EVALUATION OF PHYTOTOXICITY OF A SOIL-APPLIED
HERBICIDE, DIURON, IN APPLE AND PEAR
Katsusa MANABE and Matsuo
ITOH"
Phytotoxic symptoms from chemicals often appear on crops temporarily, but they sometimes cause serious yield reduction T r e e crop injury caused by a soil-applied herbicide, diuron was studied in apple and pear to clarify the relationship between visual phytotoxic symptom and growth reduction Diuron a t higher rates than the commercial use rate, 157kg a i/ha, caused chlorosis and/or necrosis in t h e leaves, resulted in the significant growth reduction in newly developed organs, leaves, shoots and newly developed roots Leaf and shoot weight and new root weight increased a s uninjured leaf number increased This suggests that the growth reduction from diuron on tree crops may be attributed to the reduction in photosynthesis resulted from the reduction of healthy leaves in number
Visual score in crop injury w a s positively correlated to the proportion of injured leaf number to total leaf number, and was negatively correlated to leaf and shoot weight or new root weight Thus, visual assessment for tree crop injury readily represents both the intensity of phytotoxic symptoms in leaves on a whole tree and the growth reduction from diuron
Key words : diuron, growth reduction, phytotoxicity, soil-applied herbicide,tree crops
Introduction
Soil-applied herbicides are generally treated with foliar-applied herbicides in orchards to obtain a long term control of winter annual weeds in spring'" More nutrient becomes available for tree crops a s soil-applied herbicides reduce the competition with winter weeds T h e clean culture in orchards raises soil temperature in spring, resulting in the vigorous tree growth and fine fruit quality in citrus crops However, there is a risk of tree crop injury due to the leaching of soil-applied h e ~ b i c i d e s ' ~ ~ ' Tree crop injury through root uptake sometimes becomes a serious problem because tree crops produce fruit for many years
In general, there are two evaluation methods for phytotoxicity from pesticides i e measurement of plant growth reduction and visual assessment in phytotoxicity using symptom indexes"' Phytotoxic symptoms often appear temporarily during the growing periods However, some phytotoxic symptoms sometimes bring about serious reduction of plant growth or yield T h e phytotoxic severity affecting the current and later growth or the yield depends on the
intensity of phytotoxicity, crop species and growing conditions
'
T h e relationship betweenphytotoxic symptoms and growth reduction appears to be quite complicated in tree crops partly because they are perennials There are few reports on the relationship in tree crop injury
caused by leaching of soil-applied herbicides in soil(5' Diuron is known to show phytotoxic symotoms, chlorosis, sometimes necrosis in t h e leaves of tree crops under severe condition^'^'
T h e compound is one of t h e major soil-applied herbicides, and generally considered to be quite safe in many trees except Japanese chest nut T h e present paper studied diuron to clarify t h e
relationships between phytotoxic symptoms and growth reduction in apple (Malus pumzla Mill
var domestzca Schneid ) and pear (Pyrus serotzna Rehd var culta) when diuron caused tree
crop injury through its movement in soil Accuracy of visual assessment w a s discussed for evaluation of crop injury
Materials and Methods
T h e experiment w a s carried out using 3-year-old seedlings of 'fuji' apple (Malus pumzla Mill
var domestzca Schneid) and 'Kousui' pear (Pyrus serotzna Rehd var culta) under severe
