The leaf rust disease development of hybrid corn on shaded agroecosystem

The research objective was to determine the development of hybrid corn leaf rust disease in shaded agro-ecosystems. The research was carried out in the albasia forest area of Kalices Village, Patehan District, Kendal Regency, Central Java, Indonesia in March-September 2020. Three hybrid corn varieties, namely JH 37, Nasa 29 and Bisi 18, were used as treatments, as well as three levels of shade density, namely 0, 20 and 40%. Observations were made on the incidence and intensity of leaf rust disease at the age of 40, 60 and 80 days after planting (DAP). The incidence and intensity of leaf rust disease were observed by scoring according to the modified 2012 DMRI method. The results showed that all three varieties had been attacked by leaf rust disease since 40 DAP with an incidence of 13.33–56.67% and disease intensity between 12.59–17.41% at all levels of shade density. The leaf rust disease continues to develop in all varieties with an intensity of around 30.00-56.67%. The highest incidence and intensity of leaf rust disease occurred in JH 37 variety at all levels of shade density. The development of leaf rust disease is more influenced by the genetic characteristics of each variety.


Introduction
Leaf rust is one of the important diseases of corn which is spread almost all over the world including Europe, Russia, America, Africa and Asia with tropical and subtropical climates [1].Leaf rust is endemic to corn in Nepal, Bhutan, Southern China and Northern India [2], Argentina [3], Indiana [4], Ethiopia [5,6] and China [7].In Indonesia, rust disease has spread widely in all corn producing centers such as Java, Kalimantan, Sumatra and Sulawesi [8], especially in areas with high relative humidity in Central Sulawesi, Bali, West Java, Central Java [9], and North Sumatra [10].Yield losses due to leaf rust disease on corn can reach 45-65% [9], even up to 70% [11].
Rust disease in corn is caused by three pathogenic species, namely Puccinia polysora, P. sorghi, and Physopella zeae.Pathogens commonly found in Indonesia are P. polysora, and P. sorghi [8].Rust disease attacks the lower leaves until the upper leaves and young leaves remain on the top.Symptoms of rust disease generally appear after the formation of male flowers (generative phase).Leaf rust disease has early symptoms in the form of small yellow spots.The spots are scattered on the leaf surface which causes the leaves to dry out in case of further infection [12,13].
The expansion of the area for corn development in Indonesia is directed at suboptimal land such as dry land and land under stands.Planting corn as an intercrop among plantation crops is often done by farmers to take advantage of the available land, especially on immature staple crops [21].The extent of forest tenure has the potential to expand the area for corn development.Land in forest areas is identical to shade stress, and is related to the dynamics of changes in temperature and humidity that affect the development of corn leaf rust disease.Corn leaf rust disease develops in humid and shaded conditions [18].Besides being influenced by temperature and humidity, the intensity of leaf rust disease is also influenced by the level of resistance of varieties to pathogens [3,22].Currently, there are many hybrid corn varieties that can be used as references for farmers, but their adaptability and production are not yet known if they are developed in dry land agroecosystems under shade.The aim of the study was to determine the leaf rust disease development of hybrid corn in shaded agroecosystems.

Time and location
The research was conducted in the community forest land of Kalices Village, Patean District, Kendal Regency, Central Java, Indonesia from March to September 2020.The research area used is a albasia forest area of 20,000 m² (2 ha).

Research procedure
The study was conducted using a completely randomized block design with two treatments and three replications.As treatments, shade intensity was 0, 20 and 40%, and hybrid corn varieties were JH 37, Nasa 29 and Bisi 18.Each variety was planted in 350 m2 plots with a distance of 1 m between plots.Each variety was planted in 3 plots arranged randomly at each level of shade intensity.Planting using a system without tillage with a spacing of 75 cm x 40 cm (2 seeds/hole).After planting, the planting hole is covered with drum fertilizer.The seeds used had a germination rate of >95%.Plant maintenance includes: 1) fertilization with 200 kg ha -1 Urea and 350 kg ha -1 NPK applied twice 10 and 30 DAP; 2) weeding that is carried out simultaneously with hoarding.

Parameters and data collection
Parameters observed were disease incidence, disease intensity, temperature and humidity.Disease incidence is the percentage of plants showing symptoms of disease infected by leaf rust.Disease intensity is the level of severity of symptoms caused by infection with pathogens that cause leaf rust disease.Observations were made at 40, 60 and 80 DAP.The number of plants observed was 10 plants which were selected diagonally.Disease intensity was determined according to the modified Directorate of Maize Research India [23] scoring method.Modifications were made by increasing the scoring range from 1-5 to 1-9 as shown in Table 1  Lower leaf damaged (31-40%) 7 The lower and middle leaves are damaged, lesions to the upper leaves (41-60%) 8 Lower leaves are 100% damaged, upper leaves are severely necrotic (61-80%) 9 Almost all leaves dry (dead plant) 3 Disease intensity of corn leaf rust was calculated using the following formula:

