The most economically important plant pathogens in the genus Pseudoperonospora (family Peronosporaceae) are Pseudoperonospora cubensis and P. humuli, causal agents of downy mildew on cucurbits and hop, respectively. Recently, P. humuli was reduced to a taxonomic synonym of P. cubensis based on internal transcribed spacer (ITS) sequence data and morphological characteristics. Nomenclature has many practical implications for pathogen identification and regulatory considerations; therefore, further clarification of the genetic and pathogenic relatedness of these organisms is needed. Phylogenetic analyses were conducted considering two nuclear and three mitochondrial loci for 21 isolates of P. cubensis and 14 isolates of P. humuli, and all published ITS sequences of the pathogens in GenBank. There was a consistent separation of the majority of the P. humuli isolates and the P. cubensis isolates in nuclear, mitochondrial, and ITS phylogenetic analyses, with the exception of isolates of P. humuli from Humulus japonicus from Korea. The P. cubensis isolates appeared to contain the P. humuli cluster, which may indicate that P. humuli descended from P. cubensis. Host-specificity experiments were conducted with two reportedly universally susceptible hosts of P. cubensis and two hop cultivars highly susceptible to P. humuli. P. cubensis consistently infected the hop cultivars at very low rates, and sporangiophores invariably emerged from necrotic or chlorotic hypersensitive-like lesions. Only a single sporangiophore of P. humuli was observed on a cucurbit plant during the course of the studies. Together, molecular data and host specificity indicate that there are biologically relevant characteristics that differentiate P. cubensis and P. humuli that may be obfuscated if P. humuli were reduced to a taxonomic synonym of P. cubensis. Thus, we recommend retaining the two species names P. cubensis and P. humuli until the species boundaries can be resolved unambiguously.

Currently, there are increased interests in growing grain and fiber hemp (Cannabis sativa L.) as well as in large-scale hemp products. Cannabis has been grown/utilized for thousands of years as a fiber, grain, and drug/medicinal plant. However, the strict control of cannabis cultivation to combat illegal use, the spread of new yarns and oilseeds, and the advent of cheap synthetic fibers caused a decreased/eliminated hemp production. Hemp has been banned in most of the world for more than seven decades; it missed out on the Green Revolution and the adoption of new technologies and varieties, creating a knowledge gap. After the 2014 and 2018 Farm Bill in the USA, hemp became legal and the land grand universities launched research programs. The ability to utilize the entire plant for multiple purposes creates opportunity for the market to value hemp products. Hemp production technology varies depending on the type of hemp cultivated (grain, fiber, or cannabinoids), soil characteristics, and environmental factors. Hemp has the potential to be a very sustainable and ecologically benign crop. Hemp roots have a significant potential for absorbing and storing heavy metals such as lead, nickel, cadmium, and other harmful substances. In addition, hemp has been proven to be an excellent carbon trap and biofuel crop. Hemp has the ability to successfully suppress weeds, and it is generally regarded a pesticide-free crop. The purpose of this paper is to examine historic and recent industrial hemp (grain and fiber) literature, with a focus on hemp agronomy and utilization.

Light leaf spot, caused by the ascomycete Pyrenopeziza brassicae, is an established disease of Brassicaceae in the United Kingdom (UK), continental Europe, and Oceania (OC, including New Zealand and Australia). The disease was reported in North America (NA) for the first time in 2014 on Brassica spp. in the Willamette Valley of western Oregon, followed by detection in Brassica juncea cover crops and on Brassica rapa weeds in northwestern Washington in 2016. Preliminary DNA sequence data and field observations suggest that isolates of the pathogen present in NA might be distinct from those in the UK, continental Europe, and OC. Comparisons of isolates from these regions using genetic (multilocus sequence analysis, MAT gene sequences, and rep‐PCR DNA fingerprinting), pathogenic (B. rapa inoculation studies), biological (sexual compatibility), and morphological (colony and conidial morphology) analyses demonstrated two genetically distinct evolutionary lineages. Lineage 1 comprised isolates from the UK, continental Europe, and OC, and included the P. brassicae type specimen. Lineage 2 contained the NA isolates associated with recent disease outbreaks in the Pacific Northwest region of the USA. Symptoms caused by isolates of the two lineages on B. rapa and B. juncea differed, and therefore “chlorotic leaf spot” is proposed for the disease caused by Lineage 2 isolates of P. brassicae. Isolates of the two lineages differed in genetic diversity as well as sensitivity to the fungicides carbendazim and prothioconazole.

Hop powdery mildew, caused by the fungus Podosphaera macularis, was confirmed for the first time in hop yards in the United States Pacific Northwest in 1997. The US hop industry experienced significantly crop loss and damage from the disease, but new knowledge, grower experience, and management approaches have allowed the industry to survive the introduction of this damaging pathogen. This review provides an overview of research on and progress towards economically sustainable management of hop powdery mildew in the US, and future directions for research and further innovation. Accepted for publication 2 January 2008. Published 14 March 2008.

Downy mildew of hop (Humulus lupulus), caused by Pseudoperonospora humuli, is managed in the Pacific Northwestern United States by regular application of fungicides. A degree-day model that forecasts the first emergence of shoots systemically infected with P. humuli (termed basal spikes) and a risk index for secondary spread of the disease were evaluated over four seasons in western Oregon. In surveys conducted in 34 hop yards, the predicted first spike emergence occurred on average 11.6 days (median 12 days) after spike emergence using a simple average degree-day model (base temperature 6.5°C) developed for Washington State. Predictions based on a single sine model (base temperature 6°C) provided on average 4.9 days (median –0.5 days) of advanced warning before the first spike emerged. Downy mildew severity in a previous season was negatively correlated with the degree-day emergence date of spikes the following year (r = –0.39). In experimental plots, disease severity was significantly greater where fungicide applications were timed using a risk index compared to routine fungicide applications in 2005 and 2007, but statistically similar between these treatments in 2006 and 2008. However, in 2006, 2007, and 2008, treatments initiated using a degree-day threshold resulted in an area under the disease progress curve similar to or smaller than in treatments with routine fungicide applications. Model-aided treatments required four fewer fungicide applications compared to routine fungicide applications. These studies indicate that downy mildew can be managed effectively with fewer fungicide applications than currently made by hop growers in this region if fungicide applications are timed to coincide with the predicted emergence of basal spikes and subsequent disease risk forecasts.