The risk of losing a crop is always a big part of being a vegetable grower.
With all the possibilities of pests, and diseases and deficiencies, as well as the adverse weather
conditions, sometimes it seems that the whole world is out to get our crops.
A big part of being a vegetable grower is knowing what to anticipate and knowing how
to prevent problems from arising, and then also knowing how to deal with the situation
when it does appear.
And sometimes a problem arises, and we’re not really sure how to deal with it, and just
when it looks like we're about to lose most of the crop, the situation seems to resolve
itself and we're not always certain why.
I had this happen with a crop of spinach in my Polytunnel Garden this spring.
True annual spinach has a reputation of being a difficult crop to grow, and one of the reasons
why a lot of people grow a variety of chard called perpetual spinach instead.
But I like growing annual spinach and the spring batch of this crop in my Polytunnel
Garden is usually one of my favourite crops.
It grows really well in the cool part of the season under the protection of the plastic,
producing a high yield of valuable, tasty leaves.
There is the inevitable bolting, or when the plants shift its focus away from producing
big tender leaves and into producing a flower stalk, so that it can produce seeds.
But I have found that by the time this happens I'm usually eager to replace the spinach plants
with tomato plants or some another warm season crop.
As an early season crop, spinach fits well into this contex, and I've also had reasonably
good success with it in some of the outside gardens.
Apart from bolting, and occasional issues with slug, I've had very few problems with
this crop over the past few years, until this season.
I was only beginning to harvest from my usual big bed of direct sown spinach plants, when
I noticed that pale patches were suddenly forming on the top of some of the leaves.
At first I thought it was indicating some type of mineral deficiency, but in closer
examination I realised there was a mould or a mildew on the underside of the leaves.
I don’t remember having this type of problem before, or at least not at this scale, and
it seemed to have spread really quickly to infect a lot of the spinach leaves across
the full bed.
I did a bit of reaearch and I was disappointed to learn that this was likely a downy mildew,
which can apparently be a really serious disease for spinach crops.
This disease is spread by airborne spores that are produced from an infected plant,
which in my case was probably the remains of an overwintering crop of spinach that was
occupying another part fo the polytunnel at about the same time.
Apparently if the mildew experiences a certain range of temperature and humidity, for the
right number of days, it can produce lots of spores that can travel quite a long distance
in the wind.
And when the spores land on other spinach leaves they need just enough humidity or moisture
to cause a new infection, and the inside of this polytunnel is almost always humid, especially
in the spring.
Reading about all of this and looking at the how extensive the infection was, I figured
I was probably going to lose the crop.
At that point I had a few conflicting instincts.
The first one was to pull out the whole crop right away, and to put in another quick growing
crop, or to transplant in the tomato plants a lot earleir that anticipated.
This was the more ruthless and cautious option, as it would have reduced the risk of this
downy mildew spreading, but I wasn’t ready to give up on this crop just yet.
The other approach would be to try out all the possibly remedies and solutions, to try
to delay or restrict the spread of this mildew, to try to harvest as much out of the crop
as I could.
Watering the leaves of the spinach with a solution of baking soda mixed in with water
seemed to be a very common recommendation to delay the spread, but I decided against
doing this.
I did water the whole bed with a liquid feed of a seaweed solution, which I have gotten
into the habit of using when plants seem to be having trouble.
I see it as the equivalent to taking a multi vitamin, to supply a wide range of nutrients,
some of which might be in short supply.
Apart from that I decided to take the risk and stand back and observe, to see what would
happen, and to take the opportunity to learn, and to harvest what I could from the crop.
I threw out a lot of spinach as I methodically harvested the leaves, and it did take me a
lot longer to harvest for a period of time.
But I was willing to put in the effort at this scale, which would not have been viable
at a larger commercial or market garden scale.
Over the next few weeks I was pleasantly surprised to find that the mildew didn’t seem to spread
at all, apart from the initial infection, and I was intrigued about the reasons why
it didn’t spread.
There was still mildew on some of the leaves, and there was the typical damp conditions
within the humid space of the polytunnel.
Perhaps the specific conditions of humidity and temperature that was needed for the mildew
to produce a new batch of spores, didn’t occur again, and this is a possible but I
really don’t know.
