Results
Soil physiochemical properties
The preliminary status of the soil analyzed before the commencement of the field preparatory activities revealed that the soil was subtlety fertile with regard to the physical and chemical properties (Table 1).
Assessment of disease incidence at sowing dates during each year in the trial study
In the trial study, very low and statistically significant (p < 0.05) disease incidences were recorded at sowing date one in the second year in both the tilled and zero tilled fields. On the contrary, very high and statistically significant leaf spot incidences were registered at the first and third sowing date in the tilled field during the first year of the trial investigation (Table 2).
Assessment of disease incidence between the tillage systems during each year of the study
In the investigation, extremely low and statistically significant (p < 0.05), leaf spot disease incidences were documented in both the tilled and zero tilled fields in the first and second year of the study. Contrarily, the tilled field in 2019 recorded a very high leaf spot incidence which was significantly different from the incidences registered in zero tilled field in the same year as well as from both tillage systems in the second year of the field study (Table 3).
Assessment of disease incidence during both years of the investigation
The study revealed that, the first year of the study (2019), recorded a higher T. occidentalis leaf spot incidence than the incidence observed in the second year. Statistical analysis revealed that the disease incidences differed significantly (Table 4).
Mean leaf spot disease incidence within the tillage systems
In the trial investigation, the zero tilled field globally recorded a lower and statistically significant leaf spot disease incidence than was observed in the tilled field (Table 5).
Mean leaf spot incidence at sowing dates
The field experiment divulged that, the second sowing date in the untilled field comprehensively recorded a very T. occidentalis low leaf spot incidence which was significant (p < 0.05), and differed statistically from other incidences investigated in the study. By and large, the disease incidence was rife and statistically significant at sowing date three within the tilled field (Table 6).
Discussion
The study established the vulnerability of T. occidentalis to leaf spot disease under field conditions. The nitrogen content of the soil was found to be very high. Adequate nitrogen levels are necessary for disease resistance. However21, explained that excess nitrogen may promote favorable conditions for plant disease. The authors argued that excess nitrogen promotes thinner and weaker cell walls and delays the maturity of plant tissues and therefore increases the risk of disease infection and development. Ekwere et al.22 reported that the total percentage nitrogen recommended by23, as the critical value for good crop production, is 2%. The nitrogen in the soil was therefore in excess. T. occidentalis stands were more susceptible to disease because of the excess nitrogen in the soil, except for the second sowing date, when the disease incidence was high. The prevalence of leaf spot disease was further compounded in the tilled field, which was characterized by a more humid microclimate compared to the zero-tilled experimental units.
In addition, the amount of phosphorus in the soil available for the crop was high. However, its role in resistance is variable and seemingly inconsistent. Jones et al.24 reported that increasing phosphorus rates above the level needed for plant growth increased the prevalence of Fusarium wilt in cotton and muskmelon. During the first, third, and fourth sowing dates of this study, a high phosphorus content may have contributed to the development of leaf spot disease in the field.
The findings in this study revealed that the tilled and zero-tilled fields during the second year of the investigation registered lower and statistically significant leaf spot disease incidences than the tillage systems in the previous year. This could be due to the fact that, the environmental temperatures were lower in 2020 than in 2019 (Tables 7 and 8). Thus, lower temperatures, together with higher rainfall and higher relative humidity could have been significant in reducing leaf spot disease incidence in the field irrespective of the tillage system.
Generally, in the field investigation, the zero tilled field registered a low leaf spot incidence compared to the tilled field which was significantly different.
This result is unique in its kind and innovative in the management of leaf spot disease in T. occidentalis. Previous investigations by25 obtained other results. Similar observations were reported by26 Soil tillage has been shown to have advantages by23; however, in this study the growing stages of the stands in the tilled field may have been more susceptible to infection by Phoma sorghina coupled with more encouraging microclimatic conditions.
