When temperatures are low and condensation or humidity is high, plants are at their most vulnerable to disease. Tomato plants with yellow leaves are common at this time of the season because of low temperatures and high demand for nutrients.
It’s good to reduce the amount of water plants receive when temperatures are low. Water in the morning to help avoid plants standing in cold wet soil overnight.
Foliar feeding is also best done when air humidity is at its lowest – when air is damp, leaves will not absorb or release as much moisture as when the air is dry.
Translocation and Yellow Leaves
Tomato plants are amazing!
If they need more nutrients than they can absorb through their roots, they’ll take them from their lower leaves.
It’s a bit like borrowing from Peter to pay Paul – with Peter at the bottom and Paul at the top – the growing tip always gets the best portions.
You can buy the best quality potting compost or grow bags, but unless temperatures are reasonable, both day and night, plants will often display nutrient deficiencies in their lower leaves.
This is because nutrients are only available to plant roots above certain temperatures. So when it’s too cold, the only nutrients that are available to the growing tip(s) are those in the lower leaves.
Humidity plays its part too
Another point to make is that transpiration is reduced in wet weather – when humidity is high – so the uptake of moisture and nutrients from the soil is reduced.
The lowest leaf branches can be removed. This helps prevent nutrient deficient leaves from becoming infected and passing that infection to the rest of the plant.
Yellow leaves are usually harmless but you may see all sorts of marks and many shapes and colours appear on lower leaves at this time of the season.
The best way to proceed is to remove the affected leaf branches. If the overall look of the leaves is lighter than your other tomato plants a foliar spray with a “pick-me-up” may be applied, such as:
- Liquid seaweed extract – contains a good range of nutrients.
- A general feed like Miracle Grow that contains micro nutrients.
- Magnesium (Epsom Salts) – will help make light green leaves a darker green.
- Calcium nitrate (from Chempak) contains nitrogen and calcium for strong healthy growth – also good for avoiding blossom end rot.
All of the above are best given as a foliar feed in order to avoid creating an imbalance in the soil.
When less is more
One of the most important lessons I’ve learned over the years I’ve been growing tomatoes is: it’s better to grow three plants well than six plants badly.
In other words, three well grown plants are able to produce more tomatoes, and of better quality, than six poorly grown plants.
Looking around my polytunnel I see plants that were sown early but given a lot of attention and are now doing very well. Plants that were sown at the right time but were left in their small pots for too long and therefore their recent growth has been slower than average. Some plants have a nutrient deficiency and some have a touch of fungal disease but mild enough to deal with.
When the remedy becomes the problem
It’s a strange thing that over feeding plants can cause nutrient deficiency …. why? Because when young roots are damaged by over feeding they are subsequently unable to absorb nutrients properly.
The same is true of watering. When young plants are over watered and soil remains saturated for days, if they don’t develop a disease that restricts both nutrient and water uptake, their root growth will be minimal and on a hot day they may wilt because a poor root system can’t absorb water quickly enough.
Identifying the root cause!
It can sometimes be very difficult to identify the source of a problem when plants have been over fed and over watered. Nutrient deficient leaves and wilting plants can be the result of too much rather than too little.
Tomato Plants and Aspirin
It has been suggested for a number of seasons that the active ingredient in aspirin triggers the immune system in plants and helps them protect themselves against some diseases.
Having read a recent article in which scientists have carried out tests that show this to be so, it seems a good idea to spray once a fortnight to be on the safe side. The recommended dose is around 75 to 100 mg per litre.
You can view the article here.
I read in the comments of one website that if you get a headache you should take two cherry tomatoes and if symptoms persist, consult your local gardener!
Slugs and Snails
Along with the recent wet spell, there seems to be an awful lot of slugs and snails around at the moment. If anyone has a creative idea of how to stop them (other than slug pellets) please leave a comment below.
An advantage grow bags have over large pots is their large surface area – this allows extra access for the roots to air at the soil surface.
Grow pots (ring culture) are also helpful and keep excess water away from the stem base but allow water/food (aka fertigation) to be given at the fine roots in the inner ring nearest the stem.
Most of us would love to be totally organic, but a spray with an anti-fungal treatment such as Bordeaux Mixture (copper sulphate) may be the only way to get a crop of tomatoes growing outside in a poor, wet summer.