Blossom blight diseases of caraway and coriander have a large impact on the stability of production of these crops in Saskatchewan, but their causal organisms are not well understood. Blossom blight surveys were conducted from 2015 to 2019 and revealed that generally, wetter conditions favouring blossom blight development in Saskatchewan in 2015 and 2016, whereas dryer conditions prevailed from 2017 to 2019, leading to fewer disease outbreaks. The incidence of organisms arising from isolation from blossom tissues on growth media was compared with the presence or absence of symptoms on individual umbels, and candidate pathogens were confirmed in indoor inoculations. The main pathogen of caraway, previously informally referred to as an Ascochyta species, was identified and named Didymella cari Armstrong-Cho, Banniza and Crous. Although a Heterosphaeria sp. was pathogenic on caraway in indoor testing, it was not an important caraway pathogen in disease surveys. A foliar pathogen of caraway, Septoria carvi, was detected for the first time in Canada, but only once in one field.

In coriander, heavy crop losses were associated with an odd organism with slow, yeast-like growth. Through DNA analysis and collaboration with taxonomists at AAFC Ottawa, this fungus was tentatively placed in the Heterosphaeria genus. Studies on pathogen infection chronology in coriander flower samples using qPCR technology indicated that Heterosphaeria sp. had the greatest biomass at early flowering, which increased through the season, indicating that it was the primary pathogen in most samples. In earlier research, this organism may have been either confused with, or misidentified as an Aureobasidium species. Aureobasidium pullulans isolated from caraway seed in the current study was non-pathogenic on caraway and coriander. Fusarium graminearum, F. avenaceum, Botrytis cinerea and Sclerotinia sclerotiorum were also confirmed pathogenic on coriander and may have been involved in moderate disease outbreaks at two sites in 2016. Didymella cari caused disease in coriander in only one field during the survey period, due to close proximity to severely diseased caraway.

Caraway pathogen D. cari and coriander pathogen Heterosphaeria sp. were assessed for fungicide sensitivity levels and to determine the host range of Heterosphaeria sp. Insensitivity to azoxystrobin (Quadris) fungicide was observed in D. cari isolates from two Saskatchewan farms highlighting the need to register additional fungicides against blossom blight. Fungicide insensitivity testing of Heterosphaeria sp. was complicated because by its biology, but successful testing of five isolates indicated all were sensitive to azoxystrobin. The host range of Heterosphaeria sp. included cumin, but not dill and anise. Results on chervil and fennel were inconclusive.

Greenhouse screening of caraway with D. cari identified an annual caraway accession with partial resistance; however, its oil quality and content profile renders it unsuitable for plant breeding of biennial caraway. Four coriander accessions had less disease than local cultivar CDC Major under low disease pressure in 2018. Under moderate to high disease pressure in separate D. cari and Heterosphaeria sp. screenings in 2019, three accessions with better performance than CDC Major against both pathogens were identified, and could be used for resistance breeding in coriander.

Extension Messages:

• The cause of blossom blight in caraway was identified and named as a new species, Didymella cari Armstrong-Cho, Banniza and Crous. Previously, this pathogen was colloquially referred to as an unknown Ascochyta species.
• The primary cause of blossom blight in coriander was previously misidentified as an Aureobasidium species. Our study indicated that this pathogen is an undescribed fungus, tentatively identified as a Heterosphaeria species.
• Several other fungi can also cause blossom blight of coriander (Fusarium avenaceum, F. graminearum, Botrytis cinerea, Sclerotinia sclerotiorum), but these were observed to be associated with low to moderate field symptoms in a small number of cases during disease surveys conducted from 2015-2019.
• Registered fungicide options are limited for disease control and there is a high risk of development of insensitivity to the strobilurin class of fungicides (group 11). Azoxystrobin insensitive isolates of Didymella cari, the cause of caraway blossom blight, were detected on two Saskatchewan farms.
• The potential for improvement in coriander disease resistance exists in germplasm collections, whereas disease resistance traits in caraway were only observed in annual accessions which are not suitable for resistance breeding of biennial caraway.