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Wednesday, August 31, 2016

Bacterial diseases of vegetables

M. Grabowski, UMN Extension Educator

The bacteria that cause black rot in cabbage spread through the
plant's veins in the field and in storage.
M. Grabowski, UMN Extension
The high heat and humidity this summer, combined with multiple heavy rain events have created ideal conditions for many bacterial plant pathogens to multiply and spread. This is especially evident in the vegetable garden, where black rot can be easily found on cabbage, kale, and broccoli, beans bear spots and browning from bacterial leaf blights, and tomato and peppers are spotted by bacterial spot.

Bacterial plant pathogens have several unique features that make them good plant pathogens. Many are able to infect seed and can be introduced into the garden unseen on infected seed or transplants.

Bacterial diseases of bean often result in spots on leaves and
 bean pods. M. Grabowski, UMN Extension
Bacteria are covered in a sticky coating and are easily spread through the garden on hands, tools, and insects. Many bacterial plant pathogens are also easily spread by splashing rain or sprinkler irrigation. Bacteria infect the plant through natural openings or wounds. They multiply within infected plant tissue and can survive from one growing season to the next in infected plant debris in the soil.

Many management strategies for bacterial plant diseases are designed to prevent the introduction and spread of the pathogen. For example, gardeners should buy healthy seeds and transplants from a reputable source and use drip irrigation instead of sprinklers to keep leaves dry.

At this time of year, there are still a few things that gardeners can do to manage bacterial diseases. 
  • Avoid working in plants when they are wet. 
  • When working in the garden, take care of healthy plants before working in infected plants. 
  • Clean tools with solution of 1 part household bleach and 9 parts of water after working in infected plants. Clean hands with soap and water. 
  • Up to 1/3rd of infected leaves can be removed from an infected plant to reduce disease spread. (At this time of year, this is practical for crops like Brussels sprouts and kale that are still developing a crop, but is not necessary for tomatoes, peppers, and beans, which are near the end of their natural production cycle.)
  • Remove infected plants that are no longer producing a crop. 
  • Do not save seed from infected plants. 
Seed saved from tomato fruit infected by a bacterial plant
pathogen may be infected with the bacteria.
M. Grabowski, UMN Extension 

Diseased plant material can be composted if the compost pile heats up to 148 F and all plant material becomes broken down. Many counties offer municipal compost sites that accept yard waste. If composting is not an option, infected plants should be buried in the soil soon after harvest. The soil microorganisms will begin to break down the infected plant material. The pathogen would be expected to survive in the soil and plant debris for 1 to 2 years. As a result, the same crop and related plants should not be planted at that site for the next 1 to 2 years.

Springtails in homes

Jeffrey Hahn, Extension Entomologist

A consequence of the abundant rainfall that most of Minnesota has received has been an increase of springtails found in homes and other buildings. Springtails are small insects that are associated with moisture. They range in size from 1/16 - 1/8 inch in length. Most are slender, although some are round and stout. They are typically dark colored, gray, black or brown, but some are white and some even iridescent and brightly colored. Springtails lack wings but do have the ability to jump.

Springtails as a group are very numerous, living in a variety of moist habitats including in soil, leaf litter, mulch, decaying wood, and around bark where they feed on fungi, pollen, algae, and decaying plant matter. They can live inside buildings when high moisture exits, e.g. around plumbing leaks. They can also seek shelter indoors when areas around the outside of the home become excessively wet. Fortunately, regardless of the number that are seen, they do not harm people or damage property. But they can be annoying when conspicuous numbers are present.
They may be small but springtails can occur in large
numbers and be a nuisance in homes. Photo: Unknown

They key to controlling springtails is reducing moisture; they cannot survive if sufficient moisture is not present. However, the reason springtails are present will influence how easy it is to get rid of them. If springtails entered structures to escape excessive damp outdoor areas, they are expected to go away on their own as the rain lessens and the humidity goes down. Use a fan or dehumidifier to reduce indoor humidity to help reduce springtail numbers until they are gone.

However, if there is some sort of ongoing moisture problem, springtails will continue to be an issue. Check indoors for sources of moisture and areas of high humidity. The moisture source could also originate outdoors, e.g. rainspouts that do not carry the water far enough away from the foundation or landscapes that slope towards buildings. Correcting existing moisture conditions is a long-term solution in springtail control.

Although it may be tempting for some to treat springtails with an insecticide, the products available to residents are not very effective against these insects. Moisture control is the most effective strategy.


For more information, see the University of Minnesota Extension fact sheet Springtails


http://www.extension.umn.edu/garden/insects/find/springtails/

A bad year for bur oak blight

M. Grabowski, UMN Extension Educator

Bur oak blight, M. Grabowski, UMN Extension
This summer many bur oak trees are suffering significant leaf browning and death due to bur oak blight. Frequent rain events at the time new leaves were reaching their mature size created highly favorable conditions for infection by the fungus that causes bur oak blight. For property owners with affected bur oak trees, now is the time to submit a sample for diagnosis. If disease is severe, contact an arborist now to schedule treatment for bur oak blight in spring of 2017.

