I find myself having to take notes in order to keep the information straight when I read scientific papers, so when I dug out the following review paper on the topic I decided to summarize the relevant information and add some comments to tie the science talk to backyard gardening conditions and practices. The paper itself focuses on the cellular/physiological aspects of BER, which does not concern us gardeners, but it also contains quite a bit of background info that does concern us. However, most of that background is from BER problems in commercial greenhouses, conditions and practices very different from outdoor gardens exposed to the whims of Mother Nature, so their conclusions as to what works and doesnÃ¢â‚¬â„¢t work must be considered in that context.
I do suggest looking at the article as some of the figures are informative.
A Cellular Hypothesis for the Induction of Blossom-End Rot in Tomato Fruit
LIM C. HO and PHILIP J. WHITE*
Annals of Botany 2005 95(4):571-581
My synopsis for gardeners
BER is caused by multiple cellular problems in the distal end of fruits, which result from low available calcium at that location during the first two weeks of fruit development (just after the flower fades and falls away, when most of cell division and differentiation is taking place). Because of the low number and structure of transport vessels in their fruit plum-shaped tomatoes are more susceptible than round fruited varieties. BER calcium sprays [calcium chloride et al.] applied before the second week of fruit growth can help directly, sprays are the best way to control BER under the controlled conditions of a greenhouse [But other steps might be just as effective in the more variable conditions of an outdoor garden--TZ].
Calcium can either be restricted from entering the plant (soil-root conditions) or be inefficiently transported to the distal part of the fruit once in the plant. The latter is caused by above ground conditions affecting leaf water loss and photosynthetic rates.
BER can be induced by a number of growing conditions, such as low soil calcium, low phosphorus, high magnesium, high nitrogen (particularly NH4), high potassium, high salinity, drought, water logging in the root zone, cold soil/roots, and also low humidity, high light, high temperature and high air movement in the leaf-shoot environment. When soil calcium is plentiful roots may not be able to take it in efficiently when the soil is cold or waterlogged. Once calcium is in the plant it is dependent on high turgor pressure to get to the distal part of the fruit. If water loss/transpiration is high from the leaves turgor pressure is reduced in the plant. At its most critical low pressure the plant wilts because it canÃ¢â‚¬â„¢t get enough water to its leaves. Bright light, heat, low humidity and air movement contribute to high water loss.
Movement of water up from the roots is mainly driven by negative pressure formed by evaporation within the leaves, so the leaves are literally sucking water from everything below them including the fruits. Root pressure is a much weaker force and pushes water up into the plant. It is driven by the difference in dissolved substances between the internal water and the soil water. As soil dries the concentration of dissolved substances in the remaining soil water increases, thus reducing root pressure in the plant, high salinity in moist soil has the same effect.
In addition to water transport effects, bright light also boosts photosynthesis and thus fruit growth. This is bad if the rate of calcium entering the fruit is not adequate to support the increase in growth rate.
Here is my interpretation of that information as it regards garden growing.
Some scenarios for BER
1) Cold wet cloudy conditions suddenly change to warm and sunny conditions soon after fruit set. The plant ramps up fruit growth while cold oxygen starved roots canÃ¢â‚¬â„¢t take in calcium fast enough.
2) The garden is shallowly tilled/amended over more compact subsoil layer, and the soil is high in nutrients from pre planting fertilizer application. Shallow rooted plants grow big, and then hot weather hits on schedule in early summer just as lots of tiny fruits are forming. The big leafy plants suck all the water out of the top layer of soil, reducing their internal water pressure and starving the fruits of calcium.
3) You are growing in low calcium, high fertilizer conditions with fluctuating water availability. This often resembles a five gallon bucket of potting mix, or low quality hard dirt with little organic matter, or a sand box.
4) You are growing Roma/plum tomatoes under ALMOST perfect conditions.
If you are plagued by BER, what can you do other than spray the plants with an anti-BER calcium solution when the fruits are barely visible (0-2 weeks old)?
Because soil calcium is rarely critically low, and because you cannot control cloudy, rainy, sunny, hot, dry or windy weather here are some things that you can try without having to test your soil for pH-calcium levels.
