Heterosis for Resistance against Sorghum Downy Mildew [Peronosclerospora sorghi (Weston and Uppal) C.G. Shaw] Infection in Maize (Zea mays L.)

Background: Sorghum downy mildew (SDM) of maize caused by the fungus Peronosclerospora sorghi (Weston and Uppal) C.G. Shaw is one of the most important diseases of maize and it remains vital constraint in maize production. Since maize is the highly cross pollinated crop, exploitation of heterosis is one of the breeding methods to increase the production. Negative values in the expression of heterosis indicate a contribution towards resistance which is highly needed in order to have reduced incid ence of sorghum downy mildew in maize hybrids.Methods: The materials selected as parents for the present study consisted of nine maize inbred lines which comprised of three resistant (UMI102, UMI936(w) and UMI285), three moderately resistant (UMI176, UMI13 and UMI57) and three susceptible (UMI79, UMI432 and UMI467) inbreds to SDM incidence. Nine inbred lines were crossed in all possible combinations including reciprocals in diallel fashion and the resultant seventy two hybrids along with their parents and the check (COH(M)5) were raised and evaluated for resistance against SDM under glass house condition. The extent of heterosis was estimated over mid parent, better parent and popular check hybrid.Result: Analysis of variance for sorghum downy mildew incidence revealed highly significant differences among parents and hybrids indicating greater diversity among the genotypes for sorghum downy mildew incidence under study. Out of seventy two hybrids, only three hybrids namely UMI 13 x UMI 936 (W), UMI 467 x UMI 936 (W) and UMI 432 x UMI 936 (W) exhibited three types heterosis significantly in negative direction.

Peronosclerospora sorghi (sorghum downy mildew).

P. sorghi causes sorghum downy mildew, which can result in severe economic losses of both sorghum and maize. It has been a particularly invasive pathogen: it was introduced to the Americas where it has spread on both sorghum and maize causing considerable damage (Frederiksen et al., 1973). Quarantine restrictions probably maintained the USA free of sorghum downy mildew until the early 1960s (Reyes et al., 1964; Frederiksen, 1980a). Information on the spread of the disease and the damage it causes is available in the literature reporting its spread in the USA (Frederiksen et al., 1970) and elsewhere in the Americas (Frezzi, 1970; Grobman, 1975; Burtica et al., 1992).

Potensi Pengusulan Jenis Baru Peronosclerospora sorghi Asal Nusa Tenggara Timur

Potential Proposal for A New Type of Peronosclerospora Sorghi from East Nusa Tenggara Downy mildew of corn caused by Peronosclerospra can cause real economic damage. In Indonesia, it is known that there are three species of Peronosclerospora, namely P. maydis, P. phillippinenis, and P. sorghi. Peronosclerospora maydis is the dominant species found in Indonesia. The three species can be identified from their morphological and molecular characters. However, the results of monitoring by the Kupang Agricultural Quarantine Station during 2016-2019 showed that the morphological characteristics of P. sorghi from NTT are different from the morphology from Java and Sulawesi in terms of the number of sterigmata formed from conidiophores. The number of P. sorghi sterigmata from NTT is less than in other locations. This will lead to a smaller number of conidia produced by P. sorghi from NTT. Based on the molecular analysis, the character of P. sorghi isolate from NTT is in a separated family tree from Java and Sulawesi isolates but is included in one group with P. sorghi isolates from USA. The results of morphological and molecular studies showed further study on host ranges, genetic diversity of the fungal isolates as well as shorgum should be considered in the future. Sorghum as the primary host of P. sorghi has been gathered for the genetic data of sorghum originating from NTT and then comparing to the data from Texas, USA. That information will be contributing to determine the identity of P. sorghi from NTT.

Identification of Maize Downy Mildew Pathogen in Lampung and the Effects of Varieties and Metalaxyl on Disease Incidence

Introduction: Maize downy mildew (MDM) is considered as a major problem in all maize growing areas in Indonesia including the Province of Lampung. Objectives: The objectives of this study were (i) to identify the species of Peronosclerospora causing maize downy mildew (MDM) in Lampung, (ii) to determine the influence of varieties on the intensity of downy mildew and (iii) to determine the efficacy of metalaxyl to control MDM on some maize varieties. Methodology: To identify Peronosclerospora causing MDM, the pathogens were observed under light microscope and scanning electron microscope. Maize varieties response against pathogens and efficacy metalaxyl were studied in the field with the test plants exposed to plants showing MDM symptoms as the sources of inocula to mimic natural conditions. Results: Peronosclerospora sorghi, P. maydis, and P. philippinensis were found to cause MDM in Lampung. On both varieties Pioneer 27 (P-27) and NK-22, AUDPC on F1 plants was greater than that on F2 plants. On P-27, the production of F2 was higher than that of F1 plants, but there was not significant difference in production between F1 and F2 of NK-22 variety. Seed treatment using metalaxyl was not effective to control downy mildew of maize. Conclusion: Three species of Peronosclerospora were identified as disease-causing pathogen of MDM in Lampung, but these results were tentatively pending for further studies through molecular techniques. F1 plants were more susceptible than F2 plants to MDM in both P-27 and NK-22. Metalaxyl was not effective in controlling MDM.

Identification and Validation of Candidate Genes Conferring Resistance to Downy Mildew in Maize (Zea mays L.)

Downy mildew (DM) is a major disease of maize that causes significant yield loss in subtropical and tropical regions around the world. A variety of DM strains have been reported, and the resistance to them is polygenically controlled. In this study, we analyzed the quantitative trait loci (QTLs) involved in resistance to Peronosclerospora sorghi (sorghum DM), P. maydis (Java DM), and Sclerophthora macrospora (crazy top DM) using a recombinant inbred line (RIL) from a cross between B73 (susceptible) and Ki11 (resistant), and the candidate genes for P. sorghi, P. maydis, and S. macrospora resistance were discovered. The linkage map was constructed with 234 simple sequence repeat (SSR) and restriction fragment length polymorphism (RFLP) markers, which was identified seven QTLs (chromosomes 2, 3, 6, and 9) for three DM strains. The major QTL, located on chromosome 2, consists of 12.95% of phenotypic variation explained (PVE) and a logarithm of odds (LOD) score of 14.12. Sixty-two candidate genes for P. sorghi, P. maydis, and S. macrospora resistance were obtained between the flanked markers in the QTL regions. The relative expression level of candidate genes was evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) using resistant (CML228, Ki3, and Ki11) and susceptible (B73 and CML270) genotypes. For the 62 candidate genes, 15 genes were upregulated in resistant genotypes. Among these, three (GRMZM2G028643, GRMZM2G128315, and GRMZM2G330907) and AC210003.2_FG004 were annotated as leucine-rich repeat (LRR) and peroxidase (POX) genes, respectively. These candidate genes in the QTL regions provide valuable information for further studies related to P. sorghi, P. maydis, and S. macrospora resistance.