[dropcap]I[/dropcap]n 2015, losses to tomato spotted wilt across the Southeastern production region stabilized and were similar to estimated losses from 2014 at three percent, although it was more severe in some fields than in others.
Growers can achieve excellent management of this disease, and other important diseases, using Peanut Rx. This disease risk index can help growers to better understand how careful selection of production practices can reduce the risk to disease losses.
Updates For 2016
Most of the changes to Peanut Rx 2016 from the previous year’s version can be found in the cultivar/variety section of Peanut Rx with new varieties added.
With additional data, risk points for leaf spot assigned to variety Georgia-12Y have been reduced from 20 to 15. Such a change indicates that Georgia-12Y has leaf spot resistance similar to that of Bailey, Tifguard and TUFRunner™‘727.’
Three new varieties have been added to the 2016 version of Peanut Rx, all having high-oleic oil chemistry. These varieties include TUFRunner™‘297,’ Georgia-13M and Georgia-14N. In addition to having high-oleic oil chemistry, Georgia-14N is notable for its very high resistance to the peanut root-knot nematode.
Peanut Rx 2016 also includes new discussions on the impact of irrigation on management of white mold and on steps to reduce the risk of losses to the peanut root-knot nematode. As in the previous versions, growers will note that attention to variety selection, planting date, plant population, good crop rotation, tillage and other factors, will have a tremendous impact on the potential for diseases in a field.
Irrigation And White Mold
Irrigation is a critical component of a production system and can result in large peanut yields. However, the water applied to a crop with irrigation is also beneficial for the fungal pathogens that cause common diseases such as leaf spot, Rhizoctonia limb rot and white mold.
For white mold in particular, irrigation or ample rainfall can create conditions such as more abundant moisture for growth and also greater humidity within a canopy which favors growth and spread of white mold. However, rainfall and/or irrigation are essential in the movement of foliar-applied fungicides from the leaves to the limbs and the crown of the plant where protection is needed from white mold.
Under non-irrigated conditions, growers may actually observe more white mold than for irrigated peanuts because effective fungicides are not “washed” to the parts of the plant that must be protected from this disease.
Strategies to improve efficacy of fungicides for management of white mold in non-irrigated fields include the following:
1. Apply fungicides for control of soilborne diseases ahead of anticipated rain events to facilitate movement of fungicides into the crown on the plant.
2. Apply fungicides for control of soilborne diseases at night when the leaves are folded; such application timing will increase coverage of limbs and crowns.
Root-Knot Nematode Risk
Peanut root-knot nematode is not specifically included in Peanut Rx; however, several of the factors that affect risk to other diseases also affect risk to losses from nematodes. These factors include the following.
1. Variety selection: The varieties Tifguard and Georgia-14N are highly resistant to infestation from the peanut root-knot nematode. Growers who plant these varieties in a root-knot nematode infested field will not need to use a nematicide. Use of nematode-resistant varieties not only protects the crop in the field, but also reduces nematode populations for the next peanut crop as compared to if a susceptible variety like Georgia-06G was planted.
2. Crop rotation: Like risk to other diseases, the threat from peanut root-knot nematode is greatly reduced by rotating fields away from peanut and other susceptible crops like soybeans. Cotton and corn are excellent rotation crops to reduce the risk of peanut root-knot nematodes. Corn is also a host for the peanut root-knot nematode, but is a better rotation crop than either peanut or soybeans.
3. Tillage: Though much research is still needed, there is some indication that there is higher risk to nematodes in fields are prepared with reduced tillage than with conventional tillage. This effect is much less important than variety selection or crop rotation and is not always observed. However, there is some evidence that disrupting the soil, such as occurs in conventional tillage, could help to disperse nematode populations that are present in the root zone of the developing peanut seedling.
Sclerotinia blight’s initial symptom includes a rapid wilting or flagging of the tips of infected branches. Initially, lesions are small, light green and water-soaked. As the disease develops, the lesions turn light brown, elongate and appear sunken. Older lesions may be dark brown with a distinct border between diseased and healthy tissue. Another symptom of infected plants is the presence of shredded tissue (pegs and branches). The fungus produces white, fluffy fungal growth on diseased tissue when humidity is high. As the disease progresses, the fungus produces large black sclerotia on and in infected plant tissue.
