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Effect of Genetic Variation on Growth and Gall Rust Disease Resistance in Falcataria moluccana (Miq.) Barneby and J.W. Grimes in Tawau, Sabah, Malaysia
Abstract:
Batai (Falcataria moluccana [Miq.] Barneby and J.W. Grimes) is one of the valuable multipurpose tree species for forest plantations in Malaysia and Indonesia. However, since 1993, gall rust disease had been identified as dangerous malady of batai causing severe damage to all growth stages of the plant from seedlings in the nursery to mature trees in the field. Thus, the objectives of this study were to (1) evaluate the environmental factors affecting disease development, (2) characterize and identify the gall rust fungus and gall rust symptoms, (3) screen F. moluccana seedlings from 11 seed sources for growth and gall rust disease resistance, and (4) assess the genetic variation of those seed sources and its relationship with gall rust disease severity.
Based on the results from the stepwise multiple regression analysis, it was found that open sites with flat topography, preferably without the occurrence of fog did not favor gall rust disease development. In addition, silvicultural treatments such as pruning, thinning, and clear cutting reduced gall rust disease incidence and severity. Relative humidity and wind speed were two meteorological factors significantly associated with the incidence and severity of gall rust disease. High relative humidity (RH ≥ 90%) and slow wind speed (WS ≤ 80 kmph) were found to promote gall rust disease development. In addition, it appeared that the spread of gall rust spores originated from the north, probably from the Philippines to Brumas Estate, Tawau, Sabah, Malaysia.
Gall rust disease on F. moluccana in Brumas Estate was caused by the gall rust fungus Uromycladium tepperianum (Sacc.) McAlp. (Family: Pileolariaceae, Order: Uredinales, Class: Urediniomycetes). The fungus produced teliospores, which have ridged longitudinal striations with three spores on each head. The size of the teliospores ranged from 13 to 18 μm wide and 17 to 26 μm long. The fungus completed entire life cycle on one host (i.e., F. moluccana). This study confirmed that the teliospores cannot themselves infect the host; they have to germinate to produce basidiospores, which were formed at least 10 h after inoculations. Then a penetration peg was formed by a matured basidiospore 16 h after inoculation and penetrated the host cells directly through the epidermis. Seven days after inoculation (DAI), vegetative mycelia of this gall-forming rust give rise to pycnia, recognized as small brown pustule, which breaks through the epidermis. The typical symptom of gall rust disease on the seedlings is bending of the stem or shoot, either with or without the formation of a dark red necrotic lesion. However, the symptoms on older trees range from the development of large chocolate brown, irregularly shaped, cauliflower-like or whip-like galls on the stem, branch, petiole, shoot, pod, seed, or flower stalk. The surfaces of mature galls are generally covered with cinnamon-colored spores. The older galls become reddish which eventually turned black and they are commonly invaded by tunneling insects.
Based on the rate of infection, cumulative mortality due to gall rust disease, and the effect of gall rust disease on relative growth rate, seedlings from Wamena seed source were moderately resistant. This was the best seed source of the 11 seed sources tested. Generally, the effect of gall rust disease in decreasing the relative growth rate of seedlings was only significantly different at 47 DAI, particularly for seedlings from RO5/95 and Walang Gintang seed sources. Resistance to gall rust disease was accounted by approximately 41-46 percent of genetic and 54-59 percent of environmental factors. The qualitative character of height was accounted for by approximately 50-84 percent and 16-50 percent of genetic and environmental factors, respectively. The genetic correlation between gall rust disease severity and height of seedling was high (85%) and positive, particularly at 27 DAI.
The genetic diversity of 11 seed sources of F. moluccana seedlings assessed using random amplified polymorphic DNA (RAPD) technique was small, with 1.036- 1.094 effective alleles, 34-55 polymorphic loci, 35.05-56.76 percent proportion of polymorphic loci, Shannon diversity index of 0.115-0.192, and Nei’s diversity index of 0.18-0.29. In addition, the genetic distance between seed sources was narrow (0.036-0.152). All seedlings from Brumas seed sources (RO2, RO5, R2001, and 2S/75) were closely related to those from East Timor, East Flores, Moluccas, and Java, but were distant from Wamena. There were negative and small relationship between polymorphic loci, Shannon’s diversity index, Nei’s diversity index, and gall rust disease severity at 7, 17, and 27 DAI (R2 = 4-27%). However, their relationship at 37 and 47 DAI were positive and relatively moderate (R2 = 39-49%). Thus, the correlation between genetic variation and gall rust disease severity of F. moluccana seedlings was inconsistent by times and their relationship was not strong.
Since gall rust disease resistance was accounted by genetic and environmental factors almost at the same proportion, applying integrated gall rust management control becomes essential. Site selection, using genetically resistant material, regular monitoring, pruning, thinning in the field, and chemical control in the nursery are principle methods for preventing gall rust disease of F. moluccana at Brumas Estate.