Universität Hohenheim

1. TROZ Centre for Agriculturein the Tropics and Subtropics UniversitätHohenheim Institute of Plant Nutrition (330) A BIOLOGICAL DISEASE-MANAGEMENT SYSTEM FOR CITRUS PRODUCTION IN SAO PAULO STATE / BRAZIL 1. PROBLEM During the last decade, severe distribution of the bacteriosis CVC (Citrus Variegated Chlorosis, Xylella fastidiosa; Fig. 1B) and of the virus "Sudden Death" (Tristessa Virus) has been observed in wide- spread traditional citrus production systems. Intensive use of herbi- cides (Glyphosate) and of mineral fertilizers, applied between the tree rows is characteristic for these production systems (Fig.1A). Meanwhile, more than 50% of the citrus orchards in Sao Paulo State are affected by these diseases and farmers have to change to other crops (e.g. sugar cane). 2. OBSERVATIONS In some organic farms, a biological management system with intensive mulching using Brachiaria grass (Fig. 2A) and without application of herbicides, alleviated or even avoided expression of disease symptoms on trees and fruits (Fig. 2B). Traditional Biological B A B A Mn 12.3 49.0 mg/kg Zn 13.3 57.3 DM Brachiaria mulch Fig. 2A/B) Biological production system Fig. 1A/B) Traditional production system 3. WORKING HYPOTHESIS Predominant use of nitrate fertilizers in the traditional Citrus production system limits plant availability of Mn and Zn (Fig. 1B), which is further aggravated by regular application of Glyphosate herbicides (Tab.1). A low Mn-, and Zn-nutritional status will result in low disease resistance (e.g. against CVC). In contrast, in the innovative biological production system, Brachiaria mulch inhibits nitrification, which will lead to rhizosphere acidification by preferential ammonium uptake and will thereby increase the plant-availability of Mn and Zn, which is further improved by omission of Glyphosate herbicides and inoculation with Mn-mobilizing microorganisms (e.g. Trichoderma as biofertilizers; Fig.3) M. Guldner, G. Neumann, V.Römheld - Institute of Plant Nutrition (330), Hohenheim University, Stuttgart, Germany R. Cabrera - Novais-SP, Brazil co. V. Römheld: Phone: +49 711 459 3714 T. Yamada - POTAFOS – Piracicaba-SP, Brazil Fax: +49 711 459 3295 I. Cakmak – Sabanci University, Istanbul, Turkey e-mail: roemheld@uni-hohenheim. Fig. 3) Model for the working hypothesis 4. RESEARCH APPROACH 4.1 Field Experiments in Brazil In Sao Paulo State, three field experiments (Novais, Ibira, Uchoa) with varied fertilization (Fig.4) were set up as a long-term study. 4.2 Model pot experiments in Hohenheim with Citrus seedlings, sunflower and soybean as model plants. - Incubation experiments to confirm inhibition of nitrification under Brachiaria mulch and resulting rhizosphere acidification. - Nutrient solution-, and rhizobox-experiments to demonstrate inhibition of Mn acquisition by root-released glyphosate from target plants (Table1, Fig.5) and improved Mn uptake by inocu- lation with Mn-mobilising microorganisms (Trichoderma). Conventional practice (Glyphosate) Conventional + soil and foliar Mn, Zn Conventional + (NH4)2SO4+ NI Conventional + (NH4)2SO4+ NI + Mn, Zn Conventional – Glyphosate Conventional + Trichoderma konegeir. Innovative with Brachiaria mulch Innovative + soil and foliar Mn, Zn Innovative + (NH4)2SO4+ NI Innovative + Trichodermakonegeir. (NI = Nitrification inhibitor) Treat Mn-Translocation (rel. values) ment Sunflower Soybean Soybean BTX Glyphosate 0% 100 100 100 2% 4 12 57 6 % 3 4 60 Soybean Target Sunflower Indicator Fig. 4) Field experiments in Brazil 5. EXPECTATIONS An improved Mn(Zn)-nutritional status will result in: - better disease resistance against CVC and Sudden Death - reduced application of herbicides - improved soil fertility and reduced soil degradation by mulching as an approach for sustainable citrus production Table 1) Effect of glyphosate at drift concen- trations (2% and 6% of the recommended minimum Dosage = 15%) on Mn translocation in sunflower and soybean plants at 24 h after glyphosate application. Fig. 5) Chlorosis symptoms on sun-flower leaves after simultaneous cultivation with Glyphosate-treated soybean plants