Morphological Characteristics and Identification of Algae Species on Oil Palm Leaves in an Oil Palm Estate in Riau, Indonesia

Algae infection on oil palm leaves has been observed fairly widespread in many oil palm estates in various regions in Indonesia, including Riau, Belitung, and Palembang. Based on field census, algae can be found on young to old palms and were generally more common on the lower (older) fronds. Algae infection appeared as opaque reddish-orange spots atop the leaf surface. Since algae cover the leaf surface, it may cause a decrease in the photosynthesis activities of the affected leaves. Identification of algae species is needed to know the biological properties for effective control measures. A study was carried out to identify the species of algae on oil palm leaves from an estate in Riau Region, Indonesia. According to morphological characteristics such as color, shape, and cell dimension, the algae was classified as green algae (Chlorophyta: Trentepohliales) and was identified as Phycopeltis arundinaceae.


Introduction
Algae infection on oil palm leaves has been widely reported in Indonesia.Symptoms include the presence of reddish-orange spots on the upper leaf surface.Some researchers call it Red Rust and some others call it Algae Spot [1].The severe infection of algae colonies on young oil palm leaves can reduce the rate of photosynthesis by 42.48% [2].The most common algae species reported to infect a wide range of hosts including oil palm was Cephaleuros virescens [1,3].Several species of algae from the genus Phycopeltis have also been found on several plant species in Bukit Barisan-Bengkulu, Indonesia [4].This study was carried out to identify algae species infecting oil palm leaves in an oil palm estate in Riau, Indonesia based on its morphological characteristics.

Materials and Methods
Sampling and observation were carried out from December 2022 to January 2023.Algae samples were taken from an oil palm estate in Bengkalis, Riau, Indonesia.The sampling area was located at ±42 m a.s.l.(1°6'45.24"N,101°18'49.62"E)on sedimentary soil with mean annual rainfall >2,000 mm/year.Fresh algae specimens were observed using the Digital Microscope (Dino-Lite Premier AF3113) and Biological Microscope (XSZ-107BN).Separation of algae from the leaf surface was done manually using plastic tape and inoculum needles.
Macroscopic observations of the color, shape, and size (diameter) of algae colonies were carried out directly on the leaf surface.Observations of the structure of the thallus and sporangia were carried out using a microscope at a magnification of 400x.Determination of Order and Genera was referred to [5] and species identification was referred to [6,7,8].

Macroscopic observation
Reddish-orange algae colonies were only found on the upper leaf surface (leaflets and rachis) and did not cause symptoms of necrosis on the plant tissue (Figure 1A).In general, the appearance of the algae was in the form of a round disk with smooth or slightly lobed margins, up to 3-4 mm in diameter (Figure 1B).Algae colonies were easy to remove from the leaf surface when scratched.The thallus was reddish-orange and thus it can be ascertained that the algae came from the order Trentepohliales.Even though the Trentepohliales are part of the Chlorophyta (green algae), the buildup of hematochrome (carotenoid pigments in the cells) frequently gives them an orange, yellow, or red appearance.[5].Environmental factors affect how much carotenoid is produced; it is less when light intensity and nitrogen levels are low [9].

Microscopic observation
The algae samples had round disk shape with smooth or slightly lobed margins (Figure 2A).The size of the cell was 5.9-9.7 µm in width x 24.0-43.0µm in length.Sporangiate occurred in the intercalary and was randomly scattered across the thallus surface with the size of 15.7-22.0µm x 20.0-23.9µm (Figure 2B).Unfortunately, the papilla-pore could not be observed.The observed samples also had no terminal sporangia, dorsal papillae, or erect hair (Figure 2A-F).
By comparing the morphological characteristics with the descriptions of some algae species found in Asia (Table 1), the algae collected from the oil palm estate in Riau was tentatively identified as Phycopeltis arundinacea.There were several important characteristics in Phycopeltis taxonomy such as the shape of the thallus, cell dimensions, location of sporangia or gametangia, width:length ratio, and presence or absence of sterile hairs or glandular-like cells [6], but the collected samples were more identical to Phycopeltis arundinacea.This algae species had also been identified by [4]  Based on a study by [3], Cephaleuros virescens was found to be the most common species of algae infecting oil palms.In general, C. virescens has a similar appearance to P. arundinacea, such as a thallus shape in the form of a round disk and a sporangiate-lateral structure.However, the most striking difference between these two species is that the cell size of C. virescens is much larger than that of P. arundinacea (Table 1).In addition, C. virescens also has rhizoids (heterotrichous thallus) and setae (erect sterile hair) that are absent in P. arundinacea.
Epiphytic algae, such as Phycopeltis spp., do not injure or parasitize their hosts.The absence of the rhizoid in Phycopeltis spp.(parasitic green algae) compared to Cephaleuros spp.prevented the host plant's nutrients from being absorbed.[6,7].However, the presence of Phycopeltis algae on the leaf surface may decrease the photosynthesis activities of the affected leaves.Thus, in severe cases of algae infection, treatment to control the algae is still needed.Based on our initial study, the application of chlorothalonil fungicide at 0.1% a.i. was proven to be effective in controlling algae in the field (Figure 2F & Figure 3).

Conclusion
Based on the morphological characteristics, the algae infecting oil palm leaves in an oil palm estate in Riau, Indonesia was identified as Phycopeltis arundinacea.The main characteristics that distinguished P. arundinacea were the shape of the thallus which was a round disk with smooth or slightly lobed margins, the cell size, the absence of rhizoids, erect hair, and dorsal papillae and with sporangia only in intercalary cells.Although the algae found in this study differed from those of other researchers, differences in sampling locations may have influenced the results.Molecular identification may be required for more definitive results.

Figure 1 .
Figure 1.Algae on oil palm leaves.A. Algae colonies on the upper leaf surface.B. The appearance of algae in the form of a round disk with a diameter of up to 3.7 mm.

Figure 2 .
Figure 2. Microscopic view of the algae samples taken from an oil palm estate in Riau.Bars for 2A-D = 50 µm, 2E-F = 200 µm.A. Algae forms round disk with smooth lobed margins.B-C.Sporangiate in the intercalary and randomly scattered across the thallus surface.D. Sporangiate-lateral structure consists of zoosporangium (z) and suffultory cell (sc).E. Algae colonies that were close to each other with clear boundaries.F. Algae colonies with dead cells (white color) at 7 days after chlorothalonilfungicide spraying.