Alternaria em pomares de tangerina de Minas Gerais: diversidade e sensibilidade a fungicidas

Abstract

Alternarioses are diseases that result in high economic loss in citrus crops around the world. In Brazil, especially, countryside cities and the metropolitan region of the Minas Gerais State, tangerine orchards and their hybrids have great economic and social importance. Most of these orchards are composed of cultivars susceptible to diseases caused by fungi of the genus Alternaria, and management of these diseases is based on chemical control with site-specific fungicides, however reports of resistance have intensified in recent years. As a way to evaluate the presence of Alternaria species, their distribution in orchards of the Minas Gerais State, and the sensitivity of these fungi to triazoles, strobilurins and carboxamides, leaves and fruits with characteristic symptoms of alternarioses were collected. The obtained isolates were subjected to pathogenicity testing and sequencing of the genes TEF 1α (translation elongation factor 1-alpha), GAPDH (glyceraldehyde-3-phosphate dehydrogenase), OPA (anonymous gene region) and RPB2 (RNA polymerase II). The macromorphology and micromorphology of the isolates colonies presented characteristics consistent with the genus Alternaria. To verify the sensitivity to fungicides, 58 isolates were evaluated at discriminatory doses of 1 µg mL⁻¹ (Pyraclostrobin and Tebuconazole) and 5 µg mL⁻¹ (Boscalid). Among them 30 isolates were selected for the determination of the EC₅₀ (concentration inhibiting 50% of mycelial growth or germination) for Boscalid, Fluxapyroxad, Difenoconazole and Tebuconazole. Based on the pathogenicity test, isolates of A. alternata can cause diseases in lemon and orange plants. Analysis of concatenated genes identifies A. alternata and three other pathogenic species, indicating tangerines and hybrids are susceptible to more Alternaria species than previously thought. Regarding fungicides sensitivity, the frequency of resistant isolates was: 46.6%, 36.7%, 33.3%, 16.7% and 96.7% for Pyraclostrobin, Boscalid, Fluxapyroxad, Difenoconazole and Tebuconazole, respectively. Fitness cost parameters were evaluated and, it can be observed that isolates resistant to one or more molecules did not suffer penalties regarding mycelial growth, germination, oxidative and osmotic sensitivity.