Estudaram-se clones de porta-enxertos de citros (Citrus spp.), híbridos e genitores quanto à tolerância das raízes a Phytophthora nicotianae através de inoculações em substrato de argila expandida. Investigaram-se progênies nucelares dos clones Poncirus trifoliata 'Rich 16-6', citrumelo 'Swingle' (C. paradisi x P. trifoliolata ), tangerinas 'Cleópatra' (Citrus reshni) e 'Suen Kat' (C. sunki), limão'Volkameriano' (C. volkameriana), dos genitores tangerina 'Sunki' (C. sunki) (S), limão 'Cravo' (C. limonia) (C), laranja 'Azeda' (C. aurantium) ((A), Poncirus trifoliata 'Davis A' (T), e progênies nucelares de híbridos entre eles, totalizando 2303 plântulas. Avaliou-se a taxa de sobrevivência, redução de raízes e partes aéreas, peso de raízes e de partes aéreas, diâmetro do caule e altura, comparando-se plântulas inoculadas e não inoculadas. Atribuíram-se também notas subjetivas para volume de raízes, enfolhamento, coloração das folhas e altura. Desenvolveu-se um índice total de redução (ITR) baseado na taxa de sobrevivência e nos parâmetros mencionados. Mostraram-se altamente tolerantes os trifoliatas 'Davis A' e 'Rich 16-6', o citrumelo 'Swingle', três híbridos TxS, dois SxT e dois SxA, com ITR < 20%. Laranja 'Azeda', tangerina 'Suen Kat', limões 'Cravo' e 'Volkameriano', oito híbridos TxS, quatro SxT, dois TxA e um SxA mostraram-se tolerantes, com ITR entre 20 e 40%. Três híbridos SxA mostraram-se moderadamente tolerantes, com ITR entre 40 e 60%. Tangerinas 'Sunki' e 'Cleópatra', dois híbridos SxA, um TxA e dois SxC mostraram-se intolerantes, com ITR entre 60 e 80%. Cinco híbridos SxC mostraram-se altamente intolerantes, com ITR > 80%. A metodologia de inoculação e avaliação discriminou com precisão progênies nucelares dos clones e dos híbridos, evidenciando o potencial de seleção principalmente dos híbridos SxT e recíprocos.

Clones of citrus rootstocks, hybrids and their parents were evaluated for tolerance to root rot incited by Phytophthora nicotianae. It were evaluated 'Rich 16-6' Trifoliate orange (Poncirus trifoliata), citrumelo 'Swingle' (C. paradisi x P. trifoliolata), 'Cleopatra' (Citrus reshni) and 'Suen Kat' (C. sunki) mandarins, 'Volkamer' lemon (C. volkameriana), the parents 'Sunki' mandarin (C. sunki) (S), Rangpur lime (C. limonia) (C), Sour orange (C. aurantium) (A) and 'Davis A' Trifoliate orange (T), and the nucellar progenies of hybrids between them, in a total of 2303 seedlings maintained in expanded clay. Tolerance was determined by survival rate, root and shoot growth reduction, root and shoot weight, stem diameter, plant height, visual ratings of root volume, shoot growth, leaf color and plant height comparing inoculated with non inoculated plants. A Total Reduction Index (ITR) based on survival rate and all the other parameters ranked clones and their hybrids. Trifoliate oranges, 'Swingle' citrumelo, three hybrids TxS, two SxT and two SxA performed as highly tolerant with ITR < 20%. Sour orange, 'Suen Kat' mandarin, 'Volkamer' lemon, Rangpur lime, eight hybrids TxS, four SxT, two TxA and one SxA were tolerant with ITR between 20 and 40%. Three hybrids SxA were moderately tolerant, with ITR between 40 and 60%. 'Sunki' and 'Cleopatra' mandarins, two hybrids SxA, one TxA and two SxC were intolerant, with ITR ranging from 60 to 80%. Five SxC hybrids were highly intolerant with ITR values > 80%. The methodology of inoculation and evaluation discriminated with good precision different levels of tolerance of nucellar progenies of clones and hybrids showing their selection potential, specially of the SxT hybrids and reciprocals.

Sete grupos de híbridos entre porta-enxertos elite de citros (Citrus sp.) e seus genitores foram estudados quanto a reação à infecção de tronco por Phytophthora parasitica, em plantas adultas no campo. Inocularam-se artificialmente, em duas posições do tronco, 132 plantas nucelares dos genitores e 486 híbridos entre limão (Citrus limonia) Cravo 'Limeira' (C), Poncirus trifoliata 'Davis A' (T), tangerina (C.sunki) 'Sunki' (S) e laranja Azeda (C. aurantium) 'São Paulo' (A). A classificação do grau de resistência à gomose de tronco em plantas adultas no campo foi possível somente quando baseada na média do tamanho de lesões de mais de 30 plantas nucelares, devido às grandes variações observadas em plantas individuais do mesmo clone, impossibilitando a seleção precoce de híbridos baseada em valores de plantas individuais. O trifoliata e a laranja Azeda mostraram-se bastante resistentes, desenvolvendo, em geral, lesões de tamanho reduzido. A tangerina Sunki e o limão Cravo desenvolveram lesões maiores, embora a tangerina Sunki tenha mostrado uma tendência em desenvolver lesões maiores que às do limão Cravo. Híbridos de trifoliata apresentaram, no geral, lesões intermediárias. Os híbridos de Azeda apresentaram lesões de tamanhos bastante variáveis, porém a maioria, com lesões grandes. Da mesma forma comportaram-se os híbridos recíprocos entre Sunki e Cravo.

Seven groups of elite citrus (Citrus sp.) rootstock hybrids and their genitors were field tested for gummosis caused by Phytophthora parasitica. Two trunk inoculations were made in 132 nucellar clones of the genitors and in 486 hybrids between 'Limeira' rangpur lime (Citrus limonia) (C), 'Davis A' trifoliate orange (Poncirus trifoliata) (T), 'Sunki' mandarin (C. sunki) (S) and 'São Paulo' sour orange (C. aurantium) (A). The degree of resistance to gummosis could be estimated from inoculation of adult plants in the field only when it was based on the mean lesions of more than 30 nucellar plants. This was due to the great variations observed in single individuals of the same clone although the conditions were well controlled. For this reason, it was not possible to make an early selection of hybrids based on the mean lesion area developed in single individuals. Trifoliate and sour oranges were quite resistant, displaying small-size lesions. Sunki mandarin and rangpur lime developed larger lesions, although Sunki mandarin tended to develop larger lesions than rangpur lime. In general, trifoliate hybrids showed lesions of intermediate size. Sour orange hybrids developed lesions that were quite variable in size although most of them were large. This same behaviour was observed among reciprocal SxC hybrids.

The present investigation reports reciprocal hybrids between Citrus sunki, belonging to the "True Citrus" taxonomic group, and Severinia buxifolia of the "Primitive Citrus" group. This represents the first case of sexual hybridization between genera of distinct taxonomic groups of the subtribe Citrinae, which have several complementary agronomic and phytosanitary attributes. Thus, there exists considerable interest in determining the potential of such hybrids as rootstocks, as well as a possible genetic bridge to combine, by breeding at diploid level, nuclear and cytoplasmic genes of Severinia with Citrus and other citrus intergeneric hybrids.