A Review of Chenopodium quinoa (Willd.) Diseases—An Updated Perspective

The journey of the Andean crop quinoa (Chenopodium quinoa Willd.) to unfamiliar environments and the combination of higher temperatures, sudden changes in weather, intense precipitation, and reduced water in the soil has increased the risk of observing new and emerging diseases associated with this crop. Several diseases of quinoa have been reported in the last decade. These include Ascochyta caulina, Cercospora cf. chenopodii, Colletotrichum nigrum, C. truncatum, and Pseudomonas syringae. The taxonomy of other diseases remains unclear or is characterized primarily at the genus level. Symptoms, microscopy, and pathogenicity, supported by molecular tools, constitute accurate plant disease diagnostics in the 21st century. Scientists and farmers will benefit from an update on the phytopathological research regarding a crop that has been neglected for many years. This review aims to compile the existing information and make accurate associations between specific symptoms and causal agents of disease. In addition, we place an emphasis on downy mildew and its phenotyping, as it continues to be the most economically important and studied disease affecting quinoa worldwide. The information herein will allow for the appropriate execution of breeding programs and control measures.

Identification, Pathogenicity, and Fungicide Sensitivity of Ascochyta caulina (Teleomorph: Neocamarosporium calvescens) Associated with Black Stem on Quinoa in China

Quinoa black stem is a new disease that affects the stems of quinoa plants and is more likely to develop under cool conditions (15 to 25°C, RH = 55 ± 2%). The typical symptoms include the formation of black necrotic lesions on the stem, which can completely wrap around the stem, causing lodging and blanking (development of ‘empty’ and sterile grain on the panicle). Furthermore, the pycnidia form small round protrusions on the surface of the lesions. Phylogenetic analysis revealed that representative isolates LMHS-3 and LMHS-5 were closely related to Ascochyta caulina (teleomorph: Neocamarosporium calvescens). Comprehensive morphological and molecular characterizations confirmed A. caulina as the pathogen that caused quinoa black stem. A. caulina mainly infected quinoa stems and could produce many pycnidia, but it rarely infected quinoa leaves. Pathogenicity testing showed that the most suitable temperature for the onset of quinoa black stem was from 15 to 25°C. When the temperature was increased above 30°C, the conidial germination of A. caulina became malformed, and when the temperature was decreased below 5°C, mycelium growth of A. caulina became extremely slow; thus, both extreme high and low temperatures affected the pathogenicity of A. caulina. Mancozeb and azoxystrobin fungicides were revealed to have had the strongest inhibitory effects on the conidial germination of A. caulina, and in some cases caused malformations in conidial germination. Tebuconazole and difenoconazole had the strongest inhibitory effects on A. caulina mycelial growth and less on the effects on the conidial germination. The results of the present study provide a basis for the recognition and management of quinoa black stem.

Fungal Phytotoxins with Potential Herbicidal Activity to Control Chenopodium album

This review deals with the isolation and chemical and biological characterization of phytotoxins produced by Ascochyta caulina and Phoma chenopodiicola proposed as mycoherbicides for the biological control of Chenopodium album, a worldwide spread weed which causes serious problems to some agrarian crops, including sugar beet and maize. Studies on the structure activity relationships and on the modes of actions of toxins isolated are also described, as well as the optimization of analytical methods focused on selection of the best fungal toxin producers. The attempts to scale up production of these phytotoxins aimed to obtain sufficient amounts for their application in greenhouse and field trials are also reported.

Seedlings damping-off of Chenopodium quinoa Willd.

The causal agents of damping-off of quinoa seedlings were determined in greenhouse experiments. <I>Ascochyta caulina</I>,<I> Fusarium</I> <I>avenaceum</I>,<I> Fusarium</I> spp., <I>Alternaria </I>spp. and <I>Pythium</I> spp. were isolated from infected parts of quinoa seedlings. The most frequent <I>Pythium</I> sp. was <I>P. aphanidermatum</I>. Pathogenicity tests confirmed that <I>P. aphanidermatum</I> and <I>F. avenaceum</I> were the causal agents of damping-off of quinoa seedlings under greenhouse conditions. A comparison of the reaction of quinoa with that of other susceptible plants (spinach, cabbage, sugar beet) showed that quinoa is most susceptible to the pathogen before emergence, during germination till the end of the stage of the first pair of true leaves. Germinable quinoa seeds seemed to have a lower ability to emerge from the soil. This serious problem is caused not only by pre-emergence damping-off from pathogens but more so by a complex of several adverse factors during germination when quinoa is most sensitive.