Screening for tolerance of different citrus rootstocks against zoospores of Phytophthora nicotianae in infested soil

Abstract

Citrus gummosis, caused by Phytophthora nicotianae, becomes one of the most destructive diseases in citrus production in Tunisia. Thus, the selection of resistant rootstocks is needed to reduce the damage caused by this disease. In the present study, five citrus rootstocks (Citrange carrizo (Citrus sinensis x Poncirus trifoliata), Citrumelo Swingle-4475 (Citrus paradisi x Citrus trifoliata), Citrus volkameriana (Citrus limon x Citrus aurantium), Citrange C-35 (Citrus sinensis 'ruby blood' x Poncirus trifoliata) and Sour orange (Citrus aurantium)) were screened against P. nicotianae by inoculating young citrus seedlings with the freshly zoospores suspension. After three months of inoculation, the disease severity was evaluated based on root damaged, plant height, stem and root weights (fresh and dry) and density of P. nicotianae population in infested roots. The evaluation of the rootstocks response revealed different levels of susceptibility against P. nicotianae. Regarding to all the parameters studied, results showed that the rootstock Citrumelo Swingle-4475 and the rootstock Sour orange are tolerant to P. nicotianae. The rootstock Citrange C-35 was considered as a moderately tolerant, while the two rootstocks Citrange carrizo and Citrus volkameriana were very susceptible. The index severity was ranged from 1 for the tolerant rootstocks to 3.16 for the very susceptible one. The highest percent of the stem growth rate was 75.5 % recorded with the tolerant rootstocks, while it ranged between 18.25 % and 19 %, respectively, for the moderately tolerant and for the susceptible rootstocks. In the case of the other parameters like the fresh and the dry weight of stem and root, the tolerant rootstocks showed the minimum percent of reduction (17.8 %). However, the minimum percent of reduction of the moderately tolerant and of the very susceptible rootstocks was ranged between 38.5 and 30.8, respectively. The lowest number of propagules of P. nicotianae was found in the tolerant rootstocks (4.25), the highest number was found in the very susceptible rootstocks (24.5). However, the number was 12.25 in the case of the moderately tolerant one. It is believed that the rootstock Citrumelo Swingle-4475 should be involved with the resistance of citrus plants to P. nicotianae.