Economic threshold is - Solved MCQ | Agrigyan

• Remote Sensing in Plant Disease Diagnosis:
• Spectral Bands Utilized:
• Visible Spectrum ($400-700\text{ nm}$): Healthy green vegetation strongly absorbs blue ($450\text{ nm}$) and red ($650\text{ nm}$) wavelengths for photosynthesis, reflecting green light. Diseased leaves lose chlorophyll, leading to increased reflectance in the red and blue regions.
• Near-Infrared Spectrum (NIR, $700-1100\text{ nm}$): Healthy internal leaf architecture (spongy mesophyll) scatters and reflects $\gt 50\%$ of incident NIR light. Pathogen invasion disrupts cell walls and collapses intercellular air spaces, drastically reducing NIR reflectance.
• Normalized Difference Vegetation Index (NDVI): Relies on combining reflectance in both visible and infrared regions ($NDVI = \frac{NIR - Red}{NIR + Red}$) to detect pre-visual and visual disease symptoms.
• Conclusion: Both visible and infrared wavelengths are critical (Option 3).

Correct option: (3) Both visible and infrared

• Mathematical Modeling of Bacterial Population Growth:
• The Logistic Model (Sigmoidal Curve): Bacterial growth in restricted culture media and natural host tissues follows the logistic growth model:

where $r$ is the intrinsic growth rate, $N$ is population size, and $K$ is the carrying capacity of the environment.

• Growth Phases: Incorporates lag, exponential (logarithmic), deceleration, and asymptotic stationary phases constrained by nutrient exhaustion and toxic metabolite accumulation.
• Conclusion: Bacterial growth is modeled using the Logistic model (Option 1).

Correct option: (1) Logistic model

• Forecasting Apple Scab (Venturia inaequalis):
• Mills' Infection Periods: The classical disease forecasting system developed by W. D. Mills (1944) determines the occurrence of primary ascospore infection based on three environmental parameters:
• Temperature: Average temperature during the wet period governs fungal germination and penetration rates.
• Relative Humidity (High RH hours): High atmospheric humidity is required for perithecial maturation and ascospore release.
• Leaf wetness hours: Ascospores require a continuous film of free water on leaf surfaces (e.g., 9 hours at $17-24^\circ\text{C}$) to successfully penetrate the cuticle.
• Exclusion: Cultural practices are management strategies, not dynamic weather inputs into the predictive forecasting model.
• Conclusion: Inputs (A), (B), and (C) only are required (Option 2).

Correct option: (2) (A), (B) and (C) only,

• Parasitic Weed Striga (Witchweed):
• Host Range and Parasitism: Striga (family Orobanchaceae) is an obligate root hemiparasite that infects tropical cereal crops in semi-arid regions.
• Target Crops: Major Striga species (Striga hermonthica, Striga asiatica) infect C4 graminaceous cereals including sorghum, maize, sugarcane, and pearl millet (Pennisetum glaucum), causing severe chlorosis, stunting, and drought-like symptoms by siphoning water, nutrients, and photosynthates.
• Cowpea Parasite: Striga gesnerioides attacks cowpea, but among the cereal options listed, pearl millet is the classic host.
• Conclusion: Striga parasitizes pearl millet (Option 3).

Correct option: (3) Pearimillet