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During summer, growers experience a lot of problems with tomatoes. This article deals with the effects of temperature on tomatoes – on pollination and fruit set and also on ripening.  I will deal with diseases in another post.

Tomatoes are affected by high temperatures in a number of ways. Some sensitive varieties are affected when average daily temperatures exceed 25°C, whereas more heat tolerant cultivars are not impacted until daytime (maximum) temperatures exceed 32°C. There are even some cultivars are able to set fruit at temperatures above 35°C.

Under marginal conditions fruit may set without adequate pollination but the internal fruit segments will contain few seeds and the tomato will be flat sided and puffy. Irregular pollination can also cause ‘cat facing’ (http://vric.ucdavis.edu/veg_info/catface.htm).

In general fruit set is adversely affected when temperatures fall below 10°C or rise above 27°C. Optimum temperature for fruit set is 18° to 24°C. Even moderate increases in mean daily temperature (from 28/22°C to 32/26°C day/night) result in a significant decrease in fruit set.

As a general rule, the 8 to 13 day period prior to flowering is the most critical phase. If the average maximum temperature in that time exceeds 29°C, pollination and fruit set are impacted. However as pointed out earlier, this does vary according to cultivar.

Why aren’t my tomatoes ripening?

In hot weather people expect fruit to ripen faster. But with tomatoes the optimum temperature for ripening is 21 to 24ºC. When temperatures exceed 29 to 32ºC, the ripening process slows significantly or even stops. At these temperatures, lycopene and carotene, the pigments giving the fruit their typical orange to red appearance cannot be produced and so the fruit stays green.

For tomatoes light has very little to do with ripening. Light is not needed for ripening and fruit exposed to direct sunlight can heat to levels that inhibit pigment synthesis (As explained above). Direct sun can also lead to sunburn. Do not remove leaves in an effort to ripen fruit. Also, soil fertility doesn’t play much of a role. High magnesium and low potassium can cause blotchy or uneven ripening or yellow shoulders. But slowness to ripen is generally not due to poor nutrition and adding more fertilizer won’t help.

You can remove fruit which are just showing the first colour changes (mature green), and store them at 21-24ºC in the dark, preferably in an enclosed space or in the presence of fruit that give off ethylene gas such as bananas. This may speed up the process by up to five days.

References and further reading

http://www.managingclimate.gov.au/wp-content/uploads/2012/04/Critical-temperature-thresholds_Tomato_V2.pdf

http://cvp.cce.cornell.edu/submission.php?id=91

 

 

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OK, I know I have done much for a while but this came up on a forum today so it seemed like a good opportunity.

Nematodes are tiny worm like creatures. They are a particular problem around Perth because we have sandy soils. If you live in one of the older suburbs you probably have them for sure!

How can you tell if you have them?

Plants will be unthrifty, they may simply appear unwell, nutrient deficient or they may be getting a lot of other problems. For example roses or eucalypts with nematodes often have stem cankers as a secondary problem. If you dig the plant up and examine the roots they may have knots on them – or they may not. Most people aautomatically think of root knot nematode when they think of nematodes however many other species of nematodes don’t produce galls or knots. You may just see roots that seem more branched and profuse then normal (not to be confused with proteoid roots on banksias, hakeas etc. Or on leaves, you may see angular blackened sections.

Types of nematodes

Most nematodes can’t really be seen by the naked eye – the commonest species may be up to 1mm long. Some nematodes feed from the outside of the root (dagger, needle or stubby-root nematodes), or they may go inside the plant and either stay in one place (eg root knot nematode) or move around inside the plant and feed along the way (lesion or burrowing nematodes. Some other types of nematodes move around inside the plant but feed on above ground parts such as leaves or stems (such as Aphelenchoides that infect leaves of eg Chrysanthemum or some ferns)

How do you get nematodes?

They can come in on plant material which is already infected or in soil/soils mixes.

What conditions do they like?

Ideal soil conditions vary with species. Moist soil is required by all to reproduce and move. The optimum temperature varies with species. The pore size of the soil affects nematode movement. The small pores of clay soils make movement difficult so the nematodes have to move in the spaces between aggregates. The larger pores in coarse sands may be too big to allow nematodes to gain leverage between particles.

Control

The home gardener has a different range of options to a commercial grower. Nematicides used to be available to the home gardener (Nemacur® granules) but aren’t any more. They are all S7 pesticides so pretty nasty! Over time the microbes in the soil that break the chemical down build up numbers so over a period of a few years the pesticides become less and less effective. There are natural predators of nematodes – such as fungi or other nematodes but these aren’t really commercially available. Method more suited to the home gardener include:

Rotation – the use of a rotation crop that is resistant to the particular nematode. So for root knot, that may be something from the grass family eg a grass or sweetcorn or sorghum. This will not eliminate them entirely but reduces numbers to levels that don’t cause problems.