Bur oak blight, often referred to as BOB, is a plant disease caused by the fungus Tubakia iowensis. The BOB fungus survives the winter on infected leaves that remain attached within the tree canopy.  In wet spring weather, the fungus releases spores that start infections on new leaves. Although the infections occur in spring, the most obvious symptoms do not appear until the end of July or early August. Initially, dark discolored lines can be seen forming along major leaf veins. As the disease progresses the discoloration expands into brown wedge shaped areas. Leaves may remain partially brown and green or may turn completely brown and withered. When autumn leaf drop occurs, many leaves infected with bur oak blight remain attached to the tree.

Wedge shaped brown areas and completely
brown leaves are both symptoms of bur oak
blight. M. Grabowski, UMN Extension 
Bur oak blight is a slow moving disease. The leaves in the lower canopy are usually first to be infected. Some of these leaves remain attached until the following growing season and will allow the disease to move up to higher branches. Each year, the disease infects a greater portion of the tree canopy. This slow progression of disease eventually stresses the tree and allows secondary pests and pathogens, like two lined chestnut borer and Armillaria root rot to infect and further injure the tree. The combination of these infections can lead to tree decline and death.

Many pests and pathogens can cause leaf browning in oak trees. Oak wilt, another fungal disease of oak trees, can cause similar symptoms to bur oak blight. It is important to identify exactly what pest or pathogen is causing the problem, so the appropriate management strategy can be applied. Property owners with bur oak trees in poor condition should submit a sample to the UMN Plant Disease Clinic for diagnosis.  

Browning begins on major leaf veins and expands into large
wedge shaped areas. M. Grabowski, UMN Extension 
Once bur oak blight has been confirmed, a management plan can be created. Trees tolerate BOB and do not require treatment if less than 50% of the leaves in the canopy are infected. Trees with low levels of infection should be examined each year in August to determine if severity has increased to the point that intervention is needed.

Once 50% or more of the canopy is infected with BOB, treatment of bur oak blight with a fungicidal injection is recommended to prevent tree decline. A trunk injection of propiconazole fungicide in spring when new leaves have just reached their mature size has been shown to significantly reduce bur oak blight in most cases.  This treatment must be applied by an arborist and timing of the treatment is critical. Property owners wishing to have their trees treated in 2017 should contact an arborist now to schedule the treatment.

Bur oaks are sensitive to propiconazole fungicides. This may result in browning and other leaf damage the year the application is made. Unfortunately no other fungicides have proven effective in protecting bur oaks from BOB. Once an application has been made, disease should be significantly reduced and the tree will not need to be treated again until bur oak blight once again affects 50% of the canopy.


Tuesday, August 30, 2016

Useful Tools to Determine Soil Moisture Status II: Gypsum blocks and others

Karl Foord, Extension Educator - Horticulture

Another tool for measuring soil moisture are gypsum blocks. Gypsum is a porous material that equilibrates with soil water similar to the ceramic tips of the tensiometers. In this case two electrodes are embedding in the gypsum block in a cylindrical or parallel mode (Figure 1). The commercial product often has a perforated metal covering to protect the gypsum (Figure 2).
Figure 1






Figure 2

This system functions by connecting to a meter that measures the resistance between the two electrodes embedded in the gypsum. Water is a good conductor of electricity. As soil water decreases due to plant draw down resistance increases and visa versa when soil water increases resistance decreases. The electrodes can be permanently connected to a box (Figure 3) or temporarily connected to a portable meter (Figure 4).

Figure 3

Figure 4



Placement of gypsum blocks follows the same strategy that is used with tensiometers. One block is placed in the middle of the fibrous root zone and another below this zone. The upper block indicates when irrigation is needed, and the lower block determines when to turn the irrigation off.

Other systems

There are other types of soil moisture sensors. The principles behind these sensors range from simple to sophisticated.

The most inexpensive type of moisture sensor follows the principles of a galvanic cell e.g. a battery. The shaft of the instrument is made of one metal and the tip of another, often copper and lead respectively. The soil water functions as the electrolyte facilitating the movement of electrons from one metal to the other. High meter readings are associated with high soil moisture and low readings with low soil moisture (Figure 5). One note of caution about these meters. They are designed to take a soil moisture reading and be removed from the soil. If left in the soil the "battery" will continue to discharge and the meter will quickly become nonfunctional. What can you expect from a meter that might cost $8? There are more robust meters with greater capacity due to more metal but they are significantly more expensive.

Figure 5


Other soil moisture sensors utilize other principles of physics to measure soil water content. These principles are beyond the scope of this blog. Suffice it to say that they all use water as part of their "circuitry" and respond differently to different amounts of soil moisture.

The information given herein is supplied with the understanding that no endorsement of named products is intended, nor is discrimination or criticism implied of products mentioned or not mentioned.

