The laboratory experiment was conducted in the central MS laboratory, Department of Plant Pathology, Sher-e-Bangla Agricultural University, and the field experiment was conducted at central farm of Sher-e-Bangla Agricultural University, Dhaka-1207.

The soil of the experimental site belongs to the Agro-Ecological Zone of “Madhupur Tract” (AEZ No. 28). It was Deep Red Brown Terrace soil and belongs to the “Nodda” cultivated series. The topsoil is slightly clay loam in texture. Organic matter content was very low (0.82%), and soil pH varied from 5.47-5.63.

Seed rate was 8-10 kg/ha. After layout preparation, seed was sown in lines on 1 ^st December, 2022. The selected sunflower plants were harvested at 115 DAS when all plants were fully mature. Yields of each treatment in all replications were recorded separately.

The experiment was conducted in a randomized complete block design (RCBD) with three (3) replications and eight (8) treatments. The field layout was done as per the experimental design on 22 ^th November, 2022. The field was divided into three blocks, each of which represented a replication. The unit plot size was 2m × 2m and, the plot-to-plot distance was 0.5m and the block-to-block distance was 0.75 meter.

Sunflower seeds were sown maintaining a row-to-row distance of 50 cm and plant-to-plant distance of 25 cm.

Five plants were randomly selected from each of the plots, and the data were recorded on the following parameters:1) Disease incidence (%), 2) Percent disease index, 3) Plant height (cm), 4) Number of leaves per plant, 5) Stem girth (cm), 6) Head diameter (cm), 7) Number of seeds per head, 8) 1000 seeds weight (g), and 9) Yield (g/plot).

Fertilizers and manures were applied at the doses recommended by BARI for sunflower: urea (180 kg/ha), TSP (150 kg/ha), MoP (120 kg/ha), gypsum (120 kg/ha) and cow dung (8000 kg/ha), equivalent to 81, 68, 54, 54 and 3600 g/plot, respectively. One-third of the urea and full doses of TSP, MoP, gypsum and cow dung were applied as basal dose during final land preparation. The remaining two-thirds of urea was applied as top-dressing in two equal splits at 30 and 50 days after sowing (DAS) each followed by irrigation to enhance nutrient availability and uptake.

The 1/3 ^rd of the urea and the whole amount of other fertilizers were applied during final land preparation as a basal dose, and the rest 2/3rd urea was applied at 30 DAS and 50 DAS followed by irrigation.

The experimental area was under a sub-tropical climate, which was characterized by comparatively low rainfall, low humidity, low temperature, relatively short days during November to May and high rainfall, high humidity, high temperature, and long day period during April to September.

Sunflower ( Helianthus annuus L.) a variety of BARI named BARI Surjomukhi-2, is known for its susceptibility to collar rot, was used. Seeds were collected from the Bangladesh Agricultural Research Institute (BARI), Gazipur. Seven treatments were evaluated: T₀ = Control (no treatment), T₁ = Seed treatment with Autostin @ 2 g/kg, T₂ = Biochar @ 4–5 t/ha, T₃ = Vermicompost @ 5–6 t/ha, T₄ = Tricho compost @ 2–2.5 t/ha, T₅ = Seed treatment + biochar, T₆ = Seed treatment + vermicompost, T₇ = Seed treatment + tricho compost. Tricho compost is an organic fertilizer produced by decomposing plant and animal wastes inoculated with beneficial Trichoderma fungal spores (such as Trichoderma harzianum). Treatments were arranged in a randomized complete block design (RCBD) with three replications. The plants were grown under optimum temperature while maintaining proper fertigation.

Diseased sunflower stems showing characteristic symptoms of collar rot were collected from SAU Central Farm. Samples were transported in moist zip-lock polyethylene bags to the laboratory. Seven (7) small tissue sections (2–2.5 mm) containing both healthy and diseased portions were surface sterilized in 70% ethanol for 1 min, rinsed twice with sterile distilled water, blotted on sterile paper, and, once sclerotia had formed, one sclerotium was plated onto one sterile Petri dish containing Potato Dextrose Agar (PDA) medium following [13]. Plates were incubated at 25 ± 2°C under alternating light and dark conditions for two weeks. Emerging fungal colonies were sub-cultured repeatedly to obtain pure cultures for morphological observation.