Fallow – leaving an area fallow has a similar effect to rotating with a resistant crop – numbers fall because they can’t reproduce.

Solarisation is another option. The use of clear plastic laid over tilled moist soil for several weeks during the hottest part of the year.

Bio-fumigation – there are some crops that can be grown and hoed back in that contain chemicals that will help control pests and diseases including nematodes – such as some of the mustards. These crop residues are planted densely and hoed in while in full flower. Castor oil plants and marigolds have root exudates that may kill nematodes – they can be grown as rotation crops and hoed in. The type of marigold matters, not all are useful. Tagetes patula has traditionally been the one to use but some other species also work.

Sugar and molasses – In some Brazilian work, 300g granulated sugar per litre of soil at 7 days intervals controlled root knot.

Work in Australia on field grown tomatoes found 150 m³/ha of sawdust plus urea (600 kg/ha) to be quite effective. Molasses at 375 litres/ha per week for 14 weeks helped reduce numbers but was inferior to the sawdust.

In some other trials, urea concentrations of 4% totally eliminated nematodes but adversely affected plants. A combination of urea and molasses reduced the phytotoxic effects. Some papers mention molasses in water with a final sugar concentration of about 2% reduces nematodes numbers by about half in just over a week.

Other plant extracts – many have been trialled. Things like calendula, rosemary, lantana, onion, fennel, datura and liquorice which are ground up and put in water. What works probably depends on the type of nematode and the crop. Many of these trials have been in vitro and not in field situations.

The APPS website has some good info if you’d like to do any further reading.

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A frequent cry from gardeners relates to lack of flowering and very often the response is to add potassium. But potassium is NOT responsible for flowering, any more or less than any other nutrient and the real reasons for flowering problems can be many.

No flowers at all? Ask yourself how old is the plant? Is it a seedling which may not flower and fruit for several years? Many eucalypts are in this category. I have an Illyarrie (E. erythrocorys) which is now seven years old and hasn’t yet flowered – I’m not happy but there’s not much I can do about it! Passionfruit from seedlings are similar and may not fruit for several years.

Sometimes pruning is the problem. Particularly with fruit trees, cutting off the wrong wood means no flowers and no fruit. Pears, plums and cherries produce fruit on spurs, lateral growth is often minimal. Japanese plums fruit on 1-3 year old spurs. European plums fruit on lateral spurs on 2-3 year old wood. Peaches and nectarines produce fruit on the new season’s fruiting wood. Apples produce their best fruit on fruit on wood two-years and older, though some varieties such as Sundowner™ also bear fruit on one-year wood. Figs produce their main crop on current seasons growth and an early lighter crop on the previous years wood.

Sometimes the weather is responsible. A cold snap at a critical time can prevent flowers from initiating or abort them very early. Drought can do the same. Occasionally plants don’t flower because the light hasn’t been right. Some plants such as Zygocactus will not flower if their night time darkness is interrupted by light from a street light or inside a house.

The mechanisms for flowering in plants can be complex and are generally a combination of temperature and daylength and one of these may be more important than the other. So anytime you grow a plant out of its natural habitat, you run the risk of problems depending on how pernickety it is. Banksia coccinea is one plant like this that tends to not flower well, if at all, around Perth. Banksia coccinea is also prone to the ‘pruning effect’ because it actually is initiating the next years flowers around the same time as it is flowering. So prune after flowering and you’re apt to be pruning off all the next years flowers!

It needs to be understood that flowers may have quite separate requirements for flower initiation as compared to flower development. Obviously flower development can’t happen without first having flower initiation but you can have flower initiation and no flower development afterwards. Most European bulbs require a period of chilling during winter to initiate flowers but the soil temperature during spring can also be too high and those flowers will abort. Conversely, many South American or South African bulbs that are autumn flowering have high temperature requirements that must be met for flower initiation and flower development to occur. Sometimes these requirements can be met by other means such as putting bulbs in the fridge for a few weeks but sometimes this doesn’t always work well because of these other factors at play.

Sometimes uneven flowering or flowering on one side of plant can happen and that’s usually for the same set of reasons as above.

