Pointers on Disease Resistance Management From Expo 2014 in Pinehurst; More Information on Botrytis Crown Rot (11:12am, 3/13/15)

— Written By Barclay Poling and last updated by
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Good morning!

I know that many of you are in the field this morning “working ahead of rains” coming later today, but I felt it might be very timely to share this article written by Dr. Schnabel and Dr. Peres, who both spoke at our Southeastern Expo last fall, and this was an excellent recap of that presentation. For your convenience, I also added some timely information about Botrytis Crown Rot, by Dr. Frank Louws (bottom of advisory). Finally, in the early afternoon today I received this VIP document, “2015 Fungicide Resistance Management Guidelines” from Dr. Schnabel (by way of Tom Baker):Straw_FRAC2015 (you should download this and keep it handy).

In the article by Schnabel and Peres, please note the sentence  that says,

“Botrytis control success is vastly improved if you know the resistance profile of your fields.”

It is very easy to determine what your resistance profile is by simply following the instructions at this website (you need to do this ASAP):

An ideal "dead" flower specimen for submittingFig. 1. An ideal “dead” flower specimen for submitting to Clemson’s lab to determine you crop’s resistance PROFILE.

Correct address for mailing specimens:

Guido Schnabel, Ph.D.
Clemson University
Dept. Agricultural & Environmental Sciences
105 Collings St./220 BRC
Clemson, SC 29634

Office 864 656 6705: cell 864 643 7131

This is the pdf version of article below: SchnabelPeresDiseaseAndResistanceManagement

Disease and Resistance Management in Strawberry; Top Considerations for the Coming Season

Guido Schnabel (Clemson University) and Natalia Peres (University of Florida)

The new strawberry season is just around the corner and we need to make smart choices for pest and disease management. At the Strawberry Expo 2014 in Pinehurst we talked about some important things to consider this coming season to ensure maximum disease and resistance management. Here they are in a nutshell:

Implement IPM practices in nurseries. It is not a secret that diseases often come in with transplants and we must do a better job avoiding that. Luckily, we received some funding that will enable us to work with nurseries, investigate their practices, and develop solutions to current problems. This will not happen overnight but will rather be work in progress over the next years.

Avoid Key Selectors. Some fungicides are key selectors for resistance to multiple fungicides in the gray mold fungus on the east coast. Resistance to multiple fungicides has built up in a stepwise fashion over time and resistance to some fungicides is the backbone of resistance to newer chemistries. Basically, if new resistance emerges, it is most often from a population that is already resistant to established fungicides. In particular, applications of fungicides from FRAC (Fungicide Resistance Action Committee) group 1 (e.g. Topsin M) and to some degree FRAC 11 (Abound, Cabrio, Pristine, Merivon) are frequently associated with resistance to other FRAC groups that we need for disease control. Our recommendation:

  • Avoid FRAC 1 fungicides
  • Use FRAC 7/11 premixtures (Merivon, Pristine) ONLY if gray mold AND anthracnose are a threat BUT NOT for routine gray mold control.
  • Use FRAC 11 solo products (Abound, Cabrio) only for anthracnose control
  • Do not use FRAC 7/11 premixtures or FRAC 11 solo products more than twice per season.

Spray Strategically. If applications are needed prior to bloom, thiram, captan, and maybe biologicals should be used. During bloom, stick with captan as much as possible and use the ‘at risk fungicides’ (including FRAC 1, 2, 7, 9, 11, 12, and 17) only when the weather is favorable for disease development (Table 1).

Table 1. FRAC code, trade name, and primary target of fungicides frequently used for disease control in strawberry

table 1 FRAC Code details

Botrytis control success is vastly improved if you know the resistance profile of your fields. Make sure you get the gray mold fungus tested. Download instructions.

Spray less. Spraying less is the ultimate resistance management tool because we are selecting less. But that is only an option if we do not compromise disease control success. Research has shown that we are spraying way too many times and that often more than 50% of our applications are unnecessary. We are implementing an online tool, the Strawberry Advisory System (SAS), in southern states that notifies growers when an application is truly necessary. Growers will need to be near a weather station that is hooked up through the internet to a weather database. Contact us for more information if you are interested. But if you do not have access to this system, table 2 shows the weather conditions that you may use to decide whether to spray or not. You might be surprised how many sprays you can save without compromising control, especially in a reasonably dry year.

Table 2. Decision Support Chart for Gray Mold Management in Strawberry

table 2Conversion: 17-25 C (62.6-77 F)

In conclusion, after more than a decade of applying multiple fungicides of multiple FRAC codes, resistance is now common in the gray mold pathogen Botrytis cinerea. But the resistance profile is different from location to location and depends on spray history, nursery source, and nearby crops hosting the pathogen. Knowing your resistance profile will enable you to prevent ineffective sprays and improve preharvest and postharvest disease control. We must make every effort to spray strategically and to limit the number of sprays and we must include nurseries in our efforts to control pests and diseases. Good luck!

