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Seed and Residue Serving as Pathogen Reservoirs for Wheat (04/30/20)

Planting season has begun and questions have been asked pertaining to how pathogens survive. This includes the risk of using bin-run seed and seeding wheat after a previous crop of wheat or corn. This article will review some of the most important seed-borne and residue-borne diseases in North Dakota.

Seed and Residue Serving as Pathogen Reservoirs for Wheat

Planting season has begun and questions have been asked pertaining to how pathogens survive. This includes the risk of using bin-run seed and seeding wheat after a previous crop of wheat or corn. This article will review some of the most important seed-borne and residue-borne diseases in North Dakota.

Seed-borne

An example of a pathogen that primarily survives on the wheat seed coat is Xanthomonas campestris pv. undulosa (bacterial leaf streak; BLS). The pathogen can also overwinter on other grass hosts, but seed is often considered the most important inoculum source for BLS. Several factors can influence how long or how much of the bacterium can survive. Research conducted in in the late 1980’s suggested that both storage conditions and length of storage impact the survival of the bacterium (ie: viability decreases the longer it is in storage). Also, research conducted in the southern US indicate that the transmission rate for BLS is <3%. Other factors that influence BLS include the amount of bacteria detected on the seed and host susceptibility. Preliminary results from NDSU research indicate that the most susceptibility varieties carry the most amount of bacteria. This further stresses the importance of selecting a variety with a BLS score of 5 or less (Extension Publication A574-19)

A common pathogen that survives within a kernel is the loose smut pathogen. Loose smut may be our most common seed-borne disease of wheat and the signs of this pathogen are most evident when the wheat head emerges (black dusty spores replace kernels – Figure 1). Aside from using clean seed, fungicide seed treatment (FRAC 3, 7 or 11) can significantly reduce loose smut incidence. Scabby wheat seed is another common seed-borne disease. The Fusarium fungus that causes shriveled and lifeless kernels can lead to lower germination rates and seedling blight. It is important to note that scabby seed will not lead to Fusarium head blight. If you choose to use a scabby seed source, it is best to remove (clean) as many scabby kernels as, check germination rate, and adjust plant populations accordingly. A fungicide seed treatment should also be considered if the level of scabby kernels is high after cleaning.

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The most common pathogen that can be found in seed lots is Claviceps pupurea (ergot). In the last couple years, it appears ergot prevalence has increased for certain regions of the state. There are several factors that can influence ergot risk including cool-wet weather at heading, vicinity to other grassy weed hosts, and factors that may disrupt successful pollination such as nutrient stresses, herbicide damage, and hail. However, when ergot is found in the seed lot, here are a few thoughts to consider on how it may impact the 2020 crop. The mushroom spore-bearing structures that germinate from ergot bodies tend to reach maximum lengths of ¾ inch (Figure 2). Thus, it is possible that the seeding depth of wheat will likely drive the ergot bodies far enough into the soil preventing the spore-bearing structures from reaching the soil surface. Also, unlike other sclerotia-producing diseases such as white mold, ergot bodies are prone to rapid degradation by fungi and insects and do not last more than one year.

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Residue-borne

Fungal leaf spots and Fusarium head blight are diseases that benefit most from host residue. Fungal structures for these pathogens can survive for several years, but the availability of spores is most important the year after a crop was sown in the field.

Tan spot, Stagonospora nodorum blotch and Septoria tritici blotch all survive very well on wheat residue. The pathogens responsible for these diseases can also be seed-borne, but residue tends to be the most important inoculum source. The most common fungal leaf spot that is observed in ND is tan spot, and all three can potentially occur in the same field. There are no HRSW varieties that are immune to fungal leaf spots and variety scores can be found in A574-19. Another management tool that is commonly used in ND is the use of a fungicide tank-mixed with an herbicide at the tillering growth stage. There are several effective fungicides available for use on fungal leaf spots in ND and the “response” of a fungicide will depend on several factors (more on this next week!).

The Fusarium head blight pathogen survives on small grain residue, and its most preferred host is corn (Figure 3). Fusarium graminearum can cause stalk rots, seedling blights, and ear mold on corn and uses the crop as a saprophytic host as well. Corn residue has more surface area than small grain residue and takes longer to break down, and the combination of these factors make corn a very important inoculum source. It is likely that some wheat will be planted on corn ground this year, and if that is the case, make sure to understand that you will have a higher in-field inoculum source for FHB. This does not necessarily guarantee a FHB problem as environmental conditions are an important factor for this disease. Prolonged periods of relatively humidity prior to heading and throughout the flowering process will drive FHB epidemics. Remember that host resistance is our most important management tool against FHB, and when combined with a well-timed fungicide at onset of early-flower (or up to seven days later), can reduce disease and DON levels up to 70%.

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Andrew Friskop

Extension Plant Pathology, Cereal Crops

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