Peanut Disease Update: week of May 27 – June 1.

Before getting to the state disease summary, a quick note on two diseases common at this time of year, Pepper Spot/Leaf Scorch and funky leaf spot (FLS). Funky Leaf Spot is important because the early symptoms can be confused with early leaf spot. A severe foliar disease of peanuts requiring thorough and regular sprays.

Leaf Scorch/Pepper Spot

Leptosphaerulina crassiasca is a fungus (pycnidial type) that can cause two different disease symptoms on peanut leaflets, Leaf Scorch and Pepper spot. Both symptoms produce only minor plant problems (slight defoliation) and are not worth spraying by themselves. Sprays that are targeted to other problems will almost inevitably control L. crassiasca.

Leaf Scorch is the most common symptom produced by L. crassiasca in Mississippi. Scorch symptoms typically start near the tip of the leaflet. Symptoms may or may not start as with a dark circular lesion, surrounded by a halo, making it seem like Early Leaf Spot. If the circular lesion is present, the next stage is the most eye-catching, the development of a wedge-shaped lesion with a bright yellow margin and brown center. The leaf may eventually shrivel, or fall from the plant. Examination of the lesion (holding the leaf flat) with a 12x or higher hand lens, will show some brown to black “volcano” looking structures (sometimes with an open top), the fruiting body of L. crassiasca. Last year, especially in northeast Mississippi, another fungus (an anthracnose fungus) would sometimes enter the wedge shaped lesion and caused the remaining part of the leaf, as well as the attached petiole and stem to wilt. A hand lens is necessary to separate Leaf Scorch from Funky Leaf Spot.

Pepper Spot, also caused by L. crassiasca, is distinguished by small (about 1 mm) black spots on the upper side of the leaf. They grow only slowly. This symptom was becoming more prominent in the Clarksdale area this week, whereas the Leaf Scorch symptom was found in all production areas, depending upon the field.

Funky Leaf Spot (FLS)

There is no identified cause of FLS (also called Florida Leaf Spot and Irregular leaf spot). Extensive laboratory examination of FLS symptoms have yet to find a cause, bacterial or fungal. FLS was first noticed in Florida in the 1990’s and became common though out southeastern growing areas by 2000. The leaf symptoms look very much like Leaf Scorch.

FLS leaf symptoms are common on lower plant leaves and are most common during the earlier part of the season, before the plant really gets into reproductive growth. There may be a greater incidence in fields with reduced tillage or residue amounts. I have noted that FLS is more common in areas where moisture stays on the leaves longer (eastern side of the field near trees), or where an upper leaf touches a lower leaf, and where a plants are already stressed. I recently visited a field in the Clarksdale area that had some test plots of a fungicide applied in-furrow at planting. Perhaps because of a combination of cold weather and the fungicide treatment, the plants in the test plots were stunted compared to the untreated plants next to them. A count of FLS (and some leaf scorch) symptoms in the test plots showed about 2x as many FLS as compared the the untreated plants in the rows right next to the test plot.

No one has shown a clear association between FLS and yield. Apparently, defoliation can be high in some older cultivars, but I have yet to see a serious defoliation problem in Georgia 06 G.

A number of people have looked at applying fungicides for control of FLS, with no effect. Save your money and fungicides, do not try to control this leaf spot. It will disappear on its own. Apparently some fields have been sprayed – with a storbilurin fungicide (Group 11 fungicide). Storbilurin fungicides are very effective against a wide range of pathogens, but resistance to them builds quickly, therefore, the labels limit their use when applied as the only active ingredient, to no more than three applications. It is my opinion that these expensive but very effective fungicides should be saved for events when they are truly needed, for example limb rot and aerial blight caused by rhizoctonia, for (unlikely) early and late leaf spots that might enter later in the year, or perhaps white mold.

Funky Leaf Spot, spot symptom phase

FLS wedge symptom, with yellow margin

FLS, lesion beginning to dry and wither.

Statewide

Some peanuts in both the south and north started flowering around May 23. In the North some of the flowering fields were planted prior to April 15 (about 47 calendar days of age) whereas in the south, the flowering fields were planted about May 5 (about 28 days old).

The difference in the number of days from planting to flowering is due to growing temperatures. The late planted field in the south experienced +80 F air temperatures whereas the earlier planted northern fields experienced some quite cool temperatures not long after planting. Cool temperatures weakened the plants, and allowed some soil-borne pathogens (see week of May 14- 18) to gain a weak foot-hold and further slow plant development. While the older plants have now out grown these problems, they are not as physiologically advanced as their calendar age would indicate. Since so many peanut management programs depend on calendar age, this is problem. A quick calendar adjustment can be made – by restarting the calendar at 30 days once flowering begins. You might use this for Determing your white mold applications.

South

Leaf scorch and funky leaf spot are starting in most fields in the Lucedale area. Some late seed are continuing to emerge. Compared to last year, there is very little seedling and crown rot caused by Aspergillus niger. Very little funky leaf spot or leaf scorch is apparent further north in the greater Hattiesburg area.

Northeast

I did not make it the greater Aberdeen area, but funky leaf spot was beginning to occur in Webster County and Vardaman area.

Batesville area

Some Thimet injury and some FLS.  Thimet can cause injury to younger plant leaves.  Please see: http://www.peanut.ncsu.edu/NC/PeanutNotes.aspx#&&ID=005231 for more information.  Images of Thimiet damage courtesy of Bullock’s Ag Consulting, THANK YOU TREY!

Typical Thimet injury.

Clarksdale area

Leaf scorch/Pepper spot has replaced Funky leaf spot as the predominant leaf spot in the fields. These leaf spots have not been related to yield loss and SHOULD NOT be sprayed, but apparently some fields have been sprayed – with a storbilurin fungicide (Group 11 fungicide). Storbilurin fungicides are very effective against a wide range of pathogens, but resistance to them builds quickly, therefore, the labels limit their use when applied as the only active ingredient, to no more than three applications. It is my opinion that these expensive but very effective fungicides should be saved for events when they are truly needed, for example limb rot and aerial blight caused by rhizoctonia, for (unlikely) early and late leaf spots that might enter later in the year, or perhaps white mold.

Greenwood/Tchula

Both FLS and some minor Leaf scorch present in some, but not all fields.  Some symptoms of Thimet injury.

Travel  funded in part from grants provided by the Mississippi Peanut Promotion Board and Integrated Pest Management (IPM)