Citrus trees are susceptible to a variety of diseases and can be combating more than one at the same time. In the Lower Rio Grande Valley (LRGV) of south Texas, a common oomycete causing major crop losses in citrus is Phytophthora nicotianae (Kunta et al., 2007). Two species of Phytophthora, P. nicotianae Breda de Haan (synonymous with P. parasitica Dast.) and P. citrophthora (Sm. & Sm.) Leonian are the most prevalent and highly destructive pathogens affecting citriculture worldwide (Graham and Menge, 2000). Another species, P. palmivora, causes serious damage to fibrous roots and causes citrus brown rot in Florida and Japan (Tashiro et al., 2012; Zitko and Timmer, 1994). Only P. nicotianae has been confirmed in Texas (Chaudhary, 2018; Kunta et al., 2007). Phytophthora can reproduce asexually through sporangia and zoospores as well as sexually through oospores (Meng et al., 2014). The thick-walled long-term survival spores called chlamydospores germinate during wet weather to produce sporangiophores bearing sporangia. The sporangia will develop short-lived motile zoospores with flagella that can swim in water and infect a new host plant. The chlamydospore will remain in the soil for several years and can serve as an inoculum source until the environmental conditions are optimal.
Phytophthora causes both above and below ground diseases, such as foot rot and gummosis of the trunk, brown rot of fruit, and fibrous root rot. Fibrous root rot may go unnoticed by growers until the canopy shows stunting, premature defoliation and branch dieback (Naqvi, 2004). Foot rot infection begins above the soil surface where a lesion on the bark can extend from the scion to the base of the rootstock (Savita and Nagpal, 2012). Gum will exude from diseased bark, serving as a clear characteristic of P. nicotianae infection and is referred to as gummosis (Graham and Menge, 1999). Gumming is more noticeable during dry weather vs. wet seasons when rain may potentially rinse away obvious gumming evidence (Savita and Nagpal, 2012). Plants infected with P. nicotianae may have a decayed root system leaving roots sloughed of all outer layers. The production of new fibrous roots is apparently outpaced by root death in heavily infected mature trees (Graham, 1995). Root death will ultimately decline the overall health of the host plant as seen in tobacco, ornamental plants, tomato, and citrus cultivars (Lamour et al., 2003).
The economic losses for the citrus industry worldwide exclusively caused by P. nicotianae are difficult to estimate as most trees are often affected by more than one disease at the same time (Ludowici et al., 2013). In California, the largest citrus producer in the United States, brown rot and root rot caused by Phytophthora accounted for $12.9 million annual loss (Savita and Nagpal, 2012). In Florida, the second top U.S. citrus producer, Phytophthora damage has been estimated to be ≈$30 to $60 million annual yield loss, without control treatments (Graham and Menge, 1999). In Texas, there are no reports on the impact of Phytophthora; however, because of the use of flood irrigation, Phytophthora is endemic.
Phytophthora nicotianae disease management strategies include use of resistant rootstocks, biological control, fungicides, clean material for propagation, and irrigation with adequate drainage (Cacciola and di San Lio, 2008). Metalaxyl and phosphite fungicides have been reported to be effective in combating Phytophthora diseases when applied at proper dosages and at right time frame (Graham and Feichtenberger, 2015). Phytopththora can be controlled with fungicides, but overuse may render them ineffective through resistance development (Timmer et al., 1998). Biological control efforts using Penicillium funiculosum and Chaetomium globosum on citrus cultivars (Fang and Tsao, 1995; Hung et al., 2015) have been proven effective; however, it is not preferred as a large-scale control strategy due to specific temperature, soil pH, and host preference for different biological control agents.
Use of resistant rootstocks is an ideal and long-term solution to fight Phytophthora diseases. Sour orange is one of the few rootstock that can produce high yields and yet tolerate the high pH calcareous soils common in the LRGV, and therefore, it is the predominant rootstock planted in this area. Phytophthora infected trees with typical symptoms of foot rot, root rot, and gummosis are commonly present. Field trials with Phytophthora-infected rootstocks in Florida showed Swingle citrumelo and Carrizo citrange having high seedling mortality whereas sour orange did not have significant seedling mortality (Bowman et al., 2002, 2003). Graham (1990) tested seven rootstocks including trifoliate orange (Poncirus trifoliata), Ridge Pineapple sweet orange, Carrizo citrange, Swingle citrumelo, sour orange, Cleopatra mandarin, and Volkameriana lemon; the results revealed that trifoliate orange and Swingle citrumelo were among the most tolerant to root rot. Similarly, trifoliate orange and Swingle citrumelo performed better than the other rootstocks in their ability to regenerate roots under Phytophthora infection (Graham, 1995). In Florida, Carrizo citrange and Swingle citrumelo were considered tolerant and moderately resistant rootstocks, respectively (Graham and Feichtenberger, 2015); however, the same rootstocks failed to grow under field trials conducted in the LRGV (Louzada et al., 2008) due to soil characteristics.
There is an urgent need for a superior rootstock that can provide resistance to Phytophthora diseases, tolerate alkaline soil and the weather conditions in the LRGV area, and yet produce high quality fruit. Sour orange rootstock plants show genetic diversity (Lamine and Mliki, 2015) among them which may reflect in differences in relative tolerance to Phytophthora diseases. To find an alternative rootstock possessing all the qualities needed for P. nicotianae tolerance and successfully grow in the LRGV; four citrus rootstocks including sour orange rootstock were subjected to high concentrations of P. nicotianae inoculation.
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