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Cracking up

This week the majority of the tomatoes from the Polyculture Market Garden Study had cracks around the tops or along the sides. We weighed out 6.100 kg of tomatoes in good condition and 16.325 kg of split tomatoes, ok for preserving in jars but not suitable for market. A further 2.925 kg of tomatoes were in poor condition only fit for the pig and chickens who did not seem to mind at all.

The tomatoes in our 9 yr old residential garden also suffered although not as badly.

So why have our tomatoes cracked up this week ? Here's some info on tomato cracking and how to deal with it in the ecological garden.

What is cracking?

Cracking is the splitting of the epidermis around the calyx or stem scar (top of the tomato). There are two types of fruit cracking in tomatoes.

(a) Concentric cracking, which is a splitting of the epidermis in circular patterns around the stem scar.

Cultivar 'Black Krim' shows high susceptibility both in green and ripe fruits

(b) Radial cracking which is a splitting of the epidermis from the stem scar towards the blossom end i.e from top to bottom.

Cultivar 'Paulina F1' a minor crack making the fruit unfit for market but perfectly fine for processing

Its possible for both types of cracks to appear at the same time

Why does cracking occur?

Cracking depends on the ability of the epidermal cells (plant skin) to stretch. Some varieties have an epidermis that stretches well and will have very little or no circular cracking. Other varieties can have the opposite situation where they do not stretch well and have a lot of cracking.

From the cultivars we are growing the most susceptible were 'Black Krim' an otherwise excellent tomato with incredible flavor and Rozova Magia a Bulgarian cultivar with huge fruits and wonderful taste. The table below indicates how our cultivars fared.

Cultivar name	Color	Maturity (days) E, M , L	Genetic type	Resistance to Cracking Season 2015
Alicante	Red	E - 55–70	Heirloom	excellent
Black Krim	Purple/ Brown	M-L - 69-90	Heirloom	very poor
Citrina	Yellow	M -75-90	Heirloom	good
Paulina BG F1	Red	E - 55–70	Hybrid	moderate
Marglobe	Red	M -75-90	Heirloom	moderate
Rozova Magia	Pink	M - 70–80	Heirloom	poor

When Does cracking occur?

Cracking occurs as the tomato nears maturity. More susceptible varieties crack in the mature green stage and more tolerant varieties at later stages. The earlier the cracking then the deeper and longer the crack becomes. An older crack can cause a secondary problem of fungi colonising the exposed flesh. If left too long the fruit can not be used for processing.

Some kind of fungal organism developing in a deep and wide concentric crack

Circular cracking can often occur on ripe tomatoes that are on the vine too long.

Causes of cracking

1. Alterations in the growth rate - Plants have periods where they might have very fast growth followed by slow growth and then fast again. These changes can cause fruit nearing maturation to crack. If the cells have "hardened" during the last slow growth then in the next fast growth period they may not be able to stretch enough and the epidermis cracks.

2. Fast growth - Some varieties have periods of very fast fruit growth with high temperatures and moisture levels.

3. Fruit temperature fluctuations and leaf removal - Wide fluctuations in temperature can also induce cracking. This is true especially when plants have been de-leafed too early leaving fruit without protection. The exposed fruit heats up dramatically in the sun. At night it cools relatively quickly and the differential is bigger than it would have been had the leaves covered the fruit. The expansion and contraction of the epidermis and its cells can result in cracking.

4. Rain and irrigation. Rain and excess irrigation will often cause cracking and if the fruit lacks leaf cover then the effect is even more dramatic. Tomato crops that do not receive water at regular intervals but rather receive it periodically at large intervals are likely to have cracking.

I believe the main factor contributing to the large quantity of our tomatoes having cracks is the 3 days of heavy rainfall we had following deep irrigation of the beds. Our irrigation has been somewhat erratic this year which has also resulted in poor leaf formation so this has probably exacerbated the cracking.

How we will try to prevent cracking or reduce the problem of cracking

1. Had we harvested the day before the heavy rain was forecast we could have saved a large percentage of the crop.

2. Proper water management i.e not over irrigating and watering at fixed intervals and increasing or decreasing quantities as needed.

3. Having a diversity of cultivars reduced the complete loss of a marketable crop.

4. Pick the split tomatoes early and use for canning, ketchup/sauces or dried tomatoes. These products can be marketed in the winter. We also picked a lot of green tomatoes and will see how they ripen in the sun, otherwise these can be used in pickles and chutneys.

5.Removing cracked fruit early also has the added benefit of not allowing the fungal and mold organisms to build up in your beds.

References

Grower Solutions Magazine - Lefroy Valley April 2002

The Royal Horticultural Society Pests and Diseases - 1997

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