I have a pretty unique opportunity going right now in one my test plots. The plot has a fairly decent run of Zythia going, at the same time one of the test materials is causing a slight burn on the leaves (it's an unregistered oil), so we have the chance to compare the two.
Those of you have called me concerning Zythia know that I look for the little brown to black spots representing fruiting bodies (thank you Steven Koike!) in the centers of the necrotic areas caused by the fungus to confirm it. Usually there are concentric rings of growth also, and while these aren't diagnostic they for sure point out that symptom in question is being caused by a living organism.
Contrast that with phytotoxic burn of the leaves and other plant parts. No fruiting bodies for sure, nor any rings of growth. Too, if one is familiar with how spray droplets disperse on the leaves, it should be a small matter to trace back areas of burn to where water would gather. Veins, edges of leaves and low spots on the foliage are common.
Let's go to the pictures to make my point, shall we?
View from the top, distinct areas of tan colored burns distributed around the most exposed leaves.
Spray burn close up. Distinct tan area at leaf edge.
Leaf severly infected with Zythia fungus. Note the abundance of dark colored fruiting bodies towards the center of the spot.
Comparing phyto burn (leaf on the left) with Zythia (leaf on the right).
Challenge question. Which one is the phyto? Spoiler alert - the answer is now in the comments below.
Sure enough, just a few days of real summer weather and the strawberry plants are pushing out some seriously high quality fruit. There's a lot of messed up ones yet in the field that came to fruition during the rains of the past few weeks, but once those are cleared out and all the new ones come in we are up for some big harvests of nice fruit.
Found this beautiful number in my KPAM crop termination plot. San Andreas variety, almost as big as my hand, easily tops 50 g.
While it's pretty common for Californians to set aside their complaints about rainy weather because we need the water, the flip side for the Central Coast strawberry industry is ruined fruit and plant problems.
I've been seeing a number of fields with the problem of Photo 1 below. Plants are barely growing, the foliage is discolored and the overall aspect is not great.
It's best to take a careful, step by step approach to these problems. Splitting open a crown of one of the plants from the field shown in the first photo shows no discoloration common to many soil diseases (and in this case diagnostic testing confirmed that). The discoloration of the foliage is consistent with a lack of nutrition, but that plants very close to the ones doing poorly are doing great belies the idea that the field as a whole is lacking.
Let's look at the soil of surrounding these plants for a moment. There is a lot of water associated with these areas (close observers of Photo 2 below, and I know there are many since they want to figure out whose field this is, will see that there has been standing water indicated by the cracked soil) engendered by being a bit lower than the other part of the field, or being on the heavier side of soil type. Other areas, which are not as low or not as heavy, not nearly as many sick plants, if any at all.
How then does one explain the lack of growth and more so the mineral deficiencies so evident in the leaves? Easy. An excess of water around the plant roots create a near, if not completely, anaerobic condition and interfere with the processes needed to take up minerals from the surrounding soil which in turn impede growth.
With the above steps completed, I'd feel pretty confident on making the call that this is water related. So much in fact that I'm writing a blog about it.
To be sure, disease is out there as evidenced by the third and last photo. These plants are very similar to the others, although the spotty pattern in the field gives one the sense that it's not excess moisture. Splitting open the crown revealed discoloration, and further testing at the diagnostic laboratory confirmed Fusarium.
Photo 1: Aspect of plants in affected area. Discolored, small and struggling to grow.
Photo 2: Crown split open on affected plant. No discoloration, also note how very little the roots have grown over the three months or so since planting.
Photo 3: Looks like the same deal as above, but these were confirmed as Fusarium. You'll be hearing more about these soon I am sure.
Just put out a study this morning with my colleagues from PSI screening a number of biological fungicides. These materials have been accumulating interest on the part of both growers and buyers as they seem to fit a more "natural" approach to food production attractive to consumers.
Trouble is, and I've had conversations concerning this with my colleague Gerald Holmes down south at CalPoly , that it's hard to get treatment separation in field fungicide screens for Botrytis. And much less for biological materials which tend to be of a lower efficacy.
Time to go all in to try and tease those treatment separations to come out. As (I hope) most of our industry participants know, Botrytis does not generally infect the mature fruit, and rather infects the open flower and lies dormant until the fruit reaches a certain ideal concentration of soluble solids. With that in mind, I'm thinking it's a good call to start these screens, especially of less established materials, early in the year when most of the early crop is still in the flower stage, and with the addition of these intermittent, late season rains we have the table set for a very solid piece of work.
Along with the weekly applications of fungicides, we'll be doing both in field evaluations of marketable and diseased fruit, and then take it even further with clinical evaluations in the lab once we have mature fruit.
I'm very optimistic here that we'll really bring something of value forward on Botrytis this year in strawberry.
The scene in the field this morning right before the rain. Plenty of open flowers, ready to receive spores of Botrytis.
8001 TJet Nozzle from a gang of 10. Brand new ceramic, Teflon tape and brass fittings. Doing high quality work and loving it!
I've recently had a number of calls concerning an increasing amount of leaf blotch on strawberry. This makes sense, since we've had continuing rains and the subsequent lengthy periods of free moisture coupled with the warming weather ideal for propogation of this pathogen.
For those unfamiliar with this disease, it is caused by the fungal pathogen Zythia fragaraiae. The disease appears as tan to gray blotches often occurring at the margins of the leaves, and one tends to see more disease on the older leaves than the younger ones. The blotches are irregular in shape and can cover pretty well the whole leaf and spread to the fruit calyces in advanced cases. A diagnostic feature of the disease are tiny brown to black fruiting bodies nestled within the blotches. These are the fruiting bodies of the pathogen which produce the spores to spread the disease around.
The call has always been that this is a minor disease, and not worthy of taking any action beyond keeping an eye on it. Personally, outside of minor marketable fruit loss to the unsightly brown calyces, I have yet to see major plant damage, much less total plant loss to this pathogen. That is not to say that it won't happen, and our new small fruit plant pathologist Akif Eskalen asks that I mention here how a minor disease could get to be a major issue when the right environmental conditions come up, that is to say, this year.
I couldn't agree more with Akif that the problem of leaf blotch could someday and sometime be serious, so we are developing a competence on it. Akif already has a bunch of Zythia samples that I sent to his laboratory for fungicide sensitivity analysis (in other words which materials offer promise and which ones don't). Currently he has them ID'd by DNA analysis and will soon proceed to test them.
So, while I still would not be overly concerned about Zythia on strawberry right now, it nevertheless serves our purposes to be ready should it ever be a problem. And that is exactly what we are doing.
Leaf blotch caused by Zythia fragariae in strawberry. Note the purple margin on the edge of the blotch.