Evaluation of New Oomycota Fungicides for Management of Phytophthora Root Rot of Citrus in California

Abstract

Phytophthora root rot, caused by several species of Phytophthora, is an important disease of citrus in California and other growing regions. For chemical management, mefenoxam and potassium phosphite have been available for many years, and resistance in Phytophthora spp. has been reported for both compounds. We evaluated the efficacy of the new Oomycota fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin, each with a different mode of action, against Phytophthora root rot of citrus in field and greenhouse studies. Root balls of navel orange trees on ‘Carrizo citrange’ rootstock were inoculated with P. nicotianae at planting in the field in fall 2013. Applications with 11 fungicide treatments were made 5 weeks after planting, in spring and fall 2014, and in spring 2015. Feeder roots and adjacent soil were collected before or after application. All of the new fungicides significantly reduced root rot incidence and Phytophthora soil populations to very low levels as compared with the control starting after the first application. Mefenoxam was only effective when a high label rate was used in the fourth application. Selected treatments also increased tree canopy size, trunk diameter, and fruit yield as compared with the control. A rate comparison with the four new fungicides was initiated in summer 2016 in another field trial using navel orange trees inoculated with P. citrophthora. Minimum effective rates to reduce Phytophthora root rot incidence and pathogen soil populations were determined after one and two applications in fall 2016 and summer 2017, respectively. Greenhouse studies confirmed the efficacy of the new fungicides. Based in part on our studies, fluopicolide recently received a federal and oxathiapiprolin a full registration for use on citrus, and registrations for ethaboxam and mandipropamid have been requested. These new compounds will provide highly effective treatment options and resistance management strategies using rotation and mixture programs for the control of Phytophthora root rot of citrus.

Root treatment with oxathiapiprolin, benthiavalicarb or their mixture provides prolonged systemic protection against oomycete foliar pathogens

“…Soil treatment with oxathiapiprolin was reported to be effective against soil borne oomycete pathogens, e.g., black shank in tobacco caused by Phytophthora nicotianae [12] and root rots in citrus caused by Phytophthora species [13]. In our recent papers, we reported that root treatment with oxathiapiprolin or oxathiapiprolin+mefenoxam were effective in controlling downy mildew in cucumber [4] and late blight in tomato [9].…”

“…Oxathiapiprolin is a piperidinyl-thiazole-isoxazoline fungicide (FRAC code 49) [1,2] with extremely high activity against plant pathogenic oomycetes [3][4][5][6][7][8][9][10][11] except Pythium species [2]. Oxathiapiprolin is effective when applied preventatively or curatively to the foliage or to the root system [12,13] and shows translaminar and acropetally systemic translocation [2,3,5,11].…”

A new strategy for durable control of late blight in potato by a single soil application of an oxathiapiprolin mixture in early season

“…Others showed efficacy of soil drenches with oxathiapiprolin against black shank in tobacco [ 12 ], Phytophthora root rot in citrus [ 14 ] and Phytophthora blight in bell pepper [ 13 ]. No soil drench tests with the above fungicides were reported for potato.…”

“…BENT is a low to medium risk fungicide with resistance known in Plasmopara viticola but not in P. infestans (Frac list 2018, [11]). As both fungicides are systemic [3][4][5][6]13,14] they should mutually protect each other against resistance development even during a prolonged exposure period in the field. Experimental field data are required to validate this hypothesis…”

“…Soil application of 1mg active ingredient per plant provided durable protection for up to four weeks in tomato against late blight, cucumber against downy mildew and basil against downy mildew. Other researchers demonstrated efficacy of soil drench with oxathiapiprolin against Phytophthora black shank in tobacco [ 12 ] Phytophthora blight in pepper [ 13 ] and Phytophthora root rot in citrus [ 14 ].…”