1) Soil: Make sure your soil both drains well and is water retentive. This means deeply dug with a lot of organic matter so that it provides air and even moisture to the roots in both wet and dry weather. High soil moisture offsets soil salinity and reduces the effects of high transpiration (makes wilting less likely). Compost will increase the amount of calcium available to the plants, IÃ¢â‚¬â„¢m not sure if peat moss or pine needle compost will provide additional calcium in the same way, the acidity could also cause problems. Mulch heavily to slow drying.
2) Fertilizer: Provide phosphorus early in the season (planting hole etc) but hold off on Magnesium (EpsomÃ¢â‚¬â„¢s salts) and high nitrogen fertilizer (organic or synthetic) until after fruit set. Commercial tomato farmers wait until midseason/after fruit set and then side dress the plants with nitrogen fertilizer. It boosts plant growth/health at the time the plants are growing the largest, and doesnÃ¢â‚¬â„¢t interfere with flowering or BER. It also prevents nitrogen loss from the soil through leaching and nitrification.
3) Early summer bright sun & hot days: Because high transpiration rates reduces calcium transport to fruits, selective well timed leaf pruning might be a good idea. Most of us get BER mainly with the first fruits of the season, and since it is a good idea to trim off the bottom leaves to aid airflow and reduce fungal infection anyway, it might be a good idea to wait until you have a good number of flowers and pea-sized fruit before trimming those big bottom leaves off (leaves below the first set of flowers; or about 1/4 to 1/3 of total leaf area). This will immediately shift the leaf to root ratio in favor of roots and boost the amount of calcium getting deep into the fruit. Reducing leaves will also reduce photosynthesis and slow fruit growth rate, thus reducing the need for calcium in the little fruits. So, with pruning you are attacking the problem from both directions. However this is probably best done only one time because the plant will grow roots in proportion to its leaf area/water needs, so repeated pruning will affect over all growth and production. Note: if you are growing determinant varieties only trim off some bottom leaves, do not sucker the plants because that will stop growth and production.
4) You may want to have your soil tested in general. Long term organic practices (large inputs of manure and compost, or wood ash for many years) can result in very high potassium levels, which affect calcium uptake. In this case, adding calcium can help both prevent BER and increase overall nutritional quality of the fruit. Yes, this is counter to number one above (add lots of organic matter), but life is complicated. Be careful to add the appropriate calcium source based on your soil pH (e.g. gypsum for high pH soil, limestone or dolomitic lime for acidic soil), or uptake of all nutrients could be affected.
List of factors and reason they promote BER
Below ground --Root environment:
Low CaÃ¢â‚¬â€Calcium is needed for proper cell development in fruit.
Low PÃ¢â‚¬â€Potassium is needed for root growth and fruit development, most of the plantÃ¢â‚¬â„¢s entire need is taken up while the plant is young.
High Mg, -- Magnesium competes with calcium for uptake by roots (competition is minor though), and it increases nitrogen/photosynthetic efficiency = leaf growth and fruit growth.
High N (particularly NH4), -- Nitrogenous ions compete with calcium for uptake by roots and nitrogen favors foliage growth over root growth.
High K, -- Potassium competes with calcium for uptake by roots.
High Na-- Sodium competes with calcium for uptake by roots, and displaces potassium within the plant (only relevant in irrigated desert areas and for people who are told to salt planting holes for better tomatoes).
High salinity (total salts) -- at high levels causes osmotic stress, inhibiting water uptake by plant, thus lowering turgor pressure.
Drought, --Reduces Ca availability (less water for it to be in) and increases salinity of soil water.
Water logging in the root zone, --inhibits root activity = Ca uptake.
Cold soil, -- inhibits root activity = Ca uptake.
Above ground -Leaf-shoot environment:
Low humidity-- increases transpiration thus reduces internal water pressure.
High air movement -- increases transpiration thus reduces internal water pressure.
High light --increases transpiration, increased fruit growth rate,
High temperature --increases transpiration, increases growth rate of fruit.