Whole tomato plant suddenly wilting in the day with brown streaks inside the stem

Wilting that worsens in the afternoon and partly recovers at night, lower leaves yellowing first often on one side, and brown streaks inside the stem when you slit it lengthwise, are the classic signs of fusarium wilt, a soil borne fungal disease. The fungus blocks the water carrying vessels, which is why the plant wilts even with enough water in well drained soil, and the brown vascular streaking is the key diagnostic. First rule out bacterial wilt with the water dip test, cut a wilted stem and put it in clear water, if milky white ooze streams out it is bacterial wilt instead, which changes nothing about removal but matters for variety choice. Fusarium wilt cannot be cured in an infected plant, and the fungus survives in the soil for years, so this is serious for the field if not managed. Remove and destroy wilted plants with the surrounding soil, do not compost them. The most reliable long term solution is resistant varieties, look for hybrids marked resistant to fusarium wilt, since they perform well even in infested soil. Improve drainage, keep the soil pH from being too acidic, and apply Trichoderma viride enriched compost to the soil and as a seedling root dip, which suppresses the fungus. Rotate away from tomato, brinjal, chilli and potato for three to four years, and use grafted seedlings on resistant rootstock where available. Avoid carrying infested soil on tools and water between fields. Take a slit stem and a wilted plant to your KVK to confirm fusarium versus bacterial wilt before you buy seed for next season, because the resistant variety you need depends on which one it is.