Pea is (Pisumsativum L.) the fourth important legume crop in the world. Its importance can be estimated from the fact that, 40% of total pulse trade is based over pea. Pea is also important due to its high nutritional value which contains 15.5 to 39.7% of proteins. It is also a rich source of carbohydrates, Calcium, Potassium, Sodium, Iron and Phosphorus.This crop is best for the improvement of soil fertility, particularly it reduce the nitrogen requirement of other crops upto 20-50 Kg/ha.Pulses have been an imperative part of Pakistani food for the years and known as “common man’s meat”, because they are rich source of proteins. They are cultivated in Pakistan over a large scale which covers 1,298,000 hectares. Among the total cultivated area under pulses, 84% is present in Punjab province, which is followed by 8% in Sindh, 5% in Baluchistan and 3% in KPK.
Pea is cultivated for different purposes in Pakistan because their seeds, pods and foliage all are used efficiently. Generally it is used as fresh vegetables but canning and freezing has also been introduced.This cropis cultivated as winter annuals, because it requires cool and humid climate where temperature ranges from 7 oC to 30 oC as optimum temperature for the vegetative growth of pea is 10-21 oC. When the temperature increases upto 27 oC the vegetative growth and pollination are adversely affected.The market demand of pea can be anticipated from its consumption, which goes up to 160,000 tons annually.It also ranked second, after Chickpea in Pakistan. But production level of pea is not adequate to fulfill our domestic needs, although we import 106,000 tons of Peas from other countries.
Regardless of other constraints in pea production, the diseases are most important factors which influence the production of pea. Particularly they are infected by several pathogenic fungi in which most destructive are Cladosporium pisicola, Ascochyta pisi, Fusarium oxysporum, Sclerotina sclerotiorum, Aphanomyces euteiches, Peronospora pisi, Botrytis cinerea, Erysiphepolygoni and Pythium species.
Among the other pathogens of pea, Fusarium oxysporum f. sp.pisi is more important which cause partial to complete loss of crop. It is a major siolborne pathogen in Pakistan which is widely distributed throughout the country. It was firstly reported in pea in 1994 by Hague and Ghaffar in Northern areas. It is also estimated that50% losses of fruits, vegetables and field crops in the country are due to soil born diseases in which Fusarium spp. have major contribution.
Pea wilt is caused byFusarium oxysporum f. pisi. Generally, two races are most importanti.e Race 1 and race 2. Both races attack the plants in a different pattern. “Common wilt” is caused by race 1 ofFop. It occurs in the early season crop while race 2 causes “Near wilt” disease in peas, which occurs in late or mid-season cultivars. Optimum temperature for disease development is 21-26 oC.
When thispathogen attacks the plant it invades the root system and move upward through water conducting xylem tissues. In the result the plant becomes pale. Infection starts from lower leaves and gradually moves towards the upper portion. Leaves and stipules show downward curling and plant growth is stunted. In severe cases the plant wilts and dies due to vessel clogging. Generally pods are not formed, if pod formation occurs the seed is not well developed. Sometimes infected seed is formed which act as a secondary source of disease spread.Root system is not destroyed at all, sometimes remains unaffected in mild cases. Cross section of root and basal stem shows brick red, orange or yellow the vascular discoloration depending upon the severity of infection and race type.
PHYSIOLOGY OF INFECTION
Infection of Fusarium has been thoroughly studied by using advance techniques such as Electron Microscopy, and the work has been represented in various scientific writings.Before the infection takes place the fungus germinate and hyphae hold the roots of plant.The mode of attack in Fusarium oxysporum is direct penetration for the infection process to take place.Then the mycelium progresses through cortex and finally it reaches the xylem vessels where it grows extensively. As the nutrients are available from vascular bundles of host, the Fusarium completes its life cycle and made its microconidia. These reproductive units move towards the shoots and leaves with the movement of sap. When these micro conidia germinate they make heavy masses of mycelium which cause vessel clogging. It causes a barrier in the water conductance and plant is starved due to unavailability of water and nutrients. Initial wilting symptoms are appeared due to water stress, which is further associated with the production of fungal toxins.When Host defense response is failed, it alsoinvolved indirectly in vessel clogging due to production of gels, pectins, gums etc.Fusarium oxysporum also has peculiar characteristic, it remains inside the vascular bundles particularly in xylem vessels during the life of plant. When the plant dies, it profuses to the next tissues particularly parenchyma. After complete invasion it sporulate on the surface of plant.
Generally the infection of Fusarium oxysporum can be described in the following steps:
(1) Recognition of host through biochemical and mechanical signals.
(2) Identification, attachment and penetration of hyphae on the roots of host plant.
(3) After penetrationaccession of vascular tissues by eliminating the physical barriers which include cortical tissues and endodermis.
(4) Adaptation to host environment, particularly tolerance against defense mechanism which is based over a series of antifungal compounds.
(5) Extensive production of hyphae and production of micro conidia for further distribution to upper parts of plant.
(6) Finally the pathogen secretes toxic compounds which cause mortality of tissues.
So far no chemical has been successfully applied under field conditions. Crop rotation is not implemented as management tool for the control of this disease because pathogen can exist in soil up to ten years. Resistant varieties can prove their ability against Pea wilt. So far, Disease escape is the best strategy which can be achieved by changing the location and time of harvest.
Cultivation over well drained soils and planting of chemically treated seed particularlyon raised beds, reduce the disease severity. Use of biocontrol agents such as Trichodermaharzianum and Streptomyces sp. have been proposed as a modern management strategy.
*Rana Muhammad Sabir Tariq, Dr. Shahbaz Talib Sahi, Dr. Abdul Hannan and DawoodAnser Saeed**
*Department of Plant Pathology, University of Agriculture, Faisalabad.
**Institute of Horticultural Sciences, University of Agriculture, Faisalabad.