From Profile Reports Issued by Clemson

General resistance management strategies

Use broad spectrum fungicides like captan and thiram before bloom (avoids unnecessary selection for resistance)
Spray more effective fungicides during bloom PRIOR to rain events; try not to spray calendar
­based to reduce the number of sprays

Achieve good spray coverage (reduces populations exposed to selection)
Use tank mixes of single site inhibitors (e.g.
Topsin­M, ELEVATE, ROVRAL, SCALA) with protectants (CAPTAN, THIRAM) during heavy disease pressure (reduces populations exposed to selection). If disease pressure is light (little to no precipitation), use captan or thiram alone
During heavy disease pressure (very wet conditions expected), use premixes with protectant if resistance is present to one of the two active ingredients in the premix (reduces selection pressure for other active ingredient)
Alternate fungicides from different chemical classes or FRAC groups (reduces selection time)

For additional recommendations see the Strawberry IPM Guide

Special notes from Clemson Profile report relevant to Botrytis crown rot

1)Iprodione, the active ingredient of ROVRAL , is the only dicarboxamide product currently available for control of Botrytis crown rot and gray mold in strawberry.

2) Using ROVRAL once a year prior to bloom for Botrytis crown rot and gray mold control is recommended.

NCSU INFORMATION ON:  Botrytis Crown Rot


bot cwon rot

Botrytis crown rot occurs sporadically in annual production systems. The disease occurs in early to late winter or into the early spring under cool wet conditions. The disease is favored in the winter if blooms are frost killed; if excess plant growth occurs or plants are at a high density; or if winter-killed tissue is colonized by the pathogen. Certain cultivars such as ‘Sweet Charlie’ or other early blooming cultivars are more susceptible to the problem. Sometimes, growers can be caught by surprise if the disease develops under floating row covers used to promote growth or protect against cold events.

Symptoms and Signs

If warm spells occur during winter in plasticulture systems, early blooming cultivars may push out a flush of flowers. These inadvertently are winter-killed. Botrytis will colonize these damaged flowers and start to colonize down the flower stem (peduncle) and appear as a light brown rot. The pathogen continues to colonize down into the crown area which is usually very dense, in a rosette stage, and then the pathogen continues to colonize other stems and peduncles just above the crown. Affected leaves and tissue will turn brown (Figure SS-1) and a diagnostic feature is that the tissue will easily give way from the crown when pulled.

Alternatively, flowers may not develop (e.g. in cultivars such as ‘Chandler’ or ‘Camarosa’) but warm winters favor a highly dense plant growth habit. Under cool wet conditions Botrytis will colonize strawberry tissue just above the crown and leaves will begin to turn brown and die above the colonized region (Figure SS-2). The pathogen usually does not colonize the internal crown tissue (Figure SS-2) so once the weather is non-conducive to the pathogen or if management practices are put in place, the plant recovers with new flushes of growth; however, it may be set back in terms of early yield potential.

Disease cycle

Botrytis can enter the field on transplant foliage. The fungus can live in the green tissue but be latent, or dormant, and not cause symptoms. Botrytis can affect many different crops and therefore weeds surrounding a field could be an important source of the pathogen. The pathogen can also produce dark hardened structures called sclerotia and these can persist in soils for years. Based on NC research, it appears most infections occur from initial inoculum on leaf and crown tissue moving with transplants. In the case of symptomless leaf infections, as the infected strawberry leaf begins to die, the pathogen goes into an active stage, colonizing the leaf and obtaining its nutrients from the dead tissue. Spores then form and, once environmental conditions are appropriate (between 65-75 F [18-24 C] and with damp or rainy weather), they are dispersed by water splash and/or wind onto newly emerging leaves or blossoms. Botrytis crown rot is worse in years when early bloom occurs and this bloom is winter-killed. The pathogen colonizes the flower and then grows down into the crown tissue. Alternatively, the pathogen may colonize dead and dying tissue in the lower canopy and upper crown area and begin to macerate surrounding healthy tissue. In many cases, abundant sporulation is present (Figure SS-1) and this may result in further disease spread.



1. Plant Growth and Variety Selection:

B. cinerea is commonly associated with transplant leaves and two years of research has demonstrated that there are no differences due to plant source or variety. Currently, it is not possible to obtain disease free plants and this should not be a burden on nursery growers. Plants that tend to bloom early are more susceptible (e.g. Sweet Charlie). Therefore, wise use of row covers to optimize plant growth and cold protection should also include avoidance of early bloom development or growth of plants that are too dense.

2. Monitor

Excess nitrogen will result in plants that are too dense and more susceptible to Botrytis. IF early bloom occurs, monitor for Botrytis colonization of the flowers and flower stems. These can be hand removed or a fungicide should be applied.

Chemical Control

Systemic fungicides are the most effective management tool and are best applied when Botrytis crown rot symptoms first appear or when many flowers are winter-killed. In general, we recommend a Rovral application since growers are allowed one application per season before bloom. This product has local systemic activity, is highly efficacious against Botrytis, and there is a low level of resistance to this product in the Southeast region. Switch also provides good control but it is best to save Switch and other Botryticides for bloom and fruit development sprays to manage gray mold of the fruit. Current fungicide recommendations are available from the Small Fruits Southern Region Small Fruit Consortium IPM and Production Guides.


Frank Louws
Director NSF Center for IPM; Professor Plant Pathology
Plant Pathology
 This NC State FactSheet can be viewed and printed from Gray Mold / Crown Rot of Strawberry fact sheet.
Have a great afternoon!
Dr. E. Barclay Poling
Professor Emeritus (Strawberry Plasticulture Researcher)
Department of Horticultural Science
Campus Box 7609, 162A Kilgore Hall
NC State University
Raleigh, NC 27695-7609
“An idealist believes the short run does not count. A cynic believes the long run does not matter. A realist believes that what is done or left undone in the short run determines the long run.”

Sidney J. Harris, In: Reclaiming a Lost Heritage – Land-Grant & Other Higher Education Initiatives for the Twenty-first Century

Updated on May 20, 2021
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