Poor nutrition is seldom responsible for a complete lack of flowering, although excessive nitrogen can keep a plant vegetative. Poor nutrition though, can be responsible for some peculiar flower development such as no petals/empty flowers. Calcium or boron are most often the culprits when there are fruiting or flowering problems but first check for periods of water stress because interruptions in the transport of these nutrients can be the reason. Both calcium and boron are essentially immobile (so when in short supply they can’t be moved from older leaves to places of greater need) so that is why symptoms are seen on the growing tips and why symptoms of deficiency show up in flowers/fruit or any growing points (in palms death of the growing point will kill the plant). Bent neck (should be self explanatory) is usually a calcium problem but often more to do with water stress/periods of reduced transpiration such as high humidity) rather than actual lack of calcium in the soil.

Sometimes flowers go green (gerberas show this symptom). This is usually due to infection by a mycoplasma and is incurable. Get rid of affected plants.

Occasionally flowers go mad and start producing flowers within flowers. Or whole little plants within flowers. Usually this sort of secondary development is physiological and due to erratic weather conditions. Rarely is it due to herbicides or any other chemical agent. I have seen this on banksias, roses and carnations to name a few.

You may have flowers but they remain stubbornly closed. This is often wind damage – we call it blasting. Sometimes, it’s also disease such as grey mould caused by Botrytis cinerea.

Finally, you may have had flowers but they may have aborted or dropped for some reason. And depending on when that happened, it may not have been that evident. Most often this is due to moisture stress or a sudden increase in soil salinity (either because the soil dried out temporarily or because you threw on a heap of fertiliser). Some plants also have inbuilt mechanisms for controlling flower (and hence fruit) load to ensure that all fruit set can fully mature. All cucurbits are like this (pumpkins, melons). Other fruit trees can be biennial bearing – that is they tend to have one year when they crop heavily, the next year is a lighter load.

While we are talking flowers in this blog, don’t forget the importance of pollinators where fruit is concerned. Bees, flies, moths or other insects. Lack or, or poor or uneven pollination can also cause a range of problems from no fruit to malformed fruit. But that will be a whole other episode!

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I covered the question of growing plants from seed or cuttings in another blog. Now I’d like to explain a little more about how to grow plants from seed.

Germinating seed can be as simple as scattering some seed on the ground or in a pot and Voila! It all happens. But often its not that easy and in fact can be down right frustrating. Why?

Well there’s a whole heap of factors that affect seed germination. If you’re buying packaged, branded seed such as vegetable seed then you can be fairly sure the seed will germinate provided its not out of date and you haven’t killed it! You see seed doesn’t last forever – although some lasts for many years, some species need to be germinated within about 12 months of being harvested because viability drops off really quickly (viability means it is capable of germinating ie the embryo in the seed is still alive).

Seed storage
For most seed of most European varieties, keeping the seed in the fridge is a good idea but recent research with many Western Australian species has shown they need to go through a process called after-ripening and that means room temperature storage is best. And when you think about it, Western Australian species are not used to going through a period of winter chilling so that sort of makes sense.

Other requirements and treatments
Some species require light for germination and so you should not cover the seed while it is germinating. This includes species such as lettuce and many other small seeds. Many hard coated seeds such as Acacias need to be scarified in some way – dropping them in boiling water is the easiest way but sandpaper can also be used. For larger seeds nick the seed coat to allow moisture in. Some seed contains inhibitors in the seed coat that have to be leached out. That can be done under running water. Nature does it over winter in the rain. Often seed may be dropped in acid to help it germinate – this is just another form of scarification. As is passing through the digestive tract of an emu, bird or other animal!

There are also a number of other specific chemical treatments that may be used on seed. For example priming seed with potassium nitrate often helps germination. Sometimes hormones can be used (gibberellic acid for example). If you have collected the seed yourself then there are a number of other considerations. Was the seed harvested at the correct stage of ripening? Is the seed even viable ie does it have an embryo and is that embryo mature? Many native plants have a large degree of seed predation – insects may bore into the seed and eat part of the inside. Most of this can be answered by cutting the seed open and examining it under a magnifying glass.

If you’re a native plant lover then you’ve probably heard of smoke treatment. There are certain compounds in smoke that help trigger germination in native plants. Extensive work has been done on that at Kings Park in Western Australia and also in South Africa. Both these places have plants that have evolved in an environment prone to bushfires.

If you’re having trouble germinating something, here are some references for you. And always try a search on the internet for your species, you may be surprised at the amount of research work out there that has been done.

http://www.backyardgardener.com/tm1.html

Australian seeds: a guide to their collection, identification and biology By Luke Sweedman, David Merritt, Kings Park and Botanic Gardens, 2006 CSIRO Publishing

Ball Culture Guide: The Encyclopedia Of Seed Germination by Jim Nau, 1999, Ball Publishing.

