VOLUME 22, NO. 14                                                                                                                                                                        August 7, 2008

SPIDER MITES FOUND IN DRY BEANS AND SOYBEANS

Spider mites have been found in the hot dry bean production areas in Cando and Towner counties.  Overall populations have been low.  Life cycles are fairly fast at 5-14 days with greater development rates occurring at temperatures above 91F.  Females live about 30 days and produce about 300 eggs.  Hot dry weather inhibits natural fungal diseases that control mite populations, thus populations can increase quite rapidly.  Spider mites thrive on stressed plants that are nutrient rich.  Leaf injury symptoms appear as stippling first and then progresses to yellowing, browning or bronzing as feeding injury increases, and eventually leaf drop. Feeding injury causes water loss from the plant and reduces the photosynthetic ability of the plant.  In severe cases, premature leaf senescence and pod shattering will occur and even plant death. When a severe mite infestations occur during late vegetative and early reproductive growth, a 40 to 60% yield loss between treated and untreated soybean has been demonstrated in other north central states. Spider mites can cause yield reduction as long as green pods are present.  Spider mites can be found on the underside of leaves and lower foliage.  Adults are small (<0.2inch), greenish-white to orange-red in color, have two dorsal spots and 4 pairs of legs.  Nymphs�s have 3-4 pairs of legs and are smaller.  Shaking a leaf or plant over a white sheet of paper they appear as small yellow dots that crawl slow.  Spider mites can move fast to adjacent fields by climbing to the tops of the plants and dispersing by �ballooning.�  Be sure to scout during full pod (R4) through beginning seed (R5) stages since these crop stages are the most important contributors to soybean yield. If hot dry conditions exist, mite populations can quickly increase and sprays should not be delayed.  There is no specific threshold that has been developed for two-spotted spider mite in soybean. As a result, several guidelines are available from different states to determine economic infestation of spider mites in soybeans. One of the more commonly used economic thresholds is based on different crop stages:

� Before pod development = 20-50% of leaf area is discolored/stippling on plants.
� Between R3 (beginning pod) to R5 (beginning seed) = 10 to 15% of leaf area is discolored/stippling on plants.
� R6 (full seed) to R7 (beginning maturity) = 25% of leaf area is discolored/stippling on plants.

The discoloration is caused from the feeding injury (e.g., leaf stippling) of spider mites, and is difficult to estimate in the field.  Remember to use an organophosphate insecticide (e.g. Lorsban, Dimethoate) over a pyrethroid insecticide to avoid flaring mite populations.  Early detection facilitates timely and effective rescue treatments. Current insecticides for soybeans provide short-term protection, maybe 7-10 days, from the pest. Fields will need to be monitored continually for resurging populations.

TIME TO SCOUT FOR THE BEAN LEAF BEETLES

Bean leaf beetles have been found in low levels in southeast North Dakota.  The second generation is the one that can cause potential yield impact due to pod feeding.  They cause pod scarring, pod clipping, and decreased seed quality.  Injuries can lead to fungal infections of pods causing discolored, shrunken or moldy pods.  Leaf feeding injury symptoms include round holes between leaf veins.  Adult bean leaf beetles are 5 mm long with yellow or red wing covers with four black spots and a black margin around the edges of the wing cover.  The main characteristic is the black triangle behind the head.  Sweep nets are the best way to monitor for beetles.  Treatment would be recommended when 3 to 7 beetles per sweep are found.  Economic threshold is 25% defoliation from reproductive pod fill to maturity or 10% pod feeding (or the presence of clipped pods). 

FOLICUR LABELED FOR MANAGEMENT OF SUNFLOWER RUST

Bayer CropScience has been granted a Section 3 � Full federal registration for the use of Folicur for the management of sunflower rust. This supersedes the ND Section 18 Crisis Exemption Issued on July 24^th 2008. To use Folicur on sunflowers you must have the supplemental label in your possession: http://www.sunflowernsa.com/uploads/Folicur_Fungicide_36F_7-31-08.pdf

The active ingredient in Folicur is tebuconazole, a demethylation inhibitor (DMI), FRAC group 3 fungicide.  The label states that Folicur can be applied at a rate of 4 � 6 oz. per acre with no more than 16 fl. oz. applied per season.  An application cannot be made within 50 days of harvest. For more information, the supplemental label is available at the Bayer CropScience website.

BACTERIAL BLIGHT SHOWING UP ON DRY BEANS

In the last week, bacterial blights have shown up in dry beans. Three different bacterial blights are found on dry beans; common bacterial blight, halo blight, and bacterial brown spot. Each disease is spread by rain splash, and the diseases are frequently worse when the plants are injured by wind or hail. Several strong storms have moved through some of the state in the last two weeks, and this has contributed to the development of the diseases.  Although multiple diseases can occur at the same time and on the same plants, each disease has different symptoms.

Leaf symptoms:  Halo blight appears as small water soaked lesions and often produces a large yellow-green halo. When temperatures are hot, the halo may be smaller. Early symptoms of bacterial brown spot also appear as a small water soaked lesion but have a small yellow-green halo, often with a very defined border.  As bacterial brown spot develops, the lesions turn brown and the centers fall out.  Heavily infected plants look like they were shot by a shotgun.  Common bacterial blight lesions begin on the edge of the leaf as a scalded area with water-soaked spots.  Common bacterial blight lesions enlarge quickly and are bigger (up to an inch) than the other two bacterial diseases, and often have a narrow yellow border.  Pod Symptoms:  Symptoms of halo blight and common blight are similar. Both diseases begin as water-soaked spots or streaks on the pod surface. As the lesions develop, they may be surrounded by a reddish-brown zone.  Under humid conditions, a cream-colored exudates may be present in halo blight lesions, whereas a yellow exudates may be present in common bacterial blight lesions.  Bacterial brown spot lesions begin as small water-soaked lesions, but become sunken brown spots.  Pods with bacterial brown spot infections may be bent or twisted at the site of infection.  The biology and management of each disease is similar.  Bacterial blights can survive in seed, bean residue, and sometimes weed hosts. Once the diseases are present, they can spread quickly if heavy rains and adverse weather conditions (high winds, hail, etc.) occur.  Unfortunately, little can be done this season to manage bacterial diseases. Although copper fungicides are sometimes used in other regions of the country (dryer western states like Colorado), little success has been observed in North Dakota. Management for future years is important if you have bacterial blights in your fields. Clean seed, rotation, and good sanitation are all recommended.  Although these practices are not a guarantee you will not have bacterial blights, they minimize the likelihood of getting the diseases. If you have bacterial blights in your field now, it is critical not to keep that seed for planting in the future.

CERCOSPORA LEAFSPOT IN SUGARBEET

It is the season for Cercospora leafspot, the most damaging foliar disease of sugarbeet which reduces tonnage and sucrose content.  Cercospora leafspot develops rapidly in warm, humid, and rainy weather. The Cercospora spores are produced at temperatures of 68 to 79F and relative humidity�s (RH) of 90-100 %.  Spore release is affected by rain and dew. Optimal spore germination and infection occurs when the temperature is 75-77� F and the RH is 100 % for at least 8 hours. Day temperature of 80-90 F and night temperatures above 60 F favor disease development. Leafspot symptoms may occur about 5-21 days after infection depending on prevailing weather conditions.  There are a number of fungicides available for controlling Cercospora leaf spot.  Fungicides with different modes of action and which are most effective in research trials should be used in rotation.