Cookies on Plantwise Knowledge Bank

Like most websites we use cookies. This is to ensure that we give you the best experience possible.

Continuing to use www.plantwise.org/KnowledgeBank means you agree to our use of cookies. If you would like to, you can learn more about the cookies we use.

Plantwise Knowledge Bank
  • Knowledge Bank home
  • Change location
Plantwise Technical Factsheet

groundnut scab (Sphaceloma arachidis)

Host plants / species affected
Arachis hypogaea (groundnut)
List of symptoms/signs
Growing point  -  distortion
Leaves  -  abnormal forms
Leaves  -  necrotic areas
Stems  -  discoloration of bark
Stems  -  distortion
Whole plant  -  distortion; rosetting
Whole plant  -  early senescence
Symptoms
S. arachidis is responsible for lesions of the leaves, petioles and stems of groundnut plants. The first symptoms appear on the leaves and petioles of the apical part of the plants, in the form of small lesions from 1 to 2 mm diameter, normally irregular, with central depressions and raised, light coloured borders, on the upper surface. The lesions are more commonly found near the main veins of the pinnae.

The coalescence of lesions in the petioles and stems, where they are usually larger and more irregular, results in distortions of these organs, causing necrosis or hyperplasia of the affected organs. The more severely affected stems and petioles become sinuous or crooked. In the final stages of the disease, the lesions become cork-like, practically covering the whole surface of these organs.

The disease caused by this fungus, known as scab, can be mistaken for attack by thrips, and there seems to be an increase in the incidence of this disease when this pest is not adequately controlled, the reason for this not clear. There may be transmission by these insects, the injuries allowing for the pathogen to penetrate the tissue, or it may simply be a coincidence of favourable environmental conditions (Barreto, 1997).

Prevention and control

Introduction

Adequate control measures for groundnut scab are necessary for a variety of reasons, amongst which one could cite: the prevention of crop losses when the pathogen is present in the area cultivated; prevention of losses when susceptible cultivars are used and when the climatic conditions are favourable for the development of the disease, especially during the initial stages of crop growth.

Resistant Cultivars

The use of resistant varieties is the main means of controlling scab. The first reports on variability in the resistance to the disease were made by Costa and Souza (1941), Cruz et al. (1967) and Rossetto et al. (1968). Resistant genotypes develop lesions of the pathogen on the leaves, stems and petioles, which are, to varying degrees, less coalescent, allowing for almost normal development. Soave et al. (1973) classified the resistance into four levels according to the severity of the symptoms, varying from 1 (practically showing no symptoms) to 4 (completely crooked stems, covered by scab lesions, severely affected petioles and leaf veins, yellowish or even dead plants).

Moraes et al. (1983) showed high correlation between lesions observed on the leaves, petioles and stems and genotypes showing various levels of resistance, suggesting that the disease can be visually evaluated with relative ease from one of these parts (see Detection and Inspection Methods).

The evaluation of millions of reports on the different morphological groups of A. hypogaea, suggest that many of them have a certain degree of resistance to the disease, even amongst genotypes showing good agricultural performance (Moraes et al., 1978; Moraes et al., 1983; Godoy et al., 1990; Godoy et al., 1993; Moraes et al., 1995).

In many cases, genotypes evaluated as resistant to black and brown spot, are also resistant to scab (Moraes et al., 1995). The cultivars IAC-Caiapó and IAC-Jumbo are partially resistant to black spot and to scab, although Lopes (1997) showed greater resistance of these varieties to the cercosporioses (black and brown spot), to mud spot and to scab.

Cultural Practices

Considering that S. arachidis persists in crop remains, agricultural practices such as crop rotation are important to reduce the initial inoculum and consequently the severity of the disease. It is very common to rotate groundnut with pasture land or with sugarcane, in areas of sugarcane renovation (see Agricultural Practices).

Chemical Control

A form of indirect control, if one admits an association between the degree of severity of the disease and infestation by Enneothripes flavens, is the efficient chemical control of these pests, although this requires further detailed studies to prove the connection.

In the producing regions of São Paulo, Brazil, the much used cultivar Tatu is susceptible to scab and it is necessary to control the disease with fungicides. However, the fungicides most widely used in groundnut plantations are those used in the control of black and brown spot, which do not always efficiently control scab. Another limitation is that in most cases, the fungicides (whose efficiency is usually tested with respect to controlling C. arachidicola and C. personatum) are usually tested under conditions of low disease intensity. Thus, with a higher intensity, the efficiency of some of the products may not be confirmed.

Recommended fungicides were maneb and a mixture of benomyl + mancozeb. The mixture benomyl + maneb also controls the disease (Campacci and Oliveira, 1983; Maezono and Gimenes-Fernandes, 1985). Lopes and Lasca (1992) showed good control of scab and cercosporioses with the mancozeb and Lopes et al. (1997) showed good control with a mixture composed of chlorothalonil + sulfur. More recently the introduction of new products for the control of cercosporioses may allow for the use of a fungicide which also shows good control over scab. Of these products, tebuconazole and propiconazole (Lopes and Lasca, 1992; Barreto, 1997) and difenoconazole (Dario et al., 1994; Leite et al., 1994; Barreto, 1997) were reported as being efficient in the control of black and brown spots and scab.

It must be emphasized that the recommendations for chemical control must be effected together with other control measures, as complementary actions in an integrated management programme.

 

Impact
In general this disease appears towards the end of the vegetative cycle of the culture, not resulting in much damage. It becomes important when it occurs during the initial stages of growth, impairing the development of severely-affected plants. The disease has caused serious problems in recent years, especially in the north east of São Paulo State, in the areas of sugar plantation renovation. Despite being considered an important disease under the conditions of cultivation in the State of São Paulo, Brazil, there is a lack of specific information about production losses due to this disease.

Related treatment support
 
External factsheets
Sistemas de Produção Embrapa - Publicações eletrônicas, Embrapa, Portuguese language
Zoomed image