Monday, September 12, 2011

Attention Strawberry Growers: Phytophthora cactorum

By Dr. Frank Louws, NCSU Dept. of Plant Pathology with contributions by Dr. Barclay Poling, Professor Emeritus, NCSU Dept. of Horticultural Science; Dr. Chuck Johnson, Virginia Tech, SPAREC; Dr. Powell Smith, Clemson Extension, Lexington Co.; and David Dycus, NCDA Regional Agronomist

Berry Alert No. 99

September 9, 2011 (Friday)

Subject: Frequently Asked Questions about Managing Phytophthora cactorum (Pc) in strawberry tips and plugs


Since the Labor Day weekend, a NC nursery plant supplier with runner tips grown on PEI, Canada, has been in touch with their customers to inform them of a problem with Phytophthora cactorum (Pc), which is the causative agent of Phytophthora root and crown rot.

Growers who have received such a call from their plant supplier are understandably concerned, and in many instances, they have in turn called their local extension agent, NCDA field agronomist, or various specialists here in the Mid-south at NC State, Clemson and Va Tech. This is not a new problem for our industry, and quite frankly this particular disease does lend itself to some "cures". As Dr. Powell Smith wrote to a South Carolina grower on Wednesday this week:

"If you have suspicious-looking tips (necrotic [decayed] looking), you can send those to the Clemson Plant Problem Clinic. They will be able to find the Pc on the tips if it is there. Also, any plants looking to be suffering from root rot in the plug house, send those immediately to the PPC for diagnosis. Time is of the essence; there are cures."

Indeed, there are some "cures", and this where I think many growers need realize that this is a situation that can be reasonably well managed, and one important step is to get a "proper diagnosis" if you are seeing some suspicious-looking tips. If you are having any question on where to go with strawberry tips/plugs for diagnosis, let us know!

In North Carolina, we have the Plant Disease and Insect, and here is the website to get directions for submitting a plant sample:

Here is the form for Clemson:

Phytophthora problems in plug plants. Notice the distinct black lesions. Botrytis will be brown and usually have gray fuzz. Also, note the poor root formation resulting in plant collapse as the misting frequency decreases.

(photo courtesy of Dr. Frank Louws)

In the interest of time today, we review below questions we have received in the last day or two about this situation:

Question 1. We have received some tips from the nursery that has reported having some Pc on the tips/plugs, and we are seeing plugs at our location that are doing fine, and rooting well after 8 days. We have no idea how we could dip 60,000 tips now that they have been set and we were told that Aliette was our best option. We need your advice!

Answer 1. First: Aliette is a fungicide product that is in the phosphorous acid family of fungicides. Since Aliette is off-patent, multiple other products have been registered and include Prophyt, Phostrol and others. These are commonly referred to as phosphonate or phosphite fungicides and do not provide any nutrient benefit to the plant. These products are not to be confused with products in the phosphoric acid family commonly used as a source of nutrients to the plant (e.g. Nutri-Phite and similar products). The label for the phosphite fungicides is written with perennial or field production systems in mind and therefore it is very difficult to directly interpret the label for this type of need. “Preplant dips” are allowed and applications within 14 days”after planting” are allowed. Thus, timing in plug facilitates is unclear but the label does not prohibit use of treating transplant trays or other “nursery” type setting.

Second, it is encouraging that you are not seeing suspicious-looking tips (necrotic [decayed] looking)! The moment symptoms appear, it would be productive to send samples to the local clinic for diagnosis asap. You are quite correct in questioning how you might go about dipping 60,000 tips, as you are already past that point! If you were just now receiving an order of tips for plugging, then perhaps a dip of a phosphite fungicide would be in order. In your case, the plugs have now been growing 8 days, and it turns out that phosphite fungicide treatment even at 4-7 days should be helpful, and a second treatment may be appropriate 4 to 7 days before going to the field (usually plugs root for a full 4 weeks, and for the first full week they are typically under some type of misting schedule in the day, and by about 7-8 days, the plugs have established enough of a root system so that much less misting is required). In one experiment done at NCSU, dipping the tips prior to plugging, spraying the plants soon after setting, and early dip/sprays followed by a second foliar application 14 days later all gave similar results for controlling Phytophthora problems in plug plants. Prophyt and Aliette gave similar results. As far as the actual "treatment", you would need to spray your 8 day old plugs with Aliette at the rate of 2.5 lbs/100 gallons/acre, 2 pints/100 gal (ProPhyt), or 2.5 pints/100 gal (Phostrol). Now, if you have 60,000 plugs, your plugs are covering about 2600 sq ft, or only 0.06 acre (2600/43560). Thus, using Aliette as an example, the amount of product needed to spray this much area would only be 0.15 lb, or 2.4 oz. Foliar spray volumes to deliver this phosphite product work best with sprays applied just prior to runoff, but if we were to stick with 100 gallons/acre, then for an area of 2600 sq ft you would need to apply 6 gallons of water. The key concept is that the phosphites are absorbed by foliage and roots, not just by roots as with Ridomil, and the phosphite product should be in sufficient water to promote as much uptake as possible by the leaves and other plant parts without creating drenched conditions in the potting medium or promoting splash dispersal among plugs on the pad. Aliette is known to cause damage in other crops if excess amounts go into the soil and if other products may be present (e.g. certain insecticides, copper, surfactants etc).

Question 2. Since the phosphite fungicides can be applied as a foliar spray my next question has to do with how much of a "drying period" is needed after you have sprayed the plugs? (of course, if the plugs are still under mist during the first week, then the only dry period is the evening)

Answer 2. The phosphites are unique chemicals in that they are absorbed by the roots and the foliage and they become systemic in the plant, moving up and down. They are absorbed fairly readily into the plant and as a rule of thumb for systemic fungicides, they probably require about 90 minutes of drying time for the majority of the product to be taken systemically into the plant. The fungicide has direct as well as indirect (changing the biochemistry of the plant) activity against the pathogen offering curative control (it can stop the pathogen from growing more inside the strawberry tissue). Therefore, if plants are under a mist schedule, the best time to apply the fungicide is towards the end of the day when the misting cycles are off. This allows maximum absorption into the plant and limits excess runoff into the soil. Avoid excess saturation of the growing media. Phytophthora is a water mold and the more free water there is the more the pathogen can spread.

Question 3. Once my plugs are out from under the mist in the 2nd week and thereafter, can you advise on whether a follow-up treatment with Aliette may be needed?

Answer 3. As mentioned above, a second treatment did not seem to offer an advantage. The product is systemic in the plant and seems to persist for long periods of time. However, if a second application is made, it is probably best done 4-7 days prior to planting to allow the fungicide to be absorbed into the plant but to limit worker exposure problems and any plant stress issues.

Question 4. Since I have plans to inject Ridomil Gold through the drip after planting my plugs (I am thinking about 1 week after transplanting), don't you think that it would be better to use Ridomil in that way, or do I also need to make an application with Ridomil to the plug trays before planting?

Answer 4: Ridomil is clearly not labeled for use in plug production facilities. The label specifically prohibits use in transplant trays, as a foliar application or in transplant water. There are two good reasons for this – possibly among others. First, Ridomil use prior to field setting strawberry transplants can result in slow grow-out of the plants once they are field set. Second, resistance to the fungicide can develop easily as use in transplant facilities favors the development of resistant populations which then get transferred to the field – a serious issue for future control of the pathogen in the field. Resistance to the phosphite fungicides is much less common.

Ridomil is a superior product in the beds. If growers cannot set up their drip irrigation system prior to or right after planting, then Ridomil can be incorporated into the beds when beds are being formed. This was the case for a grower who asked today. Application during bed formation in pepper fields that had Phytophthora problems worked better than applying Ridomil through the drip after planting but no data is available for strawberries. If excess overhead irrigation is not required, the product could be injected into the beds though the drip tape prior to planting. Finally, alternatively and the method most growers seem to prefer, Ridomil can be applied into the beds when excess overhead watering is done and when plant roots begin new growth (5-7 days after planting). If the transplants were not treated, the sooner the Ridomil is in the bed the better keeping in mind it will wash down into the soil if excess overhead watering occurs and will not be absorbed if the roots are not active. Detailed studies on timing for strawberries are not available. Ridomil should be considered again as a second application in the early spring when new plant growth starts. Strawberry plants grow lots of roots right at this time and protection is critical during early spring growth.

Question 5. I am having difficulty in my area in finding Aliette, so my question is whether the other Phosphites listed in the SR SFC IPM Guide are just as effective as Aliette, and should I stick with the rates suggested in the 2011 IPM Guide? I have heard that the Phostrol label calls for rates of 3.75-5.0 pints/acre when disease pressure is severe?

Answer 5. Overall, the phosphite chemistries are very similar in terms of their efficacy. Different rates relate to different concentrations of the active ingredient and in a plug facility there is concern about using high rates. In NCSU trials, we know Prophyt, Aliette and another similar product were all equally effective. In turf studies, some phosphites did prove superior to others and this is likely related to formulation but the differences were minor.

Question 6. Is there any advantage to mixing other materials with Aliette (e.g. potassium carbonate)?

Answer 6. Potassium carbonate is used to adjust the pH of the spray mixture and this protects against plant injury problems when Aliette is used before or after other products such as copper. There is no advantage to tank mixing with other products and there may be real disadvantages with mixing other products due to the presence of surfactants or other compounds that result in plant injury when combined.

Question 7. What is your general impression about the availability of extra tips this season?

Answer 7. Growers can certainly "check around" but the thing worth stressing is that in the tip and plug nursery business, it is very unusual to find much extra supply in any season, as these are grown under contract (very little speculative production). Used correctly, it is possible to "manage this problem", and with the products mentioned in this advisory, it is possible to turn this problem around. However, as Powell Smith says, Time is of the essence. We have seen low plant death problems where plants have been treated in accordance with the observations above. Although there is no experimental data, it makes sense to remove healthy plants or rogue out diseased plants to prevent the pathogen from spreading more. Excess free water in the and around the trays should be avoided and don’t work plants when wet. Moving plants from an infested plug facility, even if symptoms are no longer present, will introduce the pathogen into the field soils. Many farms already have Phytophthora in the field but this can be a long term risk, especially in fields where the highly susceptible variety Sweet Charlie will be planted in future years.

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