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This may be short but it is topical for me at the moment. And not without relevance for the home garden.

I am lucky I guess because I know my block was bush before I bought it. And still is in part. So I can grow veges and have chooks and eat their eggs without worrying what they might contain. But many people on newer blocks, or even not so new, may not know their history. Land around me that was orchard for years is being turned into housing. There is certainly land in the metro area that was a dumping ground for rubbish in the past. If you’re intending to grow veges and eat them, or run animals on your land and eat them or their products whether they be eggs or meat, it may just be worthwhile getting a soil test done first to ensure the land you’re on doesn’t have any nasties in it. Old farming land may have organochlorines. Rubbish dumps – who knows what!

Then there’s the question of water. If you’re drawing water from a dam or bore also consider having it tested. Especially if there’s any sort of commercial activity nearby.

Also consider what you bring on your property. Those manures/composts you bring in may also be contaminated via the land they are from. Particularly if the composts are not made to the Australian Standard which does test for all those sorts of things. Even just simple soil may contain almost anything and its hard to go back once you’ve got any sort of contamination, especially disease. Look at the sting nematode debacle around the metro area at the moment! And of course dieback.

Finally consider the things you grow. Some crops accumulate nasties. Brassicas are known to be good at accumulating heavy metals, spinach also accumulates cadmium, tea- aluminium and so on. Plants that take up lots of heavy metals can be used for bioremediation and that is a good thing but you wouldn’t want to eat them!

So surely if you’re buying a new place its worth doing a bit of checking and testing to see if there are any potential issues lurking in your land.

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If you’re in the business of trying to grow anything, the weather we’re having isn’t making it easy! And even if you’re not a commercial grower, the twists and turns of the weather lately makes it really hard to get the best from your plants.

I downloaded the weather data from Medina Research station for last December to today. The daily evaporation for that period ranged from 1.5 mm for the day up to 11.4 mm! That’s a ten fold difference. And I bet the settings on your irrigation controller haven’t changed in that whole time! When you sum up the evaporation over a week it doesn’t look quite as variable – 44.1 mm, 44.2, 57.9, 63.4, 54.2, 66.2 and 58.4 – which is only about a 50 % variation. Actually last week would have been less because I haven’t subtracted off the rain we had, which at home was 42 mm but for Medina was about 30mm. So you really need to take that off the weekly evaporation. Incidentally, from a commercial point of view we don’t regaard anything under 4 mm as being effective ie its not regarded as beng enoguh to do any good.

Anyway, what I’m trying to say is that unless you vary your irrigation you are either going to run into trouble with plants drying out, or you are overwatering like mad to cater for the worst case scenario, wasting water and probably washing away most of the fertiliser you put on into the bargain.

Commercial growers vary their water on a daily basis if they are doing their job properly. And at this time of year, depending on their crop they may be watering anything up to four times a day in amounts that will add up to the total daily evaporation – plus or minus depending on crop and stage of growth. Now, no home gardener can be bothered with that, plus if you have to abide by the two or three time a week watering edict, you simply can’t water every day unless you hand water. And in sand that’s a problem because it does not hold water and actively growing plants will dry the root zone out in well under a day and get stressed or even die – particularly if they are small seedlings.

What can you do to help this? Amend your soil with clay and organic matter. Be wary of the source – you get what you pay for! Don’t run the risk of getting dieback with your clay and remember that compost has quite a lot of phosphorus in it relative to nitrogen. Much of this is explained in previous posts.

The other thing you can do is mulch. But remember to use coarse chunky and COMPOSTED material (again to prevent importing disease and weed seeds) so any applied water runs straight through or you will end up with wet mulch and dry plants. Keep mulch away from the collar of the plants. Mulch does a good job of moderating soil temperatures and the trials we have been running at Murdoch TAFE show that if you start with a nice wet soil profile, mulch can do quite a good job of helping to maintain the soil moisture. Don’t layer the mulch on too thick though, keep it in proportion to the size of the plants it is around. And don’t add the same amount to it each year, just maintain the thickness you need.

Still though, if you are having trouble with plants not thriving in the garden, the best thing you can do is get out with a spade and dig. You may be surprised. I have been. Beds I thought should have been well wet have shown dry patches and even been dry about 25 cm down – well within the root zone of any plants.

Always beware of newly planted plants. Inferior potting mixes (you get what you pay for) and simply the type of mix, may mean the root ball of your plant dries out before the soil around it.

