Tuesday 25 August 2015

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 that are okay for preserving in jars, but not suitable for market. A further 2.925 kg of tomatoes were in poor condition and only fit for the pig and chickens, who did not seem to mind at all :)

The tomatoes in our 9 year old residential garden also suffered, although not as badly.   
So why have our tomatoes cracked up this week? Here's some information on tomato cracking and how to deal with it in the ecological garden.

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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  

It's 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 flavour -  and 'Rozova Magia' - a Bulgarian cultivar with huge fruits and a wonderful taste.  The table below indicates how our cultivars fared.

Cultivar nameColorMaturity
E, M , L
Genetic typeResistance
to Cracking
Season 2015
AlicanteRedE - 55–70Heirloomexcellent
Black KrimPurple/
M-L - 69-90Heirloomvery poor
Citrina YellowM -75-90Heirloomgood
Paulina BG F1RedE - 55–70Hybridmoderate
MarglobeRedM -75-90Heirloommoderate
Rozova Magia PinkM - 70–80Heirloompoor

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 cannot 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 difference 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 mould organisms to build up in your beds.


Grower Solutions Magazine - Lefroy Valley April 2002
The Royal Horticultural Society Pests and Diseases - 1997

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Monday 17 August 2015

C4 Plants

What are C4 plants?

Basically they are plants that undertake photosynthesis in a different way enabling them to continue to grow during hot and dry conditions. To better understand this lets quickly recap on photosynthesis.

All plants require carbon dioxide from the atmosphere in order for photosynthesis to occur and plants obtain this carbon dioxide via tiny openings on the underside of the plant leaf, these tiny openings are called stomata. The stomata also provide the exit of H2O from the plant.

When soil water resources are low small openings usually on the underside of the leaves called the stomata close to reduce the loss of water from the plant. This also reduces the incoming carbon dioxide as plants absorb CO2 through these same stomata. Without C02 plants cannot photosynthesis and growth halts.  When a plant is wilting it has reached this point.
Some plants have adapted to overcome this and one particular group of grasses and tropical plants, the C4 plants,  are able to close stomatal pores in order to reduce water loss whilst still obtaining carbon dioxide thereby maintaining photosynthesis in hot and dry conditions.

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C4 Plants, Examples, and C4 Families

They are found only in the angiosperms with about 8,000 members in 17 families equivalent to about 3% of all land plants. Combined, the grasses (family Poaceae or Gramineae) and sedges (family Cyperaceae) comprise roughly 79% of the total number of C4 species (Simpson 2010).

Examples of C4 species are the economically important crops corn or maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), and millets.

Other examples include, couch or bermuda grass (Cynodon dactylon), barnyard grass (Echinocloa spp.), goosegrass (Eleusine indica), Johnson grass (Sorghum halepense), cogon (Imperata cylindrica), common purslane or alusiman (Portulaca oleracea), crabgrass (Digitaria sanguinalis), several species of pigweed (Amaranthus spp.), carabao grass (Paspalum conjugatum), itchgrass (Rottboellia exaltata), and Russian thistle or tumbleweed (Salsola kali) (Llewellyn 2000; Moore et al. 2003).

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