Given its wide range of symptoms, spotted wilt disease is sometimes confused with white mold and CBR. Spotted wilt is a viral disease spread by thrips. Foliar symptoms can be one or more of the following: concentric light green to yellow ringspots (predominant on young foliage), chlorosis (yellowing), necrotic terminals and/or necrotic leaf spots, nondescript tan spots or blotches, oakleafing or streaking and mottling. Foliage of infected plants may be reduced in size. Plants infected early in the season are usually stunted, have reduced pod production and may die.
Stem rot or white mold is favored by hot, wet weather. Initially, foliage of the infected plants is chlorotic; this may include the whole plant or be limited to a branch. As the disease develops, the foliage turns brown to black. The crown area is usually light to dark brown depending on the stage of disease development. Symptoms unique to stem rot are white strands of fungus and white to brown sclerotia. These are usually found in the crown area of infected plants. Fields where stem rot is suspected should be scouted late in the season, 24 to 48 hours after a rain or irrigation event, beginning in late July and until digging.
Northern root-knot nematode symptoms are the same as those described for peanut root-knot nematode. Galls are usually smaller than those caused by peanut root-knot nematode and are limited to the roots. The root system of infected plants will be dense and bushy
Early leaf spot lesions are initially very small and light brown in color. Under favorable conditions, lesions enlarge up to one-half inch in diameter and are usually surrounded by a yellow halo (halo not always present). Lesions may be evident within 10 to 14 days after infection. During or after periods of warm temperatures in the 70s or above and during periods of high humidity, spores of the fungus can be observed on mature leaf spot lesions.
Rust is readily identified by the presence of numerous orange-to-red-colored lesions (pustules) on the underside of the leaf. Each lesion contains many orange to red-colored spores. Typically, leaves of infected plants will remain attached. Heavily infected areas in fields may have a scorched appearance.
Late leaf spot symptoms are similar to those of early leaf spot, but lesions are usually dark brown to black. Conditions for disease development are the same as for early leaf spot. During favorable conditions, spores of this fungus can be seen on the underside of the leaf. Under extreme pressure, lesions can develop on stems.
Web blotch lesions are characterized as having a netlike or webbing pattern, which is purplish, brown to tan on the upper leaf surface. Circular, tan to brown lesions may also be observed on the upper leaf surface. Lesions may grow together to cover the entire leaf. Older lesions are usually dry and cracked.
Symptoms of Rhizoctonia limb rot are usually not noticed until after digging of the plants. Infected branches that were in contact with the soil are black and rotten. Infection often starts near the tip of a branch, killing the tip and then progressing down the branch.
Peanut root-knot nematode infected plants are usually stunted and chlorotic. Typically, injury is not uniform in the field. Infected areas in a field are typically circular. Roots, pegs and pods of infected plants are galled. Galls on pods are wart-like in appearance and dark in color.
Wilting and death of lateral branches or the entire plant are symptoms of Diplodia collar rot that could easily be confused with symptoms of other diseases. Elongated dead lesions with light brown centers and dark brown margins may occur on stems. Fruiting bodies of the fungus appear as small black dots embedded in the infected host tissue. Collar rot is sporadic in occurence, but can cause severe losses when an outbreak does occur.
Cylindrocladium Black Rot (CBR) infected plants are initially chlorotic with foliage becoming brown to black as the disease develops. The symptom unique to CBR is the presence of brick red reproductive structures growing from the crown of infected plants, while the crown, itself, is usually black. Root systems of severely infected plants are deteriorated, void of secondary roots and are also black. Infection occurs at planting during cool, wet weather. However, symptom expression does not occur until mid-July.
Funky or irregular leaf spot is a problem of unknown cause. Symptoms appear early in the growing season on lower leaves and can look like early leaf spot. Some defoliation may occur, but yield losses have not been demonstrated. Fungicides do not control irregular leaf spot. Funky leaf spot has been found to be most severe on peanut varieties, such as Georgia-02C and Georgia-03L, but is not thought to cause yield loss for either. Stay on a good fungicide program and that will control the more important early and late leaf spot diseases.
Aspergillus crown rot causes pre- and postemergence damping off and sometimes kills up to five weeks after planting. Seedlings rapidly collapse and die. Dark brown discoloration is common on decayed roots and hypocotyls. Later, these areas often are covered with masses of black spores that look similar to bread mold. Aspergillus crown rot generally is of minor importance when high-quality, fungicide-treated seed are planted in well-rotated fields. Rotation and seed treatments are an effective control for this disease.