wilt. In addition, bleeding cankers and necrotic inner bark lesions at the root collar and lower stem were observed. Symptoms and signs agree with diseases caused by Oomycets such as Phytophthora. Until now, the etiological agent of this disease remains unknown. For this reason, the aim of these work was isolate and identified the etiological agent causing walnut dieback in North Patagonia.In this study 8 sites showing walnut decline were sampled. All sites were private properties located in the Mid Valley, Rio Negro province, Argentina. Isolations were made in the field from symptomatic trees by direct plating of necrotic tissues into selective media. Soil samples were also taken, and isolations were made using pears, apples, and Rhododendron sp. and walnut leaves as baiting. Presence of Oomycetes was also checked in irrigation systems by filtration (10 l/sample) and placing the filters in selective medium. Pure cultures were molecularly identified using ITS region (primers ITS 4 and ITS5). Isolates belonging to Phytophthora and Phytopythium were tested in pathogenicity trials, performed in a controlled chamber, by stem inoculation of 1-year-old J. hindsii seedlings. As result, Phytophthora samsomeana, P. litoralis, P. borealis, Phytopythium vexans and Ph. helicoides were isolated and identified. P. samsomeana, P. litoralis, Ph. vexans and Ph. helicoides produced necrotic dark brown inner bark lesions at the inoculation point , which demonstrated to be pathogenic to J. hindsii. However, P. litoralis and Ph. vexans were shown to be the most aggressive species producing deeper and extended light brown lesions. Phytophthora borealis was the unique of the isolated species that was not pathogenic to J. hindsii. As a conclusion, several species, included in two genera, are able to affect the health condition of walnut plantations in North Patagonia. Finally, an Elisa Test was used to analyse the incidence of Phytophthora and Phytopythium in walnut plantations of the Mid Valley. With this test, it was calculated that 0.5% to 3% of the plants in each plantation are annually afected. We concluded that walnut decline is a very complex process and could be caused by different species in different genera and so it should be managed in a integral way.