What Causes Tomatoes to Turn Brown on the Bottom While on the Vine?

Causes at the Roots

• A poor root system or damage to the roots is a common cause of blossom end rot. The best prevention is a well-developed root system on the tomato plant. Transplanting the tomato plant into cold soil stunts the growth of roots. Wait until the soil has warmed in the spring before transplanting the plants outdoors. Apply lime to adjust the soil pH to 6.0 to 6.5 about two to four months before planting, as indicated by a soil test. Hoeing or cultivating too close to the plants damages developed roots. Pull weeds by hand when plants are young and use mulch to keep the soil moist and discourage weeds.

Fertilizer Problems

• Fertilizers high in nitrogen encourage the growth of the vegetative part of the plant at the expense of the root system. Use a low-nitrogen superphosphate fertilizer, such as 4-12-4 or 5-20-5, to encourage healthy root growth.

Irrigation Problems

• Some gardeners mistake blossom end rot for a fungus and think that excess moisture is the problem. The decayed spots do eventually develop fungus, caused by the moisture in the tomato itself, but the fungus is not the cause of the problem. A calcium deficiency caused by lack of water is a common cause in hot weather. Fluctuations in water supply during the development of the tomato fruits also causes blossom end rot. Irrigate tomato plants more often often in extreme heat, giving at least 1 1/2 inches of water each week. Use a shade cloth to help cool the plants and decrease their water loss through the foliage.

Calcium Availability

• Insufficient calcium uptake also causes blossom end rot. If your plants are getting water regularly, a lack of calcium in the soil may be the problem. Use a special tomato formula fertilizer or check to be sure that your fertilizer contains calcium. In extreme situations, a calcium foliar spray can be helpful. Follow package directions carefully because too much calcium chloride can kill the plant.