growth conditions of soil and irrigation to detect t h e crop injury farm diuron Apple and pear seedlings were grown in 1/5000 Wagner pots filled with sandy loam containing 71% sand in glasshouse a t t h e Experimental F a r m of Kagawa University after they were transplanted on Dec 12 in 1993 T h e soil contained 1 7% of organic matter and had a C E C of 16 6 meq/100 g Chemical feretilizer (N,P,K : 14,14,14%) a t 300 kg/ha w a s incorporated into the soil before transplanting T h e commercial wettable powder formulation of diuron w a s diluted with 1,000 l/ha of water Diuron a t rates of 1 57, 4 71, 7 85 and 15 7 Kg a i / h a with three replications w a s applied to t h e soil with a pipet carefully, not to spray it on trees, on F e b 20 in 1994 T r e e crops were irrigated every 3 to 5 days when t h e soil surface w a s dried out Several organs newly developed during t h e experimental period were measured Total leaf number, injured
leaf number with chlorosis and/or necrosis including fallen leaves and shoot number were
counted on July 25 when those tree seedlings appeared to slow down to grow Shoot length w a s also measured Visual assessment for crop injury w a s made on a scale of 0 to 10 according to t h e intensity of chlorosis or necrosis on a whole tree and t h e overall growth reduction No injury w a s rated a s 0, and dead plants were scored a s 10 T r e e seedlings were dug out and heated to dry under 80°C for four weeks Newly developed leaves, shoots and roots were weighed after drying them up
Results and Discussion
Table 1 shows t h e effect of diuron applied to soil on t h e growth of apple and pear Uninjured leaf number, t h e dry weight ~f leaves and shoots and newly developed roots were reduced in both apple and pear with t h e increase of the rates of diuron Diuron a t higher rates than 1 57 Kg a i / h a caused chlorosis and/or necrosis in t h e leaves, resulted in t h e significant reduction in dry weight of newly developed organs Necrosis from diuron a t higher rates often resulted in fallen leaves in both apple and pear Diuron a t t h e use rate, 1 57 kg a i/ha, was quite safe even under confined pot conditions in this experiment T h e results here are t h e evidences that diuron w a s absorbed by roots and translocated to leaves showing t h e symptoms because t h e compound w a s applied to the soil carefully
Table 1 Growth inhibition by diuron applied to soil in apple and pear
V~suai score Total leaf injured leaf Uninjured Proportion of Shoot Total shoot Leaf and shoot New root
Treatments in injury number number leaf number injured leaves c) number lenqth (cm) weight(g) we~ght (g)
Apple
Untreated OO+OO 1 0 4 7 + 1 1 2 OO+OO 1 0 4 7 5 1 1 2 0 0 2 0 0 1 1 0 + 3 5 9 8 5 - 7 2 2 9 9 2 6 2 2 6 1 ~ 6 4 Dluron157a) 9 7 2 2 9 b ) 8 3 7 + 6 4 7 7 7 2 8 1 6 0 2 6 0 7 7 5 2 1 1 5 1 0 3 + 1 5 7 9 0 ~ 2 5 2 6 9 5 2 2 1 1 3 5 4 6 D~uron 7 8 5 4 3 2 2 3 7 1 3 2 1 2 7 2 3 7 ~ 2 1 8 4 7 7 2 3 3 8 3 2 1 2 2 8 5 1 1 7 5 3 2 5 5 7 + 1 4 5 1 7 2 5 9 9 1 8 2 2 5 3 Diuron 4 7 1 5 7 2 1 9 6 9 0 2 1 1 8 3 0 7 5 2 1 5 3 8 3 - 1 4 1 3 9 5 + 1 8 2 9 7 2 2 1 5 7 0 + 1 9 1 1 0 4 2 2 7 1 6 2 5 3 0 Dluron 1 5 7 0 8 + 1 0 8 4 3 2 4 5 0 0 2 0 0 8 4 3 % 4 5 0 0 2 0 0 1 1 3 1 2 9 8 0 9 5 2 6 7 2 4 5 + 1 9 2 0 4 + 6 1 Pear Untreated 0 0 + 0 0 6 5 0 2 1 1 4 0 0 2 0 0 6 5 0 2 1 1 4 0 0 + 0 0 1 2 3 2 4 2 G S O r l O O 2 5 5 k 2 7 12 9 2 2 7 Dluron 1 5 7 5 8 2 3 6 3 5 3 + 8 3 2 8 0 2 1 3 1 7 3 ~ 1 2 7 7 3 5 5 2 8 6 6 7 5 3 2 3 3 8 5 2 1 2 8 9 + 8 0 5 7 ~ 2 5 Diuron 7 8 5 3 3 + 1 5 5 6 3 + 7 1 1 2 0 + 1 2 8 4 4 3 ~ 1 3 6 2 4 5 2 1 7 8 1 0 3 2 2 1 4 4 7 2 2 1 2 2 0 2 5 6 2 8 9 5 3 1 o~uron 4 7 1 2 7 2 0 6 4 7 7 ~ 1 3 6 1 0 7 5 8 5 3 7 0 + 2 1 9 2 8 8 ~ 1 7 8 1 0 7 5 3 2 3 8 7 ~ 4 1 1 9 1 ~ 5 2 9 8 ~ 1 3 ~ ~ u r o n 1 5 7 1 5 k 0 9 5 7 7 k 1 7 . 7 O O t 0 0 5 7 7 h 1 1 7 0 0 i 0.0 1 1 3 ~ 2 . 3 4 7 5 ~ 2 9 3 2 2 4 ~ 5 6 9 8 ~ 3 5 a) kg a i /ha b) Mean + standard error c) Transformed by arcsinC
