Why are my tomatoes and capsicum turning black?

I have a small greenhouse in which I grow 12 tomato and six capsicum (sweet pepper) plants. They are growing in bags of dripper fed coco peat. I use a standard “bloom” liquid fertiliser at the recommended strength (4 ml each of A and B per litre water). The older fruit of both the tomato and capsicum have turned black on the bottom. The tomato plants generally look OK, but the capsicum plants are looking poorly.

Can you suggest what is wrong and what I can do to fix the problem?

Answer by RICK DONNAN

Blossom End Rot

The problem you have is called blossom end rot. It is a physiological disorder due to a lack of calcium in the fruit. It is rarely caused by insufficient calcium in the hydroponic solution, but rather because the calcium ions cannot get to the fruit, especially to the end furthest from the stem. This is usually caused by adverse climatic conditions, possibly aggravated by poor hydroponic management. Another point is that the symptoms take about two weeks to show up from when the problem occurred. In your case it seems to be an ongoing problem, so unfortunately, your younger fruit are also probably going to be damaged.

Possible causes

I expect that you have had many days of high temperatures, which result in the plants transpiring as much water as they can. The classic conditions for blossom end rot are a hot sunny day following a few days of dull weather. This results in a surge in growth and the fruit grow without being able to get enough calcium to meet their needs because it is being locked into the leaves due to the high water demand there.

I suspect that there may have also been a very strong influence from your root zone solution management.

Root zone conditions

The important solution to consider when managing hydroponic systems is the solution around the root zone. This especially applies to EC (electrical conductivity), which is the measure of solution strength, as much as to other properties such as pH, aeration, etc. Good hydroponic management requires regular checking of the root zone solution pH and especially EC. Water demand upon the plant is primarily transpiration, which in turn is mainly driven by the radiation received by the plant leaves. Other significant factors are temperature, humidity and movement of the air

Water uptake – osmosis

Water uptake by plant roots works through the natural process of osmosis. Osmosis is the process of water movement through a semi-permeable membrane, driven by a concentration difference across the membrane. A semi-permeable membrane is one, which allows the passage of water, but prevents the passage of virtually all other ions, molecules, and particles such as bacteria. Water flows from the weaker solution into the stronger one.

The root cells work so they are effectively a type of semi-permeable membrane. In hydroponics, the root zone solution normally has a strength that is lower than the solutions within the plant. Therefore the water transfer is from the root zone solution into the plant.

If the root zone solution has a very low EC (close to zero) the osmotic pressure difference will be quite high and the water transfer will be high. As the root zone solution EC is increased the osmotic pressure difference is reduced and therefore the water uptake is also reduced.

Your probable cause

Depending upon your fertiliser brand you would usually have an EC of your nutrient solution of about 2.5 mS/cm. You probably don’t have an EC meter, or if you do you don’t measure the EC of the run-off. In summer conditions the plant uses more water (trying to keep itself cool) than nutrient, hence the EC of the root zone solution rises. Typically, if there is 30 per cent run-off, an input of EC 2.5 mS/cm would rise to about 3.5 mS/cm in the run-off solution.

If the percentage run-off is significantly lower, the EC of the run-off solution will be substantially higher. I have measured the run-off from systems where the run-off was never checked and found ECs of over 10 mS/cm, the limit of my meter.

Very high ECs will severely inhibit the plant’s water uptake. At the same time the water demand is very high, but can’t be satisfied, so the plant temperature rises and damages the plant. I think this has happened in your case.

Specifically, concerning blossom end rot, the water flow, containing the calcium ions, gets dragged to the leaf to be used for transpiration. What calcium is in the water is locked into the leaves (known as a calcium sink) leaving none for the fruit. The end result is blossom end rot in both tomatoes and capsicum.

Tomato is a plant unusually tolerant of high levels of EC in the root zone. However, capsicum is much less tolerant and suffers severely with high EC. This has been the case with your plants and unfortunately your capsicum plants are unlikely to recover.

What to do?

My suggestions are as follows:

Permanently reduce the strength of your feed to about EC 2 mS/cm by adding only 3 ml each of A and B concentrate per litre. Immediately use this solution to flush through all your bags and displace the current high EC solution. Thereafter continue to use this feed solution, but regularly measure the volume of the feed and run-off and keep the percentage run-off to at least 30 per cent. If you have an EC meter, keep the run-off EC under 3, especially for capsicum.

To reduce the heat load on the plants cover the top of your greenhouse with shade cloth or whitewash and maximise any venting.  RD

PH&G February 2017 / Issue 176


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