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November Highlights

Things to watch for during November

From our experience of samples we have received during November in previous years in the OSU Plant Clinic, this page will highlight some problems that may occur this month.

Viral disorders

Leaf blackening, reddening and mottle can be sign of Grapevine Leafroll Virus which is transmitted by insects. Since there has not been any work on virus vectors of these viruses in grapes in Oregon, it is critical that growers continue to use only certified wood when establishing vineyards.

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grape

Grapevine red blotch disease, caused by a virus with similar symptoms to grapevine leafroll associated virus, was first reported in 2012 and has been found in many winegrape cultivars. Leaf reddening develops on the leaves, the red areas joining together to appear as a red blotches. Some cultivars are symptomless.

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grapevine red blotch
grape

Fungal disorders

Box blight is a relatively recent fungal disease for Oregon but is expected to be increasingly prevalent. The disease is common on the east coast as well as in Europe and New Zealand. Symptoms include spots on leaves, and streaks or cankers on stems followed by defoliation. Nurseries should be on the look out when the Fall rains begin. The OSU Plant Clinic has some new resources listed for susceptibility of boxwood varieties to box blight.

Box blightbox blight

Downy mildew on cabbage and other brassicas is caused by the fungus Peronospora parasitica. Small yellowish spots appear on the upper surface of the leaves. As these enlarge they also become apparent on the underside of the leaves and are accompanied by a white moldy growth especially in conditions of high humidity.

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cabbage downy mildew
cabbage

From Cindy Ocamb: Black leg will start its appearance in brassicas shortly: canola, cabbage, kale, and turnip are all very susceptible.  Organically-produced crucifer crops are especially vulnerable.  Disease will be spread by wind-blown ascospores releasing from infectious crop residues on the soil surface. Recent sustained rain events should result in widespread ascospore release that will cause leaf spots during November. Recent inspection of crop residues have shown activation of the asexual, water-splashed spore stage, which is not moved by wind, but can infect volunteer brassicas.  Growers should kill volunteer brassicas and radish now.  Scouting for Phoma leaf spots, accompanied by a protective fungicide program will aid in preventing field-wide epidemics. Scouting for disease and regular removal of infected leaves can reduce seed pod infections or protect organic vegetable crops; detached infected plant material is ideally buried until decomposed or otherwise destroyed.  For seed growers, disease management based on protective fungicide programs will require at least three applications during the fall through the winter, possibly even more than five applications are needed in seed crops including canola, in order to prevent the buildup of acreage with infected crop residues.  Protective fungicide sprays should be initiated with the onset of the rainy season in western Oregon. Materials registered for canola can be found here, while materials registered for other Brassica seed crops can be found here.

Horticultural producers should visit the PNW Plant Disease Management Handbook to see fungicides registered under the specific crop name (broccoli, cabbage, etc.).

phoma leaf lesion
Phoma leaf lesion

 

As you are harvesting garlic and pumpkins, remember that they are very susceptible to post-harvest rots and it is essential that they are properly stored. Here is a recent pumpkin sample that has Fusarium fruit rot that began on the side that was in contact with the soil.  Under moist conditions the fungi develop and penetrate the rind then secondary invaders follow along.

pumpkin fusarium fruit rot
pumpkin

Check out our own resource page on storage rots of garlic and this one on storage conditions.

If you are a nursery grower you can now learn about Phytophthora, including how to reduce the risk of disease caused by this organism, in the online training course "OSU Phytophthora Online Course: Training for Nursery Growers".

The OSU Plant Clinic has some resources for information on Phytophthora tolerant plants for the landscape, and planting suggestions following Phytophthora infection.

Yellowing of needles on spruce in late summer, which then turn brown or purplish-brown, are a sign of Rhizosphaera Needle Cast. Spores from the fungus causing this spread to current -season needles during wet conditions.

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Rhizosphaera needle cast
spruce

The OSU Plant Clinic has some new resources listed for susceptibility of boxwood varieties to box blight.

Abiotic disorders

Like many woody ornamentals, especially trees, douglas-firs have suffered in the drought conditions we have been experiencing lately. The entire tree may die, or the needles on sections of the tree may turn brown or reddish. Weakened and stressed trees may then become susceptible to secondary pathogens and pests. There is a good discussion of this phenomenon here.

Abiotic disorders may predispose plants to secondary infection. This sample of delicata squash had secondary Pseudomonas syringae infection (bacterial) of the fruit which we suspect may initially have endured mechanical injury.

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seconday Pseudomonas syringae
squash

Bacterial disorders

Blueberries will need some attention before the Fall rain begins in earnest, to manage bacterial canker caused by Pseudomonas syringae. Precautions taken now can reduce the severity of disease that may show up next Spring on new season canes.

blueberry bacterial canker
blueberry

Leaf spots on nursery herbaceous ornamentals may be also attributed to bacterial infections, including Pseudomonas syringae.

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coral bells Pseudomonas syringae
coral bells

Bacteria and fungi capable of inducing plant disease may occur together and it is sometimes difficult to determine if both are contributing to the symptoms. In this coral bells sample, fungal species of Phoma were also present.

Bacterial blight is an important disease of geraniums and pelargoniums. Symptoms can be seen in cuttings, on stems and on leaves. Disturbingly, some hardy perennial geraniums can carry the bacteria but remain symptomless. The disease is easily spread by physical contact, and in soil and irrigation water, especially during propagation.

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geranium bacterial blight
geranium

Also important for nursery growers at this time of year during stock propagation are the two further bacterial diseases in herbaceous ornamentals; crown gall

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wishbone flower

and shoot proliferation and leafy gall.


dianthus

The OSU Plant Clinic resource page for these two diseases has information and images of symptoms on many different plants.