Monday, August 29, 2016

Useful Tools to Determine Soil Moisture I: Tensiometers

Karl Foord, Extension Educator - Horticulture

Generally the most limiting element in maintaining uniform plant growth and high quality produce is water. A plant can be experiencing water deficit prior to our observation of wilting. Such deficits can lead to slower growth rates, pollen mortality and loss of flowers, lighter fruit weight, and blossom end rot, among others. It is best to avoid having the plant experience any water deficit.

However, how can you know that the plant is in water deficit? The truth is that you can only know this indirectly. One can estimate soil water content by taking a soil surface sample and feeling the water content with your hand; this will of course vary with soil type and again extrapolates the soil water content within the root zone. This method will require consistent attention to growing conditions including temperature humidity and rainfall.

What  other options might allow a more direct sampling of soil water content within the rooting zone of the plant?

Soil Water Potential

Soil water potential is a measure of the amount of energy required by the plant to absorb or extract water from the soil matrix. A soil water potential value of 0 (Zero) means that the soil is saturated, water moves freely, and the plant need not expend energy to absorb water at this potential. However as water is removed from the soil by the plant, more energy will be required by the plant to extract water overcoming the forces holding water molecules to soil particles. The soil water potential is measured in centibars and as the values become more negative the difficulty in obtaining water becomes greater. One tool to measure water potential is a tensiometer.

Tensiometer

A tensiometer is a hollow tube with a porous ceramic tip at one end (Figure 1).
Figure 1


The tube is filled with water and sealed with a cap, and has an attached vacuum gauge. The porous tip allows the water in the tube to be connected to the soil water matrix. As plants use water and the soil dries out, water is also drawn out of the tensiometer. Because the tube is sealed, a vacuum is created and measured by the gauge. Plants can easily extract soil water when the soil water potential value is 10 centibars (CB). At higher values such as 35 CB it is more difficult for the plant to extract water. Figure 2 shows the face of the pressure gauge.
Figure 2


Figure 3 shows the cycling of tensiometer readings over a period of time. The plants use water and the pressure increases into a range between 20 and 38. The sloped lines indicate the water draw down and the straight lines indicate a watering event. The gauge reading returns to near zero following the watering event .
Figure 3

By placing tensiometers at two depths, we have a way of knowing when to turn the water on and when to turn it off. A short tensiometer (6") is place in the middle of the active root zone, and a longer tensiometer (12") is placed below the active root zone. When the short tensiometer reads between 30 and 35 CB begin irrigation. As irrigation begins the 6" gauge will begin to move to zero. But when should you stop irrigating because the 6" gauge is in the middle of the root zone? When the 12" gauge begins to move to zero it means that the water is beginning to exit the active root zone. At this point the irrigation should be turned off (Figure 4).
Figure 4
 We will take a look at some other tools used to measure soil moisture in the second part of this blog.

Friday, August 19, 2016

Bugs on milkweed

Jeffrey Hahn, Extension Entomologist

Home gardeners have been finding orange and black insects on their milkweed and related plants. Although some people think they are boxelder bugs, they are actually insects called large milkweed bugs, Oncopeltus fasciatus.
Large milkweed bugs are colorful and conspicuous and
fortunately harmless.  Photo: Jeff Hahn, U of M Extension

Adults grow as large as ¾ inch long. They are mostly orange and black including a black horizontal band across the center of the body and black on the end of the wings (the membranous section). A portion of the head is reddish orange. The immature nymphs are mostly orange with black wing pads and smaller than the adults.

Large milkweed bugs prefer to feed on common milkweed but will also feed on other related species. They often feed in large groups making them conspicuous on the plants. Despite their appearance, they do not harm milkweed nor any insects, like monarchs, they may also be on the plants. No action is necessary if large milkweed bugs are found in your garden.

Thursday, August 18, 2016

Sap beetles in gardens


Jeffrey Hahn, Extension Entomologist

Some home gardeners have been finding sap beetles in some of their fruits and vegetables. These beetles are generally small, between 1/8 – ¼ inch long, oval, and dark colored. Some sap beetles have orange spots on their wing covers.
A picnic beetle, a common type of sap beetle.
Photo: Tom Murray
Sap beetles are attracted to fermenting smells and will attack fruits and vegetables, such as tomatoes, sweet corn, raspberries, strawberries, and muskmelons, that are damaged, overriped, or rotting. They often are just a nuisance, although it is possible for them to move to and damage ripening fruit.

The best management for sap beetles is to pick fruits and vegetables regularly as they ripen and remove any damaged or overripe produce in your garden and dispose of by burying or bagging them. This helps eliminate smells that could attract them to your garden. However, once sap beetle find your garden, they can be challenging to eliminate.

Insecticides, such as carbaryl or permethrin, can kill sap
Pick up fallen and damaged fruit to prevent
attracting sap beetles to your garden.
Photo: Jeff Hahn, U of M Extension
beetles and reduce their numbers. However, the challenge is that there is an interval of time between when a product is applied and when the fruits or vegetables can be safely harvested. You can find this information by looking for the Days to Wait to Harvest number on the pesticide label.

Depending on the product, this can take days or even weeks. By then the sap beetles have likely returned. If that is the case, try to use a product with as short of a time interval as possible.

See also Sap beetles in home gardens

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