Diseases Incidence and Severity of Sweet potato feathery mottle virus in Sweetpotato … Diseases Incidence and Severity of Sweet potato feathery mottle virus in Sweetpotato in Gianyar Regency, Bali

Sweet potato feathery mottle virus (SPFMV) (Potyvirus) was first discovered in Indonesia in 2018. No one has reported information about the incidence and severity of SPFMV disease. Therefore, this study aims to determine the incidence and severity of SPFMV in Gianyar, Bali. The research was carried out directly in the field with observational parameters such as symptoms, incidence and disease severity. Observations were made every week when the plants were 63 to 91 days after planting (DAP). The results showed that SPFMV causes purple ringspot symptoms. The incidence of the disease did not increase when the plants were aged 63 to 91 DAP, while the severity of the disease increased every week. The highest incidence and severity of the disease were 70% and 20.7%. The main cause of SPFMV infection in the field is probably due to the continuous use of stem cuttings from previous crops.


INTRODUCTION
Sweet potato (Ipomoea batatas L.) is a carbohydrate producing plants frequently consumed by people in the eastern part of Indonesia, especially in East Nusa Tenggara, Maluku, Papua, and West Papua (Badan Pusat Statistika 2017).Sweet potato's main content is carbohydrates.In addition, it also has other nutrition in relative high content such as 562 g sodium, 107 mg calcium, 2.8 g protein, 5,565 SI vitamin A and 32 mg vitamin in every 100 grams of fresh sweet potato (Ngailo et al., 2016).The high nutritional value makes sweet potato an alternative food to decrease people's dependence on rice.Several regions even have sweet potatoes as their staple food.Moreover, due to the high export demand, sweet potato is a competitive superior commodity in the industry (Pushpalatha et al., 2017).The largest sweet potato production centers in Indonesia are located in West Java, Papua, East Java, and Central Java.
According to Bali Province Agriculture and Food Security Agency 2021, the sweet potato production in Bali changed between 2016-2020 from 42.952 tons to 26.821 tons, and Gianyar Regency is one of the sweet potato cultivation sites.Based on the data, sweet potato production has a decreasing trend from year to year.Land expansion, cultivation methods, and the presence of pests and diseases attacking sweet potatoes are the cause of sweet potato dropping production.To preserve and improve sweet potato production, efforts in managing production-inhibiting factors are required, including managing the transmission of various pathogens such as viruses.
According to Ames et al. (1997), leaves showing chlorosis with purple spots are a symptom of sweet potato feathery mottle virus (SPFMV).However, on some strain, SPFMV does not show purple spot symptoms.Leaves infected by SPFMV have normal healthy-sized leaves and mostly do not show clear symptom.This makes it hard for farmers to identify healthy plants and infected ones.The virus infection symptom variations may occur due to factors like virus strain, plant strain, and environmental differences (Listihani et al., 2018;Listihani et al., 2019;Listihani et al., 2020;Listihani et al., 2021;Listihani & Selangga 2021;Selangga et al., 2021;Selangga & Listihani 2021;Listihani et al., 2022;Selangga & Listihani 2022;Selangga et al., 2022;Selangga et al., 2023).SPFMV is a virus not widely researched in Indonesia, so there is a lack of information especially for farmers, in controlling this virus.SPFMV is mostly found in Japan, Portugal, and Korea where it caused 14-52% of production loss (Adikini et al., 2015).
SPFMV was first reported in Indonesia in 2018 in Bogor, West Java with yellow spot symptoms on sweet potato leaves (Hondo et al., 2018).Until now, information on its distribution in other regions has never been reported.Data on the SPFMV incidence and disease severity in Indonesia are unavailable.Thus, this research aimed to determine the SPFMV incidence and disease severity in Gianyar Regency, Bali, on sweet potatoes.This information is useful in devising an accurate SPFMV control strategy.

Field summary and observation
The survey and sampling were performed on five locations in Gianyar Regency, Bali.In each location, 30 plants were observed, so the total number of plants observed ©2023 Ni Kadek Sudani Putri, Listihani, et. al. is 150 plants.Observations were performed every week when the sweet potato plants were aged 63 to 91 days after planting (DAP).Observed parameters are disease symptoms, incidence, and severity of purple spots in sweet potato plants.

Disease incidence
Disease incidence was calculated based on the proportion of disease infected plants without considering the severity of the infestation, by using the equation:

RESULTS AND DISCUSSION
The sweet potato field condition in Gianyar Regency had the sweet potatoes planted in a monoculture system using the local strain "Selo Sidan" obtained from the previous harvest.Weeding was performed to get rid of plant residues and weeds from the soil.In one location, there were 5-6 rows.
In one row there were 20-25 mounds and within one mound four sweet potato stem cuts were planted with spacing based on the farmer's approximation.With the plants being a climbing type makes the plant population turned out dense and many diseases were found in the field.
Based on field observation, sweet potato plants showed purple spot symptoms on young leaves (Figure 1).Sweet potato plants with purple spot symptoms have previously been treated by using reverse transcriptionpolymerase chain reaction (RT-PCR) by universal primers Potyvirus CiFor/CiRev (Ha et al., 2008) and the sequencing analysis result showed an infection by the sweet potato feathery mottle virus (SPFMV) (Listihani 2022, unpublished data).Healthy sweet potato plant has an even green color in their leaf area and normal-sized leaves (Figure 1a).Symptom variations found in the fields are mild, moderate, and severe purple spots (Figure 1).Sweet potatoes with mild symptoms have some purple spots on the surface of their leaves (Figure 1b).Sweet potatoes with moderate symptoms have purple spots on all surfaces of their leaves (Figure 1c).Sweet potatoes with severe symptoms have purple spots on all surfaces of the leaves along with yellowing (Figure 1d).The characteristic purple spot symptoms caused by SPFMV infection are shaped as a ringspot.Direct disease incidence observation was performed by observing SPFMV symptoms on sweet potato leaves.The SPFMV disease incidence observation result on several locations for five weeks is presented on Table 1 and Figure 2. Disease incidence showed that no increase was found when the plants were ages 63-91 days after planting (DAP) until before harvest.The disease incidence percentage on location one up to 5 when the plants were aged 63 to 91 DAP are 60%, 50%, 70%, 63%, and 57% (Figure 2).This showed that the disease did not spread during observation period.
Sweet potato feathery mottle virus (SPFMV) is transmitted by aphids (Myzus persicae and Aphis gossypii) non-persistently (Adikini et al., 2015).Viruses cause varied symptoms depending on the strain and growth condition.
Based on the analysis result on purple spot disease incidence, from week to week the value did not increase on all locations.This is because there were no aphids, the vector, on the sweet potato plants so no disease increase occurred during observation.Aphid populations dropped when the environmental temperate was low on average and rainfall also influence the insect population.Aphid population abundance may decrease depending on food sources, shelters, and the temperature in the field.Aphids on plants are tightly linked to plant metabolism activity and the quality and quantity of plant nutrition.Based on a journal released by CABI titled Sweet potato feathery mottle virus (internal cork disease of sweet potato), SPFMV disease distribution in the field is transmitted through infected stem cuttings.
SPFMV disease severity shows an increase when the plants were aged 63 to 91 DAP.The disease severity on 63 DAP in location one was 13.3% which turned into 20.7%, in location two was 12.7% which turned into 18.7%, in location three was 16.7% which turned into 20.7%, in location four was 14.7% which turned into 20.7% and in location five was 14.7% which turned into 17.3% (Table 1).
The disease severity continued to increase along with the observation period (Figure 3).Based on the observation, purple spot disease severity in sweet potato plants reached 20.7% (Table 1).The influence of environmental conditions to diseases caused by viruses is highly dependent on the host condition because viruses are unable to perform metabolism by themselves.The occurrence of disease symptoms in plants is caused by the movement of viruses into plant tissues through the phloem and spread into every part of the plants along with the distribution of photosynthate (Yamasaki et al., 2010;Kwak et al., 2015;Mingot et al., 2016;Wokorach et al., 2020).The faster the development and spread of viruses in plant cells, the faster the systemic symptoms appear and the higher the severity.The high disease severity is also caused by virusinfected stems/tubers and the lack of virus-resistant strain, causing infected sweet potatoes to worsen.
SPFMV has been reported for the first time in Indonesia by Hondo et al. (2018), which presented SPFMV infected sweet potatoes with yellow spot symptoms.The SPFMV found in Gianyar, Bali, in this research is the second report in Indonesia.The different SPFMV symptoms found in this research from the ones in Hondo et al. (2018) research is probably caused by the different sweet potato strains, the plant resistance, the abiotic environmental condition, and the SPFMV strain.
SPFMV has spread all over the world and is considered the most serious sweet potato viral disease (Souza et al., 2018;Tibiri et al., 2019;Zhao et al., 2020).SPFMV has caused sweet potato production loss up to 40%, while mixed infection of SPFMV + SPCSV caused even greater loss of 60-95% (Adikini et al., 2015).Although it is transmittable by aphids, the most important transmission is from the use of infected plants (Zhao et al., 2020).The use of virus infected stem cutting can spread the virus rapidly due to the repeated use of infected vine plants during replication.The use of SPFMV resistant sweet potato strain is important to reduce SPFMV infection in the field.

CONCLUSION
SPFMV infection in Selo Sidan sweet potato cultivar caused purple spot symptoms.The disease incidence in Gianyar Regency, Bali, when the plants were aged 63 to 91 DAP showed no changes occurring, whereas the disease severity showed increase every week.The highest disease incidence and severity values are 70% and 20.7%.
Disease severity was calculated based on the proportion of infected plants in a population by considering the disease severity according to an equation used by Mau et al. (2020): severity (%) N = The number of plants observed shows a certain score v = Score for sweetpotato crop N = Highest score value Z = Number of all sweetpotato plants The disease symptom scoring was performed by assessing the visual symptom variations observed in the field and inputting symptom scoring of (0-5) (Morin et al., 2012).0 = symptomless plants 1 = Mild mosaic symptoms 2 = Moderate mosaic symptoms 3 = Plants show severe symptoms or heavy purple blotches without shrinkage or leaf abnormalities 4 = Plants show symptoms of mosaic or heavy purple blotches with shrinkage or leaf abnormalities 5= Plant show mosaic symptoms or very heavy purple spots with severely shriveled or deformed leaves, stunted, or dead plant.

Table 1 .
The SPFMV disease incidence and severity on several locations Figure 3.The graph of SPFMV disease severity in sweet potatoes in Gianyar Regency at age 63 to 91 DAP ©2023 Ni Kadek Sudani Putri,Listihani, et.al.