The causal organism was identified as Sclerotium rolfsii based on colony morphology, sclerotia production, and microscopic observations, following the descriptions of [14] and [15]. Pure cultures were preserved on PDA slants sealed with paraffin wax and stored at 4°C for subsequent use [16].

Pathogenicity tests were conducted to confirm the role of Sclerotium rolfsii in collar rot development in sunflower plants. Wheat straw was cut into 2–3 cm pieces, soaked in water for 2 hr, and sterilized in 250 ml flasks sealed with cotton and aluminium foil. Each flask was inoculated with a 5 mm PDA mycelial disc of actively growing Sclerotium rolfsii from the pure culture previously isolated from the field specimen and incubated at room temperature for 30 days to allow sclerotia formation. The resulting colonized straw was air-dried and sclerotia were collected for inoculation. For artificial inoculation, sterilized soil (150 g per pot) was infested with sclerotia at a rate of 1 sclerotium per gram of soil in 16 cm diameter pots. Sunflower seeds were sown individually per pot, and soil moisture was maintained at 50% of water-holding capacity. Control pots contained sterilized, uninoculated soil. Plants were observed daily, and disease symptoms were recorded at 30 DAS. The pathogen was re-isolated from symptomatic plants to fulfill Koch’s postulates [17].

From each plot, five plants were randomly selected for data collection. The following parameters were recorded: (a) disease incidence (%) at 15, 30, 45, 60, and 75 DAS, and were calculated as follows [18]:

Where N _d is the number of plants showing characteristic collar rot symptoms, and N is the total number of plants assessed; (b) plant growth parameters: plant height (cm), number of leaves per plant, and stem girth (cm) at 75 DAS; and (c) yield parameters: head diameter (cm) at 90 DAS when the plant's capitulum reached complete maturity, number of seeds per head, 1000-seed weight (g) at 12% moisture, and seed yield per plot (g/plot). Plants were harvested at 115 DAS when fully mature; heads were sun-dried, threshed, and seeds weighed.

Data were analyzed using Statistix 10.0 software. Analysis of variance (ANOVA) was performed, and treatment means were compared using the Least Significant Difference (LSD) test at a 5% significance level.

Typical symptoms of sunflower collar rot were observed in the field throughout the crop cycle. Affected plants showed wilting and yellowing of older leaves, eventually leading to complete plant death. At the crown region, water-soaked girdling lesions developed, ranging from tan to dark brown with alternating bright and dark bands, reflecting diurnal fungal growth. The decay of the basal stem and roots was a common occurrence, often leading to soft, discolored tissues at the soil baseline and, at times, resulting in the detachment of roots from the stem. Despite this basal decay, lodging was rarely observed because the stem rind typically remained intact. Under moist conditions, a white mycelial mat formed around lesions and at the plant base. Sclerotia developed externally on lesions, initially white but gradually turning brown to black, and measured 0.5–2 mm in diameter, as mentioned in Figure 3.

Sclerotium rolfsii was isolated from the crown root portion of symptomatic sunflower plants using the tissue planting method on potato dextrose agar (PDA). The fungus exhibited rapid growth, covering a 9 cm PDA plate within five days (120h). Initial mycelial growth appeared light white, gradually becoming dull white with fan-shaped radial colonies. No clamp connections were observed. Upon maturation over a period of two weeks, the fungus developed compact mycelial aggregates that transformed into sclerotia, which resembled mustard seeds. These sclerotia were shiny and hard, varying in shape from spherical to irregular, and displayed a color range from deep brown to black. Microscopic observation revealed thin-walled, hyaline, septate hyphae with profuse branching. Based on these morphological characteristics, the fungus was identified as Sclerotium rolfsii in accordance with the description in Figure 4.

