Efficient management of late blight in tomato crops

Late blight, also known as mela, is a disease caused by the oomycete Phytophthora infestans (Mont.) de Bary, being one of the most devastating in tomato cultivation worldwide (Irzhansky & Cohen, 2006), affecting all parts of susceptible plants in all their physiological stages. Late blight occurs in practically all tomato-producing regions in the country, with 15% to 20% of production costs being used to control the disease. It is characterized by being aggressive and has a great destructive impact, and can limit or even prevent the economic cultivation of tomatoes under favorable conditions (Lopes & Santos, 1994; Dehne & Oerke, 1998).

The disease is extremely dependent on climatic conditions and its importance is linked to the simultaneous occurrence of mild temperatures (between 15ºC and 20°C) and relative humidity greater than 85%, resulting from irrigation, fog, rain, dew, etc. Humidity plays an important role in the progress of the disease, acting in two phases of the pathogen's life cycle: in the direct production of sporangia and in germination; both sporangia and zoospores (Vale et al.

Symptoms

Late blight can attack all aerial organs of the tomato plant: leaves, stems, inflorescences and green and ripe fruits. The most obvious typical symptoms appear on the leaves, initially appearing irregularly shaped and dark colored lesions. These lesions expand, tending to a circular shape (Figures 1a and 1b). Under conditions of high relative humidity, a light, white band forms on the abaxial surface of the leaf, surrounding the lesion – the sporulation ring. This ring is composed of sporangiophores and sporangia of the pathogen. The lesions develop and as the leaf tissue is consumed, these areas necrotize and acquire a dry consistency, giving a burning appearance (scorching). Hence the origin of the name requeima (Mizubuti, 2001).

control methods

Currently, late blight management in tomato crops is carried out exclusively based on the principle of host protection, through the application of preventive and/or curative fungicides, totaling 15 to 25 fungicide applications per harvest. The most commonly used fungicides are: copper oxychloride, mancozeb, chlorothalonil, cymoxanil (protectors), dimethomorph, propamocarb and metalaxyl (systemic).

One of the problems generated by the indiscriminate use of chemical control is the selection of resistant isolates, which leads to the need to further increase the number of applications, making the control method often inefficient. Therefore, there is a need for breeding programs in order to seek the incorporation of resistance to late blight.

Genetical enhancement

Genetic improvement programs for resistance to P. infesteans in tomato cultivation began more than 60 years ago (Richards & Barratt, 1946), with the transfer of a dominant gene Ph-1 of the species L. pimpinellifolium providing resistance to P. infestans race 0 (Bonde & Murphy, 1952). However, the gene Ph-1 it was quickly supplanted by a new race of this pathogen, named race 1 (Conover & Walter, 1953). Breeders' efforts subsequently focused on the partial resistance encountered at the West Virginia 700 access (L. pimpinellifolium) (Gallegly, 1960), controlled by a single gene of incomplete dominance named as Ph-2 (Laterrot, 1975; Turkensteen, 1973).

A new resistance gene called Ph-3 was reported by AVRDC (Asian Vegetable Research and Development Center) in accession L3708 belonging to the species Lycopersicon pipenellifolium (Jusl.) Mill. (Black et al, 1996a; black et al, 1996b), controlled by a single allele with partial dominance. However, accession L3708 was found to be susceptible to several isolates of P. infestans (Chunwongse et al, 2002), in the same way as that found in the Ph-4 resistance gene positioned on chromosome 2 found in the species Lycopersicon pipenellifolium (Kole et al, 2006). Another gene that also confers vertical resistance to late blight was identified and located at the distal end of chromosome 1, called Ph-5 (Foolad et al.

According to Li et al (2011) these qualitative resistances (Ph-1, Ph-2, Ph-3, Ph-4 and Ph-5) are not durable due to the rapid evolution of compatible races of the pathogen. Thus, a second type of resistance is quantitative, controlled by quantitative trait loci (QTLs), which is most desired by breeders, as it is difficult to “break” by pathogens, even given the great genetic variability of P. infestans.

The genetic basis of quantitative resistance can be studied using molecular markers and phenotypic information to map QTLs in segregating populations (Young 1996; Doerge 2002). Quantitative resistance in tomato has been demonstrated in interspecific crosses between S. lycopersicum and several wild tomato species, including S. pimpinellifolium, S. pennellii, and S. habrochaites (Frary et al, 1998; Brower et al, 2004; Smart et al, 2007; AVRDC 2008; Johnson et al, 2012). However, according to Johnson et al (2012), knowledge of quantitative resistance for P. infestans It has not yet been incorporated into commercial tomato cultivars.

In Brazil, several works have been carried out at the Federal University of Viçosa (UFV), guided by professor Derly José Henriques da Silva (Figure 2), current curator of the UFV Vegetable Germplasm Bank (BGH-UFV). BGH-UFV is the second oldest vegetable germplasm bank in Latin America. In it, more than seven thousand accessions of the most diverse species of vegetables (Silva et al, 2001). The tomato plant stands out, Lycopersicon esculentum Mill, with around 850 registered hits. Among the works related to the evaluation of resistance to P. infestans carried out with these accesses, the works of Abreu stand out et al (2008), Fiorini et al (2010a), Fiorini et al (2010b), Elsayed et al (2011) and Elsayed et al (2012).

Abreu et al (2008), studying the inheritance of resistance to P. infestans, resulting from interspecific crossing between species S. habrochaites (BGH – 6902) e S. lycopersicum (cultivar Santa Clara), verified polygenic resistance, governed by 28 genes, and also observed greater importance of additive effects for the character.

Fiorini et al (2010a), selected ten F strains₈ originated from interspecific crossing between Solanum habrochaites (BGH – 6902) e Solanum lycopersicum (cultivar Santa Clara). Fiorini et al (2010b) estimated late blight progress curves in tomato genotypes to identify groups of genotypes resistant to the disease and concluded that the cluster analysis of the disease progress curves allowed the identification of eight distinct groups in terms of resistance, in addition to the selection of several hybrids.

You shake et al (2011), through diallel analysis, observed ppredominance of effects of the general combining ability (CGC), suggesting that the effects additives were more important than the effects do not additive. They also reported the presence of resistance polygenic controlled by genes recessive. You shake et al (2012) studied the inheritance of resistance and determined the genetic factors that contribute to the resistance of the '163A' lineage resulting from interspecific crossing between the species S. lycopersicum e S. habrochaites and verified that two recessive genes act in resistance to late blight, confirming the absence or neglect of epistatic effects.

Currently, two studies are being supervised by Professor Derly da Silva, one being conducted by Bruno Soares Laurindo (Figure 3), with the aim of identifying sources of resistance to late blight within the cultivated species. Solanum lycopersicum, because, when the resistance gene is identified in a genotype of the same species that is to be improved, intercrossing is easier for gene transfer and faster recovery of agronomic characteristics. The second research is being conducted by Alcinei Mistico Azevedo (Figure 4) with the objective of identifying molecular markers and mapping QTLs associated with resistance to late blight in populations originating from crossing the BGH 6902 accession with the Santa Clara cultivar, with the aim of obtaining information for selection assisted by molecular markers.

Future perspectives

It is expected that with new knowledge of sources of quantitative resistance against P. infestans allyos greater accessibility of molecular analysis techniques is facilitated the genetic improvement of tomatoes, indicating that soon, there will be tomato cultivars with a higher level of resistance than pre-existing ones are on the market.

Click here to read the article in Revista Cultivar Hortaliças e Frutas, issue 79.