1Dept of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, South Africa

Background and objectives
Phytophthora spp. are known to cause serious root disease of citrus worldwide. Pathogenic isolates of Fusarium solani were shown to cause severe feeder root rot of citrus. Although citrus rootstocks are often screened for their tolerance against Phytophthora spp., they are seldom evaluated against Fusarium spp. In this study 20 rootstocks were evaluated against both pathogens individually and in combination. The ability of rootstocks to regenerate roots after 50% of the fibrous roots were pruned off, were evaluated to establish whether this could be a mechanism for tolerance.

Materials and methods
The following twenty citrus rootstocks were evaluated in the greenhouse: Swingle citrumelo, Wallace rough lemon, Terra bella citrange, Obovoideae, Cleopatra mandarin, Natsudaidai, Changsha mandarin, Shekwasha mandarin, Jacobsen trifoliate, Rusk citrange, Australian trifoliate, Sunki mandarin, Sunchusha mandarin, Rubidoux trifoliate, Japanese citron, Sampson tangelo, smooth flat Seville as well as three rootstocks under code numbers 1113 (Citrus reticulata Blanco), 1112 (Poncirus trifoliata (L.) Raf. var. monstrosa (T. Ito.) Swing X C. sunki Hort.) 116 (Poncirus trifoliata (L.) Raf. var. monstrosa T.Ito.) Swing X C. sunki ex Tan.).

The rootstocks were tested in two groups with Swingle citrumelo and Wallace rough lemon included in each group as a tolerant and susceptible standard respectively. Pathogenic isolates of P. nicotianae and F. solani cultured on millet seed was used to inoculate seedlings grown in a sand/peat/soil mixture. Plants were evaluated after 3 months. In a separate series of experiments rootstocks were evaluated for their ability to regenerate roots in the absence of pathogens after 50% of the fibrous roots were removed.

Results and conclusions
The performance of Swingle citrumelo which was previously reported to be tolerant to Phytophthora[1] varied with experiments, whereas Wallace rough lemon consistently showed susceptibility to P. nicotianae. Both these rootstocks varied with experiments in terms of its reaction to F. solani. In the first group of rootstocks, Obovoideae was most tolerant, Rusk citrange intermediate and Wallace rough lemon most susceptible to P. nicotianae. Jacobsen trifoliate and Rusk citrange showed tolerance against F. solani, whereas Changsha mandarin and Natsudaidai were the most susceptible. In the second group Swingle citrumelo was most tolerant and Rubidoux trifoliate intermediate against both pathogens. Japanese citron and Wallace rough lemon were most susceptible to P. nicotianae while Sunchusha mandarin and Australian trifoliate were most susceptible to F. solani. Seven of the 20 rootstocks showed significantly less damage when inoculated with both pathogens in combination as compared to Phytophtora alone. This finding indicate a protective effect of F. solani against P. nicotianae (with some rootstocks) in accordance with previous findings[2]. Tolerance to both pathogens corresponded with a greater capacity for root regeneration only in the case of Swingle citrumelo and 1116. Susceptibility corresponded with poor root regenerating capacity in the case of Australian trifoliate with both pathogens and Japanese citron with P. nicotianae.

1. Graham JH, 1990. Plant Disease 74: 743-746.
2. Strauss J, Labuschagne N, 1994. Applied Plant Science 8: 14-18.