Just released from UC ANR updated information of fungicide efficacy in strawberries, which are on page 59.

http://www.ipm.ucanr.edu/PDF/PMG/fungicideefficacytiming.pdf

As always, you are responsible for the fungicides you apply to your field.  Before using any of the products described in the handbook, check with your local Agricultural Commissioner and consult product labels for current status of product registration, restrictions and use information.

The element sulfur (S) has a large role in the management of plant disease. Growers are familiar with the biocidal formulations of sulfur, being elemental sulfur, sulfides, thiosulfate and fumigants like dimethyldisulfide (DMDS) and of course then we have sulfur dioxide which has been used as a postharvest preservative for dried fruits and vegetables.

However, beyond these outright biocidal effects we get from use of sulfur as a fungicide, there is also significant literature concerning the indirect effects of sulfur nutrition on reinforcing a plant's ability to inhibit and resist disease. Known as “Sulfur Induced Resistance” this is how one should frame the role of sulfur incorporated as a nutrient in plant response to disease. 

There could be something to this. Work has been done showing that higher rates of S fertility affected infection rates and severity of fungal disease in oilseed rape and stem canker of potato. While informative, it is significant that the results of the former were achieved by sulfur additions to a field that was deficient.

How would sulfur induce resistance or inhibit disease in a plant? Sulfur goes to many places, including the amino acids acids cysteine and methionine, which are in 99% of proteins found in a plant.  Findings cited by the chapter report that sulfur deficiency in the plant result in lower protein bound cysteine and free cysteine, which as the precursor to all relevant sulfur containing metabolites must have something to do with the ability to resist or inhibit disease.

Sulfur also goes to non-protein reservoirs in the plant, one of the main ones being glutathione. Glutathione, known as a phytoalexin because it is not formed prior to disease incursion, nevertheless accumulates rapidly after pathogen attack.  It is involved in detoxifying signals necessary for fungal growth and could also be serving as a messenger to carry information to yet unaffected plant cells.

Phytoanticipins, in contrast to phytoalexins, are molecules in the plant which are preformed antibiotics- i.e. the plant produces them whether or not there is disease. Glucosinolates (of which our well known isothiocyanates are a cleaved product), on which sulfur plant nutrition has a strong influence, are one of these antibiotic phytoanticipins. Interestingly, low concentrations of glucosinolates don't necessarily equate with higher disease susceptibility, making them more of a qualitative defense for the plant.  

Bottom line:  The role of sulfur in disease resistance and inhibition in plants is a very important one, but it's a pretty sure thing that these systems function perfectly well in sulfur sufficient soils, which describe pretty well every one in the Pajaro and Salinas Valleys.  As such, while the value of sulfur as a foliar fungicide is indisputable for certain diseases, I am not seeing the value of pursuing sulfur work experimentally as a soil disease mitigant.  

The above is a summary of some of the aspects of Chapter 8: Sulfur in Plant Disease from “Mineral Nutrition and Plant Disease” edited by Lawrence Datnoff, Wade Elmer and Don Huber.

I spent some time these past few days reviewing a chapter of the excellent publication Mineral Nutrition and Plant Disease.  This book is a review of the scientific literature, and while we should not see any of this as recommendations for crop production practice, it does give us plenty to think about as far as what direction we can go experimentally:

Manganese (Mn) has a consistent record in the scientific literature as reducing vascular diseases in crop plants caused by soil pathogens such as Verticillium and Fusarium.  Physiologically, this makes sense because of the role that manganese plays in the production of phenolic compounds, phytoalexins and other plant defense mechanisms.

An increase then in the amount of Mn applied to the soil (foliar applied does not work well, since Mn does not move downwards in the plant to the roots) seems to be a straightforward approach in obtaining the disease reducing qualities of this element into the plant.   For example, one study (Dutta and Bremner, 1981), achieved greatly reduced Verticillium wilt by dipping tomato roots in a Mn solution before transplanting.

However, there are other strategies to increase plant tissue Mn, especially in the soil right around the roots. Lowering the pH of that soil through the use of ammonium fertilizers such as ammonium sulfate (ammonium acidifies more than nitrate), has reduced Verticillium wilt from moderately infested soils in eggplant (Elmer and Ferradino, 1994), and the combination of a rotation with oats with ammonium fertility (Elmer and LaMondia, 1999) has been shown to double the Mn tissue concentration in subsequent plantings of strawberry and reduce black root rot incidence (but not, unfortunately, increase plant growth or yield).

Again, some of this might be worth a look experimentally in strawberry on the Central Coast.

Several of the papers mentioned in the text above are available online – links provided below.

