A Great Year for Tomato Blossom End Rot

2010 is shaping up as a great year for backyard tomatoes...with blossom end rot. During tomato planting time here (late April-early May), the weather was cooler than normal, and much wetter. That pattern continued through the rest of May; our typical valley heat didn't arrive until late June. Tomatoes took their time. There are still gardeners waiting patiently for their beefsteaks and other main season tomatoes to turn red (true here, too).

For those tomatoes that are ripening, many gardeners are now contending with a major outbreak of blossom end rot. Tomato plants with blossom end rot show small, light brown spots at the blossom end of immature fruit. The affected area gradually expands into a sunken, leathery, brown or black lesion as the fruit ripens. Hard, brown areas may develop inside the fruit, either with or without external symptoms.
 
According to the tomato heads at UC Davis, the problem occurs when tomato plants have grown rapidly during the early part of the season and are then subjected to hot dry weather when the fruits are in an early stage of development. You know, sort of like the weather in late June.



Some tomato varieties are more susceptible to blossom end rot, including plum and pear-shaped tomatoes.

Although the weather and the variety of tomato sets the table for blossom end rot, many other factors are major contributors. 

Mainly, you. 

Yes, blossom end rot is related to a deficiency of calcium in the tomato fruit, but that occurs for several reasons that can be classified as “operator error”. Among them:

• Too much water.
• Not enough water.
• Irregular soil moisture, brought on by, um, irregular watering.
• Too much nitrogen fertilizer.
• Planting in soil whose pH is not conducive to calcium uptake by the plant (below 5.5 or above 8).
• Planting in poorly drained soil.
• Planting in too sandy of a soil.
• Improper planting (spreading out the roots when planting helps the plant adapt better)
• Excessive levels of potassium.
• Excessive pruning.
• Lack of an organic mulch. (organic mulch helps moderate soil temperature and moisture fluctuations)
• Using a plastic mulch which might raise the soil temperature too high.
• Planting certain tomato varieties that are prone to blossom end rot, especially narrow paste tomatoes.

The most critical mistake gardeners contribute to blossom end rot: not monitoring the soil moisture at root level.  
Although the surface of the soil may appear dry, the moisture level a few inches down may be correct. If more water is added at that time, then the soil becomes so moist that oxygen is unavailable for root growth and calcium will not be absorbed. Why? Excess soil moisture, combined with a lack of soil oxygen, speeds the formation of Casparian strips, deposits on the young root tips that have become suberized, waxy substances through which water and nutrients cannot move.

If the soil in the root zone is too dry, then the calcium will not move to the roots. Dry soil and hot, dry, windy days create a water and calcium deficiency in the plant. Even a brief soil water deficit can disrupt water and nutrient flow in the plant. If this occurs while fruits are developing, blossom-end rot will likely develop.

Automatic irrigation timers may save you time, but it may not save your tomato plants from blossom end rot. Watering schedules need to be adjusted to the weather to maintain even soil moisture.

And when it comes to garden problems, many folks think the answer is, “buy something and put it on the plant”.  

Buying stuff won’t necessarily end blossom end rot.
 
Among the “store-bought remedies” that are frequently suggested that have been proven to be of little or no value to ending blossom end rot:

• Applying a foliar calcium spray to the tomato leaves. In University tests, studies showed that calcium does not move from leaves to the fruits. Thus, foliar sprays of calcium won't correct blossom end rot. Tomato fruits do not have openings in the epidermis (skin) through which calcium can be absorbed. Contrary to past belief, the direct application of calcium as a spray is ineffective.

• Adding a calcium supplement, such as gypsum, limestone, or eggshells. Perhaps. But it depends on your soil. 

• Limestone can raise the pH in soil to a range more favorable to tomatoes and calcium uptake, around 7.0. But if your soil is already in that range, adding limestone may raise the pH to the point where calcium uptake is again, slowed.

• Adding crushed eggshells to the soil well before transplanting time may help overcome any calcium deficiency already in the soil. But it ain’t gonna help your tomatoes if they are already in the ground.

• And gypsum (calcium sulfate)? Dr. Linda Chalker Scott of Washington State University's Horticulture Department, and author of the award winning book, “The Informed Gardener”, says home gardeners are wasting their money. “Most urban soils are not improved by adding gypsum,” she states in her on-line newsletter, “Horticultural Myths”. “Adding gypsum to sandy or non-salty soils is a waste of money, natural resources, and can have negative impacts on on plant, soil and ecosystem health.” However, she points out, gypsum can improve the structure and fertility of heavy clay soils; but consider another undesireable result to adding gypsum: Gypsum can have negative effects on mycorrhizal inoculation of roots.

Maintaining the proper balance of potassium, phosphorus and other soil nutrients and avoiding excessive growth due to over-fertilization with nitrogen is recommended. Several university studies, including a study from Cornell University and one from North Dakota State University, suggest that a low nitrogen, high phosphorus, low potassium fertilizer (such as a 4-12-4) may help control blossom end rot.

Excess levels of ammonium (NH4-N), magnesium, potassium and sodium have been reported to reduce the availability of calcium. A University of Nebraska  study reported that the use of nitrate nitrogen (NO3) stimulates Calcium uptake while ammonium nitrate (NH4) reduces the uptake of Calcium.

Their best advice: A soil test should be conducted to help determine what needs to be added and what should not be added to your garden soil.
 

And, use a moisture meter.