Santchou is characterized by a terrain that is low land and extensively flat, which makes drainage following intense precipitation within this period virtually impossible, if not difficult. As the rainy period progressed and became increasingly intense, the land became inundated. This period coincided with the short season when the study was being conducted. It was impossible to avoid flooding during this time because of the lack of drainage channels due to the flatness of its terrain, which caused water levels to rise significantly. In effect, the field was submerged in water. In addition, the soil became completely soaked and soggy, with the result that the microclimate of the field was modified with conditions rendered more humid and sustained throughout the short season. A buildup of these uninterrupted humid conditions could have been particularly significant where the field was tilled. This observation is consistent with the findings of27, who reported that the high incidence of scab and anthracnose was probably due to a relatively humid microclimate, which favors epidemics of these diseases. Soil tillage loosens the soil, creating pores and spaces for water to readily seep and flood the topsoil. It’s possible that the leaf spot pathogen multiplied quickly due to the high and persistent humidity, and since the growing stages of the crop were vulnerable, new infections were unavoidable due to the high and persistent spore production and the susceptibility. As a result, the tilled field saw an increase in the incidence of leaf spot disease.
Furthermore28, reported that species of the fungi pathogen are widely distributed in the environment, most commonly found in aquatic systems and soil. There could have been a large number of inoculums in the heavily flooded field, and the weather conditions were favourable for the pathogen to prevail and infect the plants. Earlier findings by29 and later30 confirmed that the optimum temperature for the growth of mycelial spores of Phoma sorghina lies in the range of 20–25 °C. The average temperature in Santchou that prevailed within the study period was within the range of 20–25 °C, and it is probable that this temperature highly favored conidial germination and further multiplication of the pathogen within a very short time period. To put it another way, a large inoculum density, coupled with the vulnerability of perennial vegetables, could have resulted in the high disease incidence that was observed in tillage fields.
The first and second sowing dates in the second year of the study within the tilled and zero tilled fields respectively registered very low and statistically significant leaf spot incidences. Helen and Michele31 reported that changing the usual sowing time of a crop can exploit weather conditions that are not favorable for the spread of pathogens and reduce crop losses due to diseases caused by pathogenic microorganisms.
Globally in the study, the second sowing date registered an extremely low and statistically significant T. occidentalis leaf spot disease incidence. This result, in this study, revitalizes the fact that, adjustment of sowing dates is strategic in the prevalence of crop diseases under field conditions. Previous investigations by32, revealed similar results. The author reported with empirical evidence that disease incidence was significantly affected by different planting dates.
The findings in this study confirm the holistic aspect (tillage systems and adjustment of sowing dates) of the management of plant diseases by creating an environment more suitable for plant growth but not for disease development. The second sowing date within which T. occidentalis leaf spot disease incidence was minimal could have coincided with growing stages of the crop that were less susceptible, more resistant, to infection and spread of the leaf spot pathogen, resulting in leaf spot disease avoidance. The results in this study are also in agreement with previous investigations carried out and reported by33. The researchers affirmed that the adjustment of planting dates is an important cultural practice that can be exploited to minimize crop losses due to disease. According to the authors, powdery mildew severity in sunflower decreased as a result of a strategic sowing date manipulation. The authors ascertained that such a cultural technique avoided coincidence with the susceptible stage of the crop, consequently resulting in disease escape. Subsequent reports by34 confirmed that strategic alteration in planting dates was effective in the control of some plant diseases. Previous reports35,36, established that sowing dates significantly influenced the epidemiology of crop diseases under field conditions. The observations established in this study and in other empirical studies, as reported by30,31,36, further confirm the fact that this cultural disease management technique, manipulation of planting time, is vital in reducing food crop diseases under field conditions.
The very high disease incidence observed on the third planting date in the tilled field could be due to a more favorable microclimate and very high vulnerability of the growing stages of crop plants to infection. Therefore, the more conducive microclimate coupled with a conceivably high initial inoculum population could have encouraged the proliferation of the already populated fungal spores and their germination and rapid multiplication, which favored new and rapid infections, resulting in extremely high leaf spot disease incidences and severities. Earlier studies by12 found that disease spread was aided in cucurbits under field conditions by humid and warm weather conditions. In addition37, acknowledged that leaf spot diseases are favored by humid weather conditions, where they destroy a greater portion of the foliage. Humid conditions are required for spore germination. With warm and massively humid conditions, as was the case in this study, the spores readily germinated within a brief period of time, resulting in further spread of the disease among the more vulnerable stands.
Source: Ecology - nature.com