Pear
-
L
I
1 1 I I 1 - 10 20 40 60 80 100 120 0 20 40 60 80 100 120
Uninjured leaf number/plant
F i g 1 Relationships between uninjured leaf number and leaf and shoot weight
or n e w root weight in apple and pear **p<O 01
Most growth measurements except shoot number were decreased a s the rate of diuron increased Shoot number w a s independent of diuron application in both apple and pear except that the highest rate of the compound caused t h e great growth reduction in pear Total leaf number and total shoot length of apple were also independent of diuron application, while those measurements of pear were reduced a s the rate of t h e compound w a s increased
Leaf and shoot weight and new root weight increased a s uninjured leaf number increased
0 2 4 6 8 10
Visual crop injury
Fig 2 Relationships between visual score in injury and proportion of injured
leaves to toral leaves in number in apple and pear **P<O 01
root weight, there were significantly positive relationships in both tree crops (r=O 739 in apple and 0 868 in pear,p<O 01) No clear relationships were found between leaf and shoot weight or new root weight and shoot number or total shoot length
T h e primary action of the urea herbicides including diuron is considered to be inhibition of the Hill reaction of photosynthesis Chlorqsis induced by diuron may reduce the photosynthesis ability in leaves, resulted in the growth reduction of leaves, shoots and roots in weight'" T h e number of shoots and their total length may have not been affected by photosynthesis of newly developed leaves in small pot conditions, because no clear relationships were found between uninjured leaf number and shoot number or total shoot length T h e results mentioned above indicate that phytotoxic symptoms from diuron in leaves of tree crops resulted in reduction of the top and underground growth Significant growth reduction of tree crops may
be attributed to the reduction in photosynthesis in the leaves showing chlo~osis and/or
Table 2 Correlation coefficients between visual score in injury and several growth characters
Visual score in injury
---
Characters
---
Apple Pear--
Total leaf number Injured leaf number Uninjured leaf number
Proportion of injured leaves a) Shoot number
Total shoot length Leaf and shoot weight New root weiaht
a) Transformed by arcsinj:.
Table 2 exhibits relationships between visual crop injury and several growth characters T h e
proportion of injured leaf number to total leaf number was statistically most correlated to visual score of tree crop injury Visual rating in injury increased tightly as the proportion increased (Fig 2) Only one individual in pear was out of the relationship Negative relationships were found between visual crop injury and leaf and shoot weight or new root weight (Table 2) Thus, visual score in injury from diuron can be primarily interpreted a s the intensity of phytotoxic symptoms in leaves on a whole tree, secondly a s the growth reduction Visual assessment for tree crop injury 1,eadily represents the growth reduction from diuron
Acknowledgements
T h e authors are grateful to Mrs M Miki, T Takagi and H yamazaki a t the Experimental
From a t Kagawa University for their technical support to complete the experiments
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