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Tolerance of hybrids and rootstock clones of citrus to root rot infection caused by Phytophthora nicotianae

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.

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