Artificial inoculation of sunflower plants with Sclerotium rolfsii produced initial water-soaked lesions at the soil line, showing alternating light and dark brown bands. White mycelial strands developed around diseased areas. Within two days, infected stems near the soil line disintegrated, resulting in wilting and death of the entire plant. Pathogens were re-isolated from these infected tissues, and their morphological characteristics were identical to the original culture, confirming S. rolfsii as the causal agent.

Effect of treatments on collar rot incidence

*Values in a column with the same letter (s) do not differ significantly (p=0.05)

Table 1: Effect of Seed Treatment and Different Soil Amendments on Disease Incidence of Collar 230 Rot of Sunflower At 15, 30, 45 and 60 DAS

The growth responses of sunflowers varied significantly among different seed treatments and soil amendments presented in Table 2. At 75 DAS, plants in the control plots consistently showed the weakest performance, with the shortest height (108.16 cm), fewer leaves (22), and the thinnest stems (3.95 cm). In contrast, plots treated with tricho compost, whether used alone (T₄) or in combination with seed treatment (T₇), showed the most vigorous growth across all measured traits. Plants in T₇ reached the tallest height (132.24 cm), produced the thickest stems (5.36 cm), and had the highest leaf count (29).

Seed treatment with Autostin (T₁) moderately improved growth compared to the control (T₀), but the increase in height, leaf number, and stem girth was less pronounced than with organic amendments. Biochar-based treatments (T₂ and T₅) and vermicompost-based treatments (T₃ and T₆) exhibited intermediate growth, with values consistently higher than the control (T₀) but lower than those of the trichocompost treatments. The results show that trichocompost, especially when combined with seed treatment, strongly promotes sunflower growth. Plants in these treatments were taller, produced more leaves, and had thicker stems, whereas control plants showed the poorest growth.

*Values in a column with same letter (s) do not differ significantly (p=0.05)

Table 2: Effectiveness of Seed Treatment and Different Soil Amendments on growth and growth Contributing Characters of Sunflower at 75 DAS

Yield and yield-contributing characters of sunflower also differed prominently among these treatments presented in Table 3. Trichocompost-treated plots, both alone (T₄) and combined with seed treatment (T₇), consistently outperformed other treatments across all measured parameters. T₇ produced the largest heads (17.77 cm), highest number of seeds per head (557), and heaviest 1000-seed weight (63.08 g), resulting in the highest plot yield (658.03 g/plot). In contrast, the untreated control exhibited the lowest values for all traits, indicating poor crop performance in the absence of chemical or organic amendments. Seed treatment alone (T₁) and biochar-based treatments (T₂, T₅) improved yield compared to the control but did not reach the levels achieved by trichocompost treatments. Notably, combining seed treatment with trichocompost (T₇) nearly doubled plot yield compared to the control, highlighting the numerically better effect of this combination. These findings demonstrate that organic amendments, particularly trichocompost, play a key role in enhancing sunflower productivity.

*Values in a column with same letter (s) do not differ significantly (p=0.05)

The grain yield of sunflowers showed a clear positive relationship with various growth and yield-related factors presented in figure 5. In our analysis, we found that plant height (cm), the number of leaves per plant, and stem girth (cm) were all closely linked to the yield, with strong correlations indicated by R² values of 0.688, 0.7066, and 0.5914, respectively. Additionally, we observed that head diameter (cm), the number of seeds per head, and the weight of 1000 seeds (g) also had shown even stronger associations with yield, with their R² values at 0.7923, 0.7855, and 0.6254, respectively. These relationships were further supported by scatter plots, which displayed consistent upward trends. Therefore, sunflowers that were taller, had more leaves, thicker stems, larger heads, and a greater number of seeds, as well as heavier seeds, tended to yield more grain. However, plants with lower growth and yield traits produced considerably less. Overall, it's clear that both the vegetative growth of the plants and the various components contributing to yield play a crucial role in determining sunflower grain yield.