Small Grain Disease Update – April 8, 2016

Nathan Kleczewski, Extension Specialist – Plant Pathology;

Cool to cold temperatures persist, which has squashed most disease progress in small grains. Most wheat is around Feekes 6-7 in many parts of the Delaware, and some barley is at boot or early head emergence. In wheat, diseases are scarce or nearly absent in many fields. Powdery mildew is still observed in some fields, mostly at low levels, and recent freezes should significantly push back the disease. Some subtle virus symptoms are out there at very low levels in some fields.

In barley you will see net blotch or the spot form of net blotch to some degree (Figure 1). Without using molecular tools, there is no way of distinguishing the two, but for our purposes we can treat them the same. Symptoms are jet black circular blotches to elongated, cross-hatched blotches. This is another disease that can be residue-borne, persisting in residues of barley and volunteer barley. No-till fields with high levels of residue therefore, will show greater symptoms. Contaminated seed is another source of the disease. Disease progresses when it is very wet and temperatures fall between 60-80 F. Planting barley early can exacerbate the disease in young plants, as it allows the pathogen to establish and damage germinating seedlings. Rotation, encouraging residue decomposition via disking, and planting clean seed are the best means of managing this disease. In areas where continuous small grain production is used (Western US, UK) this disease can cause some significant damage. Here it is more of an annoyance. However, if anyone is considering looking into malting winter barley in the future this disease should be on your radar. We also will need to be able to distinguish the spot from net blotch forms as there is no good resistance for the spot blotch form, but some good resistance sources for the net blotch form. We are working on generating some of that preliminary data this season.

Also, you may see some frost damage in barley as heads emerge, particularly in fields that were far along. Time will tell.


Figure 1. Net blotch of barley.

As far as scouting wheat, make sure to see where you are at disease-wise in wheat at flag leaf emergence (Feekes 8-9). There are reports of stripe rust and common leaf rust in North Carolina and Tennessee, and recent weather patterns may have started to move spores closer to areas of Maryland and Delaware. Stripe rust is a beast at cooler temperatures, whereas leaf rust tends to be more of a warm weather disease. Stripe rust does not always form stripes (Figure 2). However, the spore color is significantly different from common rust. Common rust pustules are brown to red brown, whereas stripe rust pustules are yellow to orange. Make sure you are scouting your fields, because you do not want to be stuck playing catch up to stripe rust in a susceptible variety.


Figure 2. Typical stripe rust symptoms on wheat foliage.

Expect to see symptoms of barley yellow dwarf in some barley and wheat fields. We have observed putative BYDV symptoms indicative of Fall infection at very low levels in some fields. Typically, characteristic symptoms appear after the flag leaf emerges, particularly if temperatures are cool and sunny. In barley you may see bright yellow flag leaves that can become puckered to necrotic, and in wheat flag leaves may have a purplish to orange appearance. BYDV follows patterns of infected aphids, so it often occurs in small patches or areas of the field. Plants can also be stunted, particularly at the center of these patches. Seldom are severe BYDV issues detected in Delaware and Maryland, but issues can occur from time to time. This virus is a big deal in southern areas where temperatures remain warmer for longer and out west where large areas of pasture exist for grazing by cattle, etc. The result is greater populations of aphids coupled with more potential sources of overwintering virus. The other piece is that we historically see the moderate/mild strains of the virus, so even if you see symptoms, the likelihood for any significant yield hit is low. Bottom line: know that the infection occurred long ago, and that you cannot fix infected plants. If you have areas that tend to seem to be hot spots for BYDV, avoid planting early and select varieties rated excellent for BYDV in the Virginia Tech small grains guide.

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