In my experience many plant problems come down to the basics and on our sands its odds on its watering or lack of nitrogen and those two are inextricably tied together. Too much watering means you wash all your nitrogen away. Assuming you had enough in the first place – nitrogen is the one nutrient always in short supply on our sands, and manures and composts never have enough of it while at the same time they are overloaded with phosphorus.

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The simple answer is because they are short of water. But its not always that simple. Plants can be short of water for many reasons and its not always that there isn’t adequate water around, sometimes it’s because they can’t get to it. Or get as much as they need.

So lets assume you have a wilting plant and the soil appears moist. First thing to check is – is it really moist? Remember sand doesn’t hold much moisture – so it may be moist but and actively growing plant may still struggle to extract all it needs. And in clay – especially fine clays, the soil may feel moist but may be bound so tightly the plant can’t get to it.

If you’re on sand it always pays to have a check around the root zone. I’ve seen wilting plants amongst a well watered garden that are dry as a bone because there’s a big lump of non-wetting sand right underneath them. So they appear to be being watered but a little dig reveals the truth. Non-wetting sand and the soil is as dry as a bone. So out with some wetting agent or at least a hose and some slow diligent watering, making ‘mud pies’ to throughly mix the sand and water. Once wet, if it stays wet it will keep wetting up, it’s when it dries out you have problems. Which is exactly why infrequent watering in sands is an issue.

If you’re digging around make sure you don’t just look at the top 5-10 cm either. If the plants roots are mostly at 15-30 cm then it needs to be wet down there as well. With infrequent and inadequate watering its quite common to have the top few cm wet and the rest dry. and if the plant is recently planted check the root ball ie the roots in the potting mix as well as the surrounding soil. There can be quite a difference in the characteristics of each and that needs to be accounted for.

If your plant is on drip irrigation and only has one dripper then is it actually getting enough water in that one spot to provide for all its needs? I have covered this problem under previous posts on watering. and whatever you do DON’T move the dripper in mid-summer. The roots will have grown towards the water supply and if you move the dripper they will die before new roots get a chance to establish in the new dripper position.

Now we have assumed at the moment that all the plant is wilting. But what if only one side or one branch is wilting? Well the first thing to look for there is root binding. If the roots are strangling the plant that is how it often shows up. And it often show up with the first few hot summer days. Again, get down there with a trowel and have a bit of a dig. If it is the problem then you’d better get prepared to replace it. The plant might last through summer if you water it enough, and through winter, but eventually it will die.

If its something like a wattle (Acacia) then look for borers in the stems lower down. Very common. Tell-tale signs are gumming, and of course holes! And borers do attack other species too so its worth a look.

The other reason plants may start to wilt and die on one side (or both) are root diseases such as dieback (Phytophthora). Vascular discolouration will be evident (that means scraping some bark off or cutting off a stem low down). You should see some browning as a ring inside the bark or in the middle of the cross-section. This means some of the food (phloem) and or water (xylem) conducting vessels are damaged. Again its time for a replacement and you may have to think about what species if it is disease, as its successor may also succumb.

Plants also may get collar rot (Rhizoctonia). This may come in as a secondary pathogen on plants that are a bit top heavy and being buffeted by winds (eg howling easterlies). Once plants have Rhizoctonia collar rot its generally game over. Mulching too close to the stems of plants may also cause collar rot. And if you’ve been fertilising recently with granular fertilisers make sure the granules haven’t lodged somewhere down in the base of the plant and burnt it, minor damage can permit collar rot fungi to enter via the wounds, or simply the burning outright damages and kills the tissue.

Finally, plants may also wilt because they have root damage from waterlogging – which is really low oxygenation in the soil. This can happen in summer when soil temperatures are high and you are watering madly to prevent, well, umm, wilting! Once roots are damaged, it’s easy for fungi such as Pythium to enter the roots and start root rot. In fact, damaged roots exude chemicals that actually attract Pythium spores which move in soil moisture and swim towards and infect them.

Well, I think that’s about most of the things to look for. Oh, apart from simply hot days ie the midday sun. Some plants will wilt on hot days and that’s simply because their poor little roots can’t take up enough water fast enough to counteract what is being lost from the leaves, to prevent wilting. In some cases they will recover without damage eg spinach. In other cases, they will recover but the leaves will be burnt eg gourmet lettuce varieties. In other cases if the tissue is damaged as they wilt they may not recover eg gladiolous. You may be able to stake the plant but more often you need to cool it in the middle of the day by overhead watering or misting in summer or grow it under shade.

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