Leaf rust disease incidence
The results showed that all varieties had been attacked by leaf rust disease since 40 DAP with a diseases incidence of 13.33-56.67%at all levels of shade density (Table 2).This indicates that at the research site a source of pathogen inoculum is available and the initial infection of rust disease has occurred in the previous week.Symptoms of corn leaf rust generally begin to become apparent when the plant approaches tasselling [8].The results of Sari et al. [19] showed that leaf rust symptoms appeared at 5 weeks after planting (WAP).Leaf rust infection is supported by temperature and humidity in the crops.Temperature of 26-28 o C and humidity of 85-92% were optimal conditions for infection and development of corn leaf rust disease [17].The disease incidence of leaf rust increases with the age of the plant and the higher the level of shade density.The disease incidence of leaf rust has reached 100% in all varieties at the age of 60 DAP (Table 2).This indicates that all individual plants of each variety were infected with leaf rust before 60 DAP.Leaf rust disease that developed at 40 DAP became a source of inoculum for infection in surrounding plants.It is also suspected that the source of the inoculum came from corn plantations around the research plot which is a corn plantation area.Puspawati and Sudarma [18] stated that the urediospores of Puccinia sp. is the first inoculum that causes infection, both early infection in the planting and during the growing season.Urediospores are spread by wind and raindrops.Under the right conditions, uredospores can spread rapidly and cause a new infection within seven days.
The results showed that all varieties were attacked by leaf rust disease at all levels of shade density.The highest disease incidence was occurred in the JH 37 variety at all levels of shade density at 40 DAP (Table 2).Although the value of disease incidence varied between varieties in all levels of shade density at 40 DAP, all individual plants in all varieties were 100% infected with leaf rust disease at 60 DAP.Debnath et al. [14] stated that young leaves are more susceptible to infection than old leaves, and maize planted late has a greater risk of infection if urediospores are available and favorable weather conditions are available.Leaf rust infection that occurs in the vegetative phase causes higher yield losses than infection in the generative phase [20,24].

Leaf rust disease intensity
The results showed that the symptoms of leaf rust disease had appeared in 40 DAP in the form of small yellow-brown spots on the leaf surface and were still included in the disease severity score with a value of 2. The spots were increasingly spreading on both leaf surfaces on the next day of observation.until it reaches the highest disease severity score with a value of 7. Dey et al. [25] stated that the typical symptom of corn leaf rust disease is in the form of dark brown pustules (uredinia) which are spread on both leaf surfaces so that the leaves look rusty.Pustules are circular bands that develop through the leaf epidermis and release reddish-brown spores (urediospores).Pustules that have matured will release brownish black spores (teliospora).Severe disease development, characterized by chlorosis and drying of the leaves.The result showed that the leaf rust disease intensity was developed continually in all varieties (Figure 1).This is in line with the results of research by Prasetyo et al. [12] that the development of the severity of corn leaf rust disease shows an increase in each week of observation.The result of Suriani et al. [24] indicated that there were some variations on rust severity development on every accession, and increment of rust severity at everytime of observation at 50, 60, 70 DAP.The research results of Muis et al. [26] showed that the increase in the intensity of corn leaf rust disease based on the area under the disease progress curve (AUDPC) was highest between 45-75 DAP.Furthermore, the results of Sari et al. [19] showed that the intensity of the severity of corn leaf rust disease increased continuously every week since the initial symptoms appeared at the age of 5 WAP to 11 WAP.
Temperature and humidity in the crop favor the development of corn leaf rust disease.Sopialena et al. [17] stated that the development of corn leaf rust disease increased with increasing humidity.The rate of infection of the disease is largely determined by the increase in temperature and humidity.The higher the temperature, the lower the infection rate, while the higher the humidity the higher the infection rate [18].The optimum temperature for P. polysora urediospore germination ranges from 25-32°C and spreads during the day in the temperature range of 27-28°C [9].The disease intensity of rust in all varieties under three shade densities ranged from 30-56.67%.The highest disease intensity occurred in JH 37 varieties at 40, 60 and 80 DAP (Table 3).Although all individuals in each variety were infected with leaf rust disease at 60 and 80 DAP, they showed different disease intensity between varieties at all levels of shade density.Under the same temperature and humidity conditions and the same shade density conditions, there was variation in disease intensity between varieties.This indicates that there are different levels of resistance between varieties.Bekeko [5] stated that in resistant hybrid corn varieties, the disease intensity of rust increased slightly over time, whereas in susceptible hybrid corn varieties, disease intensity increased remarkably over time.Dey et al. [20] stated that yield loss depends on the level of intensity of leaf rust disease in corn, and the level of resistance of corn varieties determines the level of disease intensity of leaf rust.

Conclusion
The incidence and intensity of leaf rust disease increases with the age of the plant and the higher the level of shade density.The incidence of leaf rust disease has reached 100% in all varieties at the age of 60 DAP with disease intensity between 22.22-31.11%.The leaf rust disease was developed continually in all varieties with an intensity of around 30-56.67%.The highest incidence and intensity of disease were occurred in the JH 37 variety at all levels of shade density.Based on the incidence and intensity of disease in each variety and the level of shade density, it suggested that leaf rust disease development is more influenced by the genetic characteristics of each variety.
I: Disease Intensity n : Number of plants affected by the disease with a certain severity score v : Score value on each plant that is attacked by the disease Z : Highest severity score N : Number of plants observed2.4.Data analysisData of the incidence and the development of leaf rust disease were presented in the form of tables and graphs.Descriptive data analysis is supported by related literature.

Figure 1 .
Figure 1.Leaf rust disease development of hybrid corns in three shaded density level.

Table 1 .
as follows: Score of corn leaf rust symptom severity.

Table 2 .
Disease incidence of leaf rust on hybrid corns in three shade density level.

Table 3 .
Disease intensity of leaf rust on hybrid corns in three shade density level.