Early Blight of Potato
Early blight is caused by the fungus Alternaria solani. The pathogen overwinters in infected plant debris, volunteer plants, or tubers. In spring, primary inoculum (conidia) spreads via wind and rain to healthy leaves, favored by warm (15-28°C) and humid conditions. Spores germinate and penetrate the host directly or through stomata. Initial symptoms are small, dark lesions with concentric rings ("bull's-eye") on older leaves. These spots enlarge, causing defoliation, especially under plant stress. Conidia produced on these lesions become secondary inoculum, rapidly spreading the disease. Tubers can also show dark, sunken lesions.

Control measures involve cultural practices like a 2-year crop rotation, destroying infected debris, using disease-free seed, and balanced nutrition. Avoiding overhead irrigation reduces leaf wetness. Resistant varieties offer a solution. Fungicides, both protectant (e.g., chlorothalonil, mancozeb) and systemic, are applied preventatively or at early symptom onset for effective management.

Blast of Rice
Rice blast is a highly destructive disease caused by Magnaporthe oryzae (formerly Pyricularia oryzae). The fungus survives in infected seeds, crop residues, or weeds. High humidity (>90%), prolonged leaf wetness (>10 hours), and moderate temperatures (25-28°C) favor spore (conidia) production and wind-driven dispersal. Conidia land on rice plants, germinate, and penetrate the tissue directly via an appressorium. Lesions develop as spindle-shaped spots with gray centers and dark borders on leaves (leaf blast), stems (nodal blast), and most severely, the panicle neck (neck blast), causing grain loss or sterility ("whiteheads"). Secondary infection cycles are rapid due to abundant spore production.

Management strategies prioritize prevention. Planting resistant rice varieties is the most economical approach, although pathogen races evolve. Field sanitation, including removing infected residues and weeds, reduces initial inoculum. Certified disease-free seeds are essential. Balanced nitrogen fertilization, avoiding excess, and maintaining a continuous flood can help. Fungicides are often used preventatively during critical growth stages like panicle initiation.