1. Dutta, B.K. and Bremner, E. 1981. Trace elements and plant chemo therapeutants to control Verticillum albo-atrum wilt.  Z. Pflanzenkankh. Pflanzenschutz 88:405-412

2. Elmer and LaMondia. 1994.  Comparison of Ammonium Sulfate and Calcium Nitrate Fertilization Effects on Verticillium Wilt on Eggplant.  Plant Disease 78: 811-816  https://www.apsnet.org/publications/PlantDisease/

3. Elmer W.H. and LaMondia, J. A. 1999. Influence of ammonium sulfate and rotation crops on strawberry black root rot.  Plant Disease 83: 119-123.  http://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS.1999.83.2.119  

Magnesium (Mg) has several purposes in the plant, one of the main ones being the central position in the ring of the chlorophyll molecule. Thus, it is very important for plant photosynthesis.  The other roles magnesium has in plants are parts in respiration and energy metabolism.  Magnesium is considered a secondary plant nutrient, because it is needed in fairly large amounts by plants. 

Magnesium moves with the water to the roots of the plant as the cation Mg²⁺, meaning the plant does not actively take it up and making root surface size and area an important factor in accumulation.  Therefore, Mg²⁺ deficiency can occur when root growth is comprised or the soil is too dry.  Interpreting soil analyses for magnesium content is not easy, because it is usually not the total amount that determines whether a deficiency can occur, but the relative proportion of Mg to calcium and potassium in the soil.  Furthermore, high levels of magnesium can cause deficiencies of these other nutrients as well.

When interpreting soil analyses, bear in mind that “exchangeable Mg^2+” and “saturated paste analysis Mg²⁺” are not measuring the same thing.  Only a percentage of exchangeable Mg²⁺ is available to the plant at any given time, the rest being bound to soil particles, colloids and organic matter.  The saturated paste analysis better represents what the plant root would encounter at the time of sampling, but this number will vary with the amount of rain or irrigation at that time.

A rule of thumb would be that saturated paste soil test values for Mg²⁺ above 0.5 meq/l represent sufficient amounts. Pajaro and Salinas Valley soils tend to be well above this threshold and thumbing through my numerous soil analyses taken from these areas from the past few years, I'm not finding a single one underneath it.

Being as they are fundamentally chemistry labs, many soil analysis reports will be expressed in other units. Some labs report soil magnesium as ppm exchangeable Mg, for example “82 ppm”.  For magnesium, meq/100 g x 120 = ppm.  Therefore, 82 ppm Mg is the same as 0.7 meq.  Luckily, most labs also provide some sort of graph that shows where the nutrient lands on the sufficiency index.  Most soils are not considered deficient unless exchangeable Mg is less than 25 – 50 ppm. 

Magnesium comes from rock and clay particles as they weather over time. The needs of strawberries and caneberries are around 40 -100 lb of Mg²⁺ per acre per season, meaning that soil analysis prints above several hundred pounds or even in the thousands in the top six inches indicate sufficiency (Mg ppm x 2 = lbs/top 6” of soil).  Again, since this is measured as exchangeable magnesium, not all of it is available at any given time.

A low soil pH under 5.4 can be restrictive for plant magnesium availability, and to a lesser extent (because there is so much magnesium already around) other cations such as potassium and calcium can also contribute to deficiency. For example, plant deficiencies of magnesium can occur in soils where the calcium to magnesium ratio (Ca/Mg ratio) exceeds 7 on a meq basis.  High levels of exchangeable potassium can also interfere with magnesium uptake.  Conversely, the opposite is true.  For example, in soils derived from serpentine rocks exchangeable magnesium can exceed calcium.  This can cause some interesting plant growth characteristics in other crops, such as yellow shoulder in tomatoes where these are grown on high magnesium soils common to the west side of the San Joaquin Valley.

Looking at the revision of plant nutrient levels produced by Tim Hartz et al in 2012, sufficiency levels of Mg in strawberry leaf tissue are 0.33 – 0.45 % prior to the onset of fruiting and then at 0.2 to 0.4 % during the harvest season.  In caneberries, according the Dr. Bernadine Strik at OSU, the optimum range is just a bit higher, with recommended values being 0.3 to 0.6 % during fruiting.

http://www.oregon-strawberries.org/attachments/2013-May_Nutrient_Management_Berry_Crops_OSU.pdf

Really a take home conclusion here is that most, if not all, of our Pajaro and Salinas Valley soils are well supplied with magnesium, and any deficiencies, should they arise, are stemming from physiological problems of the plant or chemical properties of the soil.