I hadn't reduced the density of the plants very much, so there was still a fair amount
of competition in the garden bed, and not nearly enough air movement that is recommended
to keep the leaves dry.
Perhaps the liquid feed of seaweed did the job, by supplying some nutrients or trace
elements that were missing, or boosting the health of the plant in some other way.
This would be an easy thing to claim, as the seaweed solution likely had some positive
impact, but correlation is not causation and I felt I needed to avoid the tempting of seeing
this as the solution or a cure.
The fertility and mineral balance of this soil has become quite good in the past few
years, which I have confirmed in soil tests and by the abundant and healthy growth of
most crops.
But over the years I have come to suspect that mineral deficiencies often appear in
the calcareous soil that I have when soil becomes too dry.
It seems that high levels of calcium in the soil will cause the availability of other
minerals to be restricted as the soil dries out.
I realised that I probably hadn’t adjusted the amount of watering that I was doing to
account for the warmer conditions and the faster growth of this crop.
It is possibly that the soil was dry enough for a few days to cause stress to the spinach
plants when they couldn’t get the water or nutrients they needed from the soil, and
this may have been a contributing factor to the initial infection.
There seems to be this general idea out there that pests and diseases are just waiting to
overwhelm our crops, and it is our job to defend against this onslaught.
This way of looking at things would cause us to take a more defensive approach, to avoid
the specific conditions that would allow the mildew to spread, and growing varieties of
spinach that are bred to be more resistant to downy mildew.
But the variety of the spinach that I grew is apparently resistant to four races of downy
mildew, so either this is another variation of mildew, or something else is also happening.
Another way of looking at all this is that may problematic issues can arise when plants
don’t get everything that they need in order to be truly healthy, including appropriate
amounts of sunlight, space, warmth, water, and the right mix of minerals from the soil.
In the case of my crop of spinach, I’m leaning towards the possibility that a lack of sufficient
watering on my part, led to the drying out of the soil, which caused a restriction in
the availability of essential nutrients, at a key time, which led to moderate stress in
the plants, which allowed the mildew to take hold.
Once the soil moisture levels rose again, nutrient availability possibly balanced out
again, and the spinach plants were able to get everything they needed from the soil,
and they became less susceptible to the infection from the mildew spores.
Of course it is possible that the specific combination of temperature and humidity that
is needed for another batch of spores to develop, just didn’t happen again.
Or it could be that by correcting the deficiencies experienced by these plants, they were able
to boost their inbuilt resistance.
I don’t know which one it was, or if there was some other factor, but I do know that
I need to avoid the temptation of jumping to conclusions on the basis of one observation,
but it would be interested to see if this mildew issue appears again.
I ended up with quite a good crop of spinach out of all of this, even if it took me a lot
longer to separate the good leaves from the bad.
I threw out a lot of leaves which had spots of mildew on them, but I still harvested about
26kg, or about 57 pounds of spinach from this bed, 5kg of which I had harvested after removing
about half the spinach plants to make way for tomato plants.
As this bed of spinach was 4 square meters, or about 43 square feet, this translates to
about 6.5 kg/m2 or about 1.3 lbs/sqf, which is a pretty good yield for a high value spring
salad crop like this, and on par with yields that I have harvested from previous seasons.
The yield could have been even higher if I had paid more attention, and taken more care,
or if the mildew hadn’t spread in the first place.
I’ve finally installed a sprinkler system in this polytunnel, which will help ensure
that soil moisture levels don’t drop low enough to cause nutrient availability to suffer
in the future.
But I wonder if this overhead watering system could increase the likelihood of creating
the conditions that would enable the mildew spores to develop and spread, and perhaps
a drip line system would have been better.
Hopefully this issues won’t appear again, but I’ll be keeping an eye out for signs
that I am not doing what I should be doing.
And I am really glad that I didn’t remove the crop when the problem first appeared,
the approach of waiting and seeing what happened defiantly paid off, this time.
I’m really delighted at how this polytunnel garden has developed this season, it’s been
my best year so far, and I’m still amazed at how much food you can grow in a polytunnel
in the spring, given the right conditions.
I’m planning to make another video about all of that in the near future, so keep an
eye out for that if you’re